Selecting Your StructureS       etting out to build a high tunnel   Single-bay high tunnels come in two       tunnel owner...
gain (See Mazria’s “The Passive            To get the most from your investment,       caterpillar tunnels can be construc...
The Trouble with PVC: A Cheap but Unsustainable Material                           Due of the dangers of exposure to      ...
and insulates 40% better than glass,       unheated structures. In heated             cover inside the structure. He alsob...
farmer with a disposal problem.             at very high temperatures and are          When highly contaminated plasticsPo...
With an initial aim of reducing             company has installed a $10 million       easier choice for other farmers in t...
tunnels. Haygrove’s lacing system          Haygrove tunnels concentrate the run-      farmers in the Eastern U.S. to growh...
instance, there are five pieces in each   poly cover is secured to the                 vegetables, cut flowers, greens, an...
cover and the way it is secured to the     temperatures. During the coldest           Farm Tek, “the only source of verygr...
gets a high quality fruit that bears well   minimum of two people to install a          pounds per plant on their 500 plan...
them later in the year after the plants    easier field production. The O’Haras       that these plants are winter annuals...
and with the length of this low tunnel,     For several years, Bryan has been         • Low tunnels allow farmers to growt...
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Transcript of "High tunnels selectingstructure"

  1. 1. Selecting Your StructureS etting out to build a high tunnel Single-bay high tunnels come in two tunnel owners are wise to remove is no different from any other primary shapes: Quonset and Gothic their plastic for the duration of the capital project. The steps prior arch (see diagram below). The snowy season.to construction—planning, financing Quonset shape is relatively short andand funding, and ordering—are often squat with a rounded roof and sloped Match your structure’s design load tothe most difficult. The actual building sides, while the Gothic, like a local conditions of snow and wind.project is relatively simple and can be cathedral, has a high pointed peak and Some greenhouse suppliers can selectexciting. The completion of any straight sidewalls. Unheated Quonset design specifications appropriate toproject has rewards of satisfaction and structures can also serve as cold your county. In the first winter, onefuture profit. frames for overwintering nursery northern farmer’s high tunnel— stock. Multi-bay high tunnels, purchased from a southern supplier—In shopping for a high tunnel, and including the Haygrove structure was destroyed in a snowstorm, alonghere we focus on single-bay described in this manual, are usually a with the perennial plant materialstructures, a grower will need to select series of interconnected Quonset- inside.from an array of designs, sizes, and shaped tunnels. Gothic type tunnelsspecial features. Different materials have several advantages compared to The taller sidewalls of Gothic tunnelsand construction methods can also be Quonset models. In many offer more usable space along theused. Experienced farmers have circumstances, these advantages sides for crop production and growthweighed the advantages and easily offset their greater cost. and for working comfort. For trelliseddisadvantages of these options for crops like tomatoes, Gothic-styletheir climate, markets, production A Gothic-shaped structure readily tunnels provide adequate height bothaims, and bottom line. sheds snow because of the steep pitch for the interior and perimeter rows. of its roof. Quonsets, especially those The headroom over the edge beds in aQuonset or Gothic-shaped with PVC bows, need to be swept free Quonset tunnel may be so low thatStructures? of snow to prevent collapse. When even a short person is uncomfortableThe shape of a tunnel affects its snow threatens, some growers set up when using a walk-behind seeder, forperformance. It will have an effect on 2" x 4"s as temporary props under the instance.lighting (and shading), energy gain, ridge pole, purlins, or bows of theirgrowing space, and ventilation. Quonset-shaped high tunnels. PVC The Gothic shape also contributes to better air exchange and moisture control, and thus a superior growing environment. The greater height of Gothic tunnels allows for better ventilation through higher gable-end vents. Gothic arch roofs tend to have enough of an angle to help shed water that condenses on the interior, instead of dripping on the plants below. In Quonset tunnels, since the whole structure is curved, opened roll-up sides expose some of the crops growing along the side to precipitation and other adverse weather conditions. This defect can be partially alleviated by purchasing extended ground posts. Footprint High tunnels come in many sizes, in widths from 14 to over 40 and in incremental lengths. A width to length ratio of 1:2 is ideal for passive solar28 – High Tunnels
  2. 2. gain (See Mazria’s “The Passive To get the most from your investment, caterpillar tunnels can be constructedSolar Energy Book”). However, you it might make more sense to start with of this weaker material. It should beget more for your money with a 30 x a short, wide house (i.e., 30 x 48) and noted that PVC is a persistent poison96 high tunnel than a 30 x 60 add on later than to buy a 16 x 96 that harms human and animal healthstructure. high tunnel that may quickly outgrow and pollutes the environment during its usefulness. manufacture and disposal (See “TheNarrower tunnels will experience Trouble with PVC” on page 31).more heat loss than wider tunnels Also when determining the size ofbecause of their perimeter to growing tunnel to build, take into account how Bow spacing will depend upon thearea ratio. A 10 x 90 tunnel has a the structure will fit on your property. overall design of the tunnel, the200 lineal perimeter and a 900 square What are the site possibilities? How anticipated snow load, and thefoot growing area, whereas a 30 x 70 much land is available and what is its strength and capacity of its componenttunnel also has a 200 lineal perimeter topography? It is critical to have steel. Although a 4 bow spacing isand 2,100 square foot growing area. enough room around the tunnel for used fairly commonly, with trussesThe second structure has 133% more easy access with vehicles and placed on every other bow, growersgrowing area, and less than half the equipment, snow removal, water are increasingly turning to 5 bowsratio of perimeter (or heat loss drainage, and ventilation; to avoid that utilize trusses on every bow.potential) to growing area. Wider shading; and to allow for futuretunnels also tend to be taller and expansion (See “Site Considerations” (See “Bending your Own Frames” onprovide improved ventilation and on page 26). page 43.)interior air circulation. Baseboards and Gable-Under northern growing conditions, endseven 30 wide high tunnels can besufficiently ventilated with roll-up Selecting Materials Most high tunnels use wood for baseboards, hip boards and gable-endsides and large gable-end vents and framing. For the gable-end, anotherdoors. In warmer climates, in tunnels The Frame option is steel. While more costly, itwith tall, dense crops like tomatoes, Historically, wood was commonly will not need to be replaced and isnarrower tunnels (20 to 26) may used to frame greenhouses, but it is easy to work with. For the baseboard,more effectively reduce stale air in the has gone out of fashion because of its recycled plastic lumber, that is rot andmiddle of the structure without relatively high maintenance cost and insect resistant, is anothermechanical ventilation. the availability of steel greenhouses. comparatively expensive alternative. Pete Johnson, a year round grower inSize North-central Vermont, is an In selecting the type of lumber to beHow big should your high tunnel be? exception. He is currently growing used for the baseboard and hip board,Consider not just how much growing tomatoes and salad crops in a very consider their rates of decay. Findingspace you expect to use now, but also large high tunnel which he framed lumber that is affordable, durable, andwhat you’ll want a few years into the with Eastern white cedar and other sustainably harvested is a challengefuture. A well-built tunnel will last at local trees. For most growers, steel is and trade-offs are inevitable. Forleast 20 years. the best material for a high tunnel example, redwood is very expensive frame. The best greenhouse structures and unsustainable.Another important factor in are made of high tensile strength steeldetermining the size of high tunnel to covered with a very good galvanized Other more accessible choices,purchase is the amount of additional coating to prevent rust. (depending on location) include rot-workload you are prepared to take on. resistant wood species such as theSteve Moore estimates that a 30 x 96 An alternative to steel for the cedars, cypress, black locust, andhigh tunnel that is intensively planted structural members of a high tunnel is Osage Orange. White oak can also bewith multiple crops can take 10 hours polyvinyl chloride (PVC) pipe. Price used, though it will need to beor more of labor each week after is PVC’s only real advantage replaced after about eight years, theinitial set up and skill development. compared to steel. Mainly it is used in second time you replace the tunnel’sMarketing time is additional. When a farmer-built tunnels. Tunnels whose polyethylene film. Cheaper but lessfarm uses a simple cropping pattern, bows are made of PVC pipe are more attractive options are hemlock, pine,like Slack Hollow Farm’s winter prone to collapse under snow load and and spruce, listed from most to leastspinach system, regular management wind. Only narrow high tunnels with a rot-resistant.time can be greatly reduced. Quonset-shape and smaller walk-in or29 – High Tunnels
  3. 3. The Trouble with PVC: A Cheap but Unsustainable Material Due of the dangers of exposure to human and animal health, CCA Polyvinyl chloride (a.k.a. vinyl or PVC) is a chlorinated hydrocarbon, like (chromated copper arsenate) pressure- DDT and dioxin. During PVC production and disposal, very large quantities treated wood has not been produced of hazardous organochlorine chemicals are produced unintentionally and for most residential and general released into the environment. As a result of its intrinsic hazards, PVC is consumer uses since 2004. Two under intense scrutiny in many countries, by governments as well as Canadian government agencies, environmental and health advocates. Environment Canada and Health and PVC is virtually impossible to recycle because each PVC product contains a Welfare Canada, consider arsenic to unique mix of additives. Even in Europe, where PVC recycling is more be a “non-threshold toxicant” (i.e., a advanced than in the U.S., less than 3% of post-consumer PVC is recycled. substance for which there is believed And even most of this tiny fraction of PVC is merely “down-cycled” into to be some chance of adverse health other products, so the amount of virgin PVC produced is not reduced. effects at any level of exposure). Arsenic leaches out into soil and is The production of chlorine for PVC uses as much energy as 80 medium- taken up by plants. Organic farmers sized nuclear power plants would generate. Chlorine production is an are prohibited from using arsenic- extremely energy-intensive process that consumes about 1% of the world’s containing pressure-treated lumber. total electrical output. Among the toxic chemicals produced during the PVC lifecycle are Borates are one of the arsenic-free chlorinated dioxins, chlorinated furans, PCBs, hexachlorobenzene, and wood treatment alternatives already octachlorostyrene. Workers and communities are exposed to high levels of on the market. Treating oak with the toxic substances released into water, air, and soil due to PVC production. approved borate products may These toxic substances are also released when PVC is incinerated. lengthen its lifespan to 12 years. Steve Moore has treated oak with sodium As a group, toxic by-products of PVC production are highly persistent. These tetraborate and sodium octaborate, chemicals do not degrade in nature, so they build up in the environment. both of which may meet organic Transported by wind currents and water, their presence has been documented standards if from mined sources. He even in the most remote regions of the globe. uses these food grade borate products Since these chemicals are fat-soluble, they build up in the tissues of living as they can be readily and organisms. In species high on the food chain (like humans), the levels of these inexpensively obtained from local chemicals can be millions of times greater than in the environment. In mammals, chemical suppliers. Borate wood these chemicals easily cross the placenta and concentrate in breast milk. preservatives such as Boracare can be purchased from pesticide distributors. Some of the toxic chemicals produced when PVC is made are dangerous They penetrate into the wood much even at extremely low doses. For instance, at doses in the low parts per more effectively than the food grade trillion, dioxin causes damage to fetal development, and the immune and materials dissolved in water. endocrine systems, and impairs reproduction. Cancer and brain and nervous system damage are also associated with exposure to these organochlorines. End walls can be made of plywood In its pure form, PVC is rigid and brittle. To make flexible vinyl products, (painted is best), twin wall large quantities of plasticizers called “phthalates” are added. Phthalates cause polycarbonate sheets ($1.50/ft2), other cancer, infertility, testicular damage, reduced sperm count, suppressed structured sheets, or just polyethylene ovulation, and abnormal development of male reproductive tract in lab film ($0.10/ft2). Of course, opaque animals. Like the organochlorine byproducts of PVC production, phthalates materials like plywood prevent light are pollutants that bioaccumulate (i.e., persist in the environment) and are transmission. found in the tissues and fluids of the human population. As they are not chemically bonded to PVC, but merely mixed in when the vinyl is Twinwall polycarbonate is an formulated, they eventually leach out into air, water, and other substances extruded ribbed high-tech plastic with with which the vinyl comes in contact. double walls for added insulation. It is sold as structural sheeting. For high Pure PVC is not stable, so lead, cadmium, and organotins are added to tunnels, it has an application for the stabilize vinyl used in construction and for other extended-life applications. gable-end walls. As a hoop house These stabilizers, which are toxic and persistent in the environment, are covering, it compares favorably with released when vinyl is formulated, used, and disposed. both polyethylene and glass. It (Source: Thorton, PhD, Joe. Environmental Impacts of Polyvinyl Chloride transmits up to 83% of light (more Building Materials. A Healthy Building Network Report.) than two layers of polyethylene film),30 – High Tunnels
  4. 4. and insulates 40% better than glass, unheated structures. In heated cover inside the structure. He alsobut weighs one-sixteenth as much and structures (where the interior frost is hoped to gain the benefit of enhancedwon’t shatter. It is very durable and not present), infrared AC plastics light transmission and save money bycuts easily with a saw. It is far more undoubtedly retain heat better. not using another layer of high tunnelexpensive than poly but costs less covering. But neither of these theoriesthan glass. New designer greenhouse films are proved correct. In the dead of winter, now entering the market. When the the double walled tunnel stayedPlastic Film Cramers replaced the film on their warmer and outperformed the singleBy far the most common covers for Haygrove multi-bay tunnel, they layered tunnel.high tunnels of all types are made of chose Luminance THB (thermal heatpolyethylene. UV resistant barrier) poly which costs 10% more Double Versus Singlegreenhouse-quality polyethylene is far than ordinary greenhouse film (See Layerssuperior to common construction- www.bpiagri.com/hort-luminance.htm). A double layer of poly film withgrade polyethylene. It transmits light Using infrared blockers, this enhanced inflation between the layers providesbetter; is more resistant to wind, heat, poly reduces excess daytime heat and insulation and reduces heat loss byand yellowing; and has a longer life. scorching while also helping to 40% according to Aldrich and Bartok minimize heat loss at night. It also (see NRAES publication,It is important to replace poly film as increases light diffusion, making more “Greenhouse Engineering,”). Alongrecommended. For instance, after four light available to plants to increase with increasing heat retention, theyears, standard 6-mil plastic loses photosynthesis and yields. In second poly layer reduces the lightabout 15% of its ability to transmit particular, it is recommended for level by about 10% so a balance mustlight. This is particularly significant ornamentals and nursery stock and has be reached. Low light levels causeduring winter production, especially been shown to improve tomato yields. plants to become weak and leggy, andin cloudy climates. However, the manufacturer cautions slow down growth. As an alternative that it is not the best choice for early to double poly layers with an inflationGreenhouse film treated with anti- spring growth. fan, some farmers use multiple layerscondensate additives prevents of floating row covers, which morecondensation drips. Infrared re-radiant Steve Moore experimented with drastically decrease light(IR) materials are added to film to several types of plastic film over a transmission. Unless these covers arereduce overnight heat loss. In the multi-year period on two adjacent removed during the day, cropU.S., metal halides are typically used high tunnels. He compared double production may suffer.to treat the film, while in Europe layers (inflated) of the standard 6-milphosphorous and boron compounds 4-year film with infrared re-reflectants Many farmers have found it sufficientfill this function. and anticondensate to single layer to use a single layer of polyethylene Coeava, a 7.8 mil film with reportedly on their high tunnels. However, in theChris Wien, at Cornell University, an 8-year useful life. winter, these high tunnels will havepoints out that “films that lack an greater heat loss and will be colderadditive which blocks infrared In south central Pennsylvania, a high than tunnels with a double layer ofradiation can allow so much heat to tunnel with two layers of 6-mil, 4-year poly. And where a heating system isescape on cold clear nights that standard poly was warmer by an used, significantly more fuel will betemperatures in high tunnels are lower average of over 6°F during the winter, needed if just a single layer of polythan they are outside the tunnel. You and had superior plant growth film is employed. Using a doublecan have instances in which the plants compared to high tunnels with a layer requires electricity to run a smallinside freeze before plants just outside single layer of 7.8 mil high blower fan. An alternative to beingthe greenhouse. It is very important performance plastic. Steve suspects connected to the electrical grid is athat polyethylene films used on high that this difference in thermal modest solar power system. (Seetunnels have an infrared blocker performance between the two types of “Photovoltaic Inflation Systems” onadded to prevent such a problem.” film would be less significant in a page 48 and “Getting the Right Pressure warmer climate or under late fall or on Your Inflation” on page 46.)Anecdotal evidence suggests that the early spring conditions.frost forming on the inside of the Disposal and Recycling ofplastic on a high tunnel is an excellent In carrying out this experiment, Steve Agricultural Plastic Filmsreflector of infrared radiation. Steve had thought that the lower insulating Plastic film must be disposed of whenMoore thinks that it may be equal in ability of a single layer of poly could its useful life is over, presenting thevalue to special infrared AC plastic in be offset by using more layers of row31 – High Tunnels
  5. 5. farmer with a disposal problem. at very high temperatures and are When highly contaminated plasticsPolyethylene covers on high tunnels equipped with mandatory pollution are used to make the pellets, moreand greenhouses make a significant control devices. An early 1990s study toxic emissions result, sometimescontribution to the growing problem for the US EPA reported that 20 times exceeding regulatory limits. Oneof waste plastics. Polyethylene film as much dioxin, 40 times as much hypothesis is that dirt and debris limithas rapidly replaced more durable particulate matter, and many times air movement to the combustiblematerials in many farm applications. more metal emissions resulted from materials—a problem that increasesPlastic bags and covers are used to open burning of household waste in when plastic mulches are wet.store hay and ferment animal feeds barrels compared to municipal wastesuch as silage and haylage, and incinerators. Originally researchers envisioned “co-palletized goods are shrink-wrapped. firing” small burners with these wasteMost of these plastic films are Since that study, the gap in emissions plastic pellets and coal for heatingproduced from low-density from open burning versus incineration greenhouses and for other farmpolyethylene (LDPE #4) resins. has grown exponentially due to applications. But pollution concerns stricter pollution control standards for may require that the pellets be burnedThe most environmentally sound incinerators imposed since 1995. as a tiny portion of the total mix in adisposal option for these plastics is Between 1990 and 2000 emissions of large industry or municipalrecycling, which is discussed below. toxic compounds called dioxins and incinerator. The toxicity of ash fromMost agricultural plastics are not furans declined by about 99% and burning plastic fuel pellets on therecycled, however, in part because heavy metals by more than 90% in 66 farm is another area of concern.recycling opportunities are few and large municipal waste incinerators.far between. Surveys in New York Recycling is a superior way to disposeand Pennsylvania suggest that about On-farm burial removes refuse from of agricultural plastics. Recyclinghalf of agricultural plastics are burned sight, but decomposition is extremely programs are underway in theand the remainder are buried or slow and the potential exists for Northeast US and Canada fordumped on-farm. These states, like movement of water-soluble handling most types of plastic resinsmany others, do not restrict the on- breakdown products into used in agriculture, though theirfarm disposal of plastics. Therefore, groundwater. Leaving discarded geographic coverage is spotty. In thefarmers have no economic or legal plastic lying around is unsightly and absence of such a program, someincentive to transport these materials gives a negative impression about farmers are stockpiling their wasteto a regional transfer station or other agriculture. Pools of water that form plastic film until they have access to acentral collection facility for recycling in the plastic film serve as mosquito recycling program. A trip to purchaseor controlled disposal (at a landfill or breeding grounds. equipment or supplies may serve as anincinerator). occasion to bring waste plastic to a Municipal disposal in landfills or distant recycling program.Burning agricultural plastics in open incineration is a better alternative forfires or burn barrels is a burdensome disposing of waste plastics than on- In the Northeast, two successfuland unpleasant job. It is also highly farm burning, burial, or non-disposal. agricultural plastic recycling programspolluting and carries short- and long- Farmers often reject this option stand out as models for governmentterm health risks. A chemical fact because of the expense, time, and initiatives and individualsheet cautions firefighters to wear a logistics involved. entrepreneurs. A polyethylene nurseryself-contained breathing apparatus film collection program was started inwhere there is a risk of exposure to Another disposal method—pelletizing New Jersey around 1997. This state-burning polyethylene and states that waste agricultural plastic into fuel run collection program collectsfumes from molten or burning nuggets—has been under “clean” bundles of nursery andpolyethylene can cause respiratory investigation by at Pennsylvania State greenhouse films. Its Mt. Holly siteirritation, headache, and nausea. University researchers since 1995. accepts film from out-of-state The “densified” fuel pellets would growers. In its first five years, withLow temperature combustion of generate energy when burned with 100 to 125 growers participating,plastic film that occurs in on-farm coal in boilers. These pellets would almost 1.8 million pounds of nurseryburning produces soot and other also eliminate the problem of densely film was recycled. The $20 to $25 pernoxious emissions and also leaves compacted plastic bales causing “hot ton fee is substantially less that thetoxic residues. These emissions are far spots” that damage incinerator $60 per ton landfill tipping fee.in excess of those released by equipment.municipal waste incinerators that burn32 – High Tunnels
  6. 6. With an initial aim of reducing company has installed a $10 million easier choice for other farmers in theirfarmers’ tipping fees, Lancaster wash line to clean dirty plastics and region.County farmer Daniel Zook started a also has a pick line to remove debris.viable agricultural polyethylene This section was adapted in part fromcollection business that is now The black and white agricultural films Levitan, Lois, and Barros, Ana.actually paying $80 to $100 a ton for used for mulch bags and wrapping are “Recycling Agricultural Plastics inwaste polyethylene. In 2005 (its fifth not much in demand by plastic New York State.” March 11, 2003.year), Zook Plastic Recovery sold recyclers as the pigments must be 2nd ed. Environmental Risk Analysis500,000 pounds of plastics. Zook set greatly diluted to manufacture Program. Cornell University.up the program with Trex, a company marketable plastic lumber. This is http://environmentalrisk.cornell.edu/Cthat reprocesses the material into relevant to high tunnel growers &ER/PlasticsDisposal/AgPlasticsRecplastic lumber for decking. Besides because, in many regions, the bulk of ycling/. Personal communicationsguaranteeing a market, this firm agricultural film is used by dairy with plastic recyclers and reportsguided Zook through start-up and farms and feed producers, and from Penn State provided additionalhelped him purchase a cardboard greenhouse film alone might not be background.baler. Zook holds month-long waste abundant enough to support aplastic collection drives each spring collection program.and fall and for a couple weeks in thesummer. He also accepts plastic atother times. After he bales this plastic, Agricultural plastics are also more costly and inefficient to collect than Multi-Bay Tunnelshe has it hauled to a Trex factory in urban plastics, because they areVirginia. dispersed across the countryside and Multi-bay high tunnels are a special farmers dispose of these plastics category of tunnel. The mostA 2003 Cornell report lists several sporadically and seasonally. Since prominent manufacturer of this typeother agricultural plastic recycling concentrated quantities of agricultural of tunnel is the British company,efforts. A plastic lumber re-processing plastics are more attractive to Haygrove Ltd. As fruit farmers infacility based in Prince Edward recycling markets, recycling programs Herefordshire, England, HaygroveIsland, Canada, is capable of handling may need to be organized into found single bay tunnels inadequate“dirty” LDPE plastics used in regional or statewide programs. for field-scale crop protection. Withdairying. A nationwide, industry- multi-bay tunnels in Spain as ansponsored network collects high- Farmers are more likely to participate inspiration, it developed its owndensity polyethylene pesticide in recycling where there is a nearby model.containers. In the US and Canada, an collection system, where burning andindustry-sponsored program based in dumping are prohibited, and where In Spain, where the climate is mild,Ontario, Canada, picks up, pays for, tipping fees for solid waste disposal multi-bay tunnel frames can be ratherand re-processes polystyrene nursery are significantly higher than for weak. Haygrove tunnels improved onflats and trays. recyclables. Regional and statewide the original Spanish tunnel concept by recycling programs may be most making them appropriate for use inThe recycling of agricultural plastics appropriate given the capital costs for the U.K. Since they were firstlags behind other types of recycling recycling equipment, the sporadic and introduced in 1996, their use hasdue to several barriers. Municipal seasonal nature of plastic removal, the spread to 30 countries, from Norwaywaste transfer stations cannot easily dispersed feedstock for recycling, and to Australia, and by 2006, they wereaccommodate loose plastic film, the critical mass of materials needed protecting over 7,000 acres of crops.which is bulky and difficult to handle. to attract industry.Any collection site must be equipped Like other high tunnels, Haygrovesto bale the material separately. Despite these challenges, enable the farmer to extend both ends opportunities for new private and of the growing season. They alsoThe quality of agricultural plastic film public recycling efforts abound. While prevent rain, wind, and hail damage,is often compromised by greater farm use of polyethylene is reduce pesticide use, and avoid thecontamination of dirt, moisture, or increasing the magnitude of the loss of harvest days due todebris. Contamination reduces the disposal challenge, high oil prices are precipitation.marketability and/or the price paid. also creating greater demand for wasteFor example, the Trex Company pays plastics. Farmers who use agricultural In terms of environmentalless than one-fifth its standard rate for plastics should consider taking an management, Haygroves vent moreplastics that require pre-cleaning. This active role in making recycling an fully than typical single-bay high33 – High Tunnels
  7. 7. tunnels. Haygrove’s lacing system Haygrove tunnels concentrate the run- farmers in the Eastern U.S. to growholds the polyethylene cover securely off in the leg rows between the bays, crops in a multi-bay high tunnel. Hein place while allowing a grower to and this water must be controlled. grows fall raspberries, everbearingvent the tunnels up to 8 or 10 high. Prior to construction, most growers strawberries, and cut flowers (forThis design feature virtually form shallow swales that are covered October weddings) in the structureeliminates the problem of excess with weed barrier cloth. The legs are and also has five heated and unheatedtunnel heat and humidity. then drilled into the soil through small single bay high tunnels. slits in the cloth.Haygroves are three-season structures. After going on the Haygrove growerThey are not designed to support a Haygroves are sold in various bay tour in England, he was sold on thisheavy snow load. In temperate widths and heights to accommodate a type of high tunnel. Based solely onregions, the polyethylene film should range of needs. The structures are tall economic considerations, he opted tobe removed for the period of time enough for tractor access and dwarf buy a cheaper model from anotherduring which snowfall is possible. tree-fruit production. The Haygrove company (which we will refer to asThe film is “hibernated” by pulling it design allows adjacent bays to be “Brand X”), instead of a Haygrove,off the hoops, down onto the Y of the managed for different microclimates, which it superficially resembled. Helegs, and covering it with black plastic with curtains dividing bays with cool purchased a four-bay structure thatthat is secured with twine. This and warm crops. The configuration of covers a quarter acre for undereliminates any chance of snow these tunnels provides for easy $7,000; each bay measured 26 x 100.damage to the tunnel and protects the expansion of growing area to the sides A Haygrove covering a third acrefilm from UV exposure when not in or ends. would cost $12,000, in part becauseuse. At the beginning of the next the shipping is the same whether onegrowing season, the film is remounted These tunnels retail for about half the buys an acre or a fraction of an acre.on the hoops and secured with the cost of traditional, single-bay highrope lacing. tunnels. The minimum Haygrove “There are many subtle differences purchase is a half-acre; however, a that you can’t see in the catalogue,”Haygroves can withstand relatively full trailer load contains two acres and says Rob, explaining hishigh winds. Each Haygrove leg and the delivery cost is the same whether disappointment at the weaknesses ofanchor has an auger welded to it, and the trailer is full or not. the structure he selected. He has hadthese are drilled 30" into the soil; to modify and reinforce the highcement is not used. This keeps Construction time is faster for these tunnel in order for it to function.Haygrove tunnels from lifting out of multi-bay structures than for Contrasting the two companies, hethe soil, something that can happen traditional high tunnels. Typically, characterizes Haygrove as “farmerswith single bay tunnels that are not growers report 250 to 300 man-hours turned metal benders” and Brand X ascemented into the ground. During to construct an acre-sized Haygrove “metal benders who happen to makehigh winds, Haygroves should be left compared to 75 or 100 hours for a 30 products for farmers.”fully vented. This decreases pressure x 96 tunnel (1/12th acre).on the tunnel by allowing some of the Construction involves relatively few While on the tour in England, hewind to move through the tunnel tools and no lumber. Along with watched how quickly the Haygrovewhile still shielding the crops from its delivery, Haygrove provides on site went up. The Brand X structure tookfull force. hoop bending and construction much longer to assemble and training for customers. Since these construction was far from smooth.Most growers in temperate climates tunnels follow the natural contours of “There is no comparison,” Robvent (open) and close their tunnels the land, there is no need for heavy observes. A major difference is thatevery day for the first six weeks or so equipment to prepare a level site. Haygroves can be constructed out ofof the season and then leave them extra-long pieces because they arecompletely vented for the majority of Inferior Multi-Bay High shipped from the United Kingdom inthe growing season. No special Tunnels a container and then delivered as aattention to venting is required during One farmer decided to buy a less unit to the customer. Haygrovethe period when severe thunderstorms expensive multi-bay structure to save personnel come and bend the hoopsare most likely. Daily venting and money, and regrets his purchase. Rob on the farm.closing is again practiced during the Hastings, the owner-operator offall. Rivermede Farm in Keene Valley, Brand X’s multi-bay high tunnel New York (in the Adirondacks), may consists of multiple pieces that need be one of the most Northern-most to be bolted or screwed together. For34 – High Tunnels
  8. 8. instance, there are five pieces in each poly cover is secured to the vegetables, cut flowers, greens, andBrand X hoop while a Haygrove hoop framework with the same rope lacing herbs.comes as a single piece. “I had to find system used on Haygrove’s multi-baya flat surface on the farm to assemble tunnels and therefore providesthe hoops so they would not be excellent venting. With a minimumtwisted.” Each of these joints presents 10 height at the peak, it has a large aira weak spot, and Rob has experienced volume for its size. Being a single-baya plethora of failures around these tunnel, the Solo seals well forjoints. “I had to double screw every maximum season extension. Likehoop where it joins the Y post,” he other Haygroves, it is sold as a three-says. “It either tore out the tek screw season structure and is not engineeredor bent them.” for snowload.Rob has identified various annoying Solos can be erected on hills ordesign flaws in the Brand X. In a uneven sites and compared to otherHaygrove the hoops rest on the Y post single-bay tunnels, much less The size of walk-in tunnels isand no water can get in and freeze. construction time is required. As with variable. They range from 8 to 18 inThe Brand X structure allows water to Haygrove’s multi-bay high tunnels, no width and cover two to four beds.enter and freeze. Haygroves have a carpentry or extra materials are Their length may be from 24 to 300peg at the top of each Y post for tying required. While Solos are single-bay or more, depending on the length ofthe rope to that keeps the plastic film tunnels, they can be built on double beds they are intended to cover andon. The Brand X lacks the peg and prong anchors (8" between bays) to limits to the sizes of available covers.there is nothing to tie to. cover large areas. The flexible tunnel length enables a grower to construct a walk-in tunnelThe Haygrove has an effective way to A Solo package, including the poly virtually anywhere on the farmfinish off the end walls ands film cover and 12 mil roll-up doors because it can be sized to fit into asimultaneously provide wind bracing. for both ends, costs about $1 per farm’s existing bed system. TheThe Brand X structure comes with a square foot, or about half the cost of a tunnels are tall enough to walk in andreinforced plastic end wall which has more conventional high tunnel. are accessed by ducking under thenot been satisfactory. “The flopping sides anywhere along their lengthplastic acts like a gigantic sail tied (hence their name).down with an anchor,” observes Rob.He plans to do some cross bracing toshore up the plastic. Walk-in Tunnels Bows for walk-in tunnels may be made from PVC, electrical conduit, orThe lack of technical support is (Caterpillars) galvanized steel hoops. To erect the tunnel, the bows are either slippedanother major frustration. “Brand X over ground stakes made of rebar orbarely knew I had bought one,” he Walk-in tunnels are inexpensive tubular steel, or the bows are setsays. “When I called, I was talking to alternatives to the greenhouse-like directly into the ground about a footsomeone who was clueless and who structures that come to mind when deep. Bows are spaced six to ten feetwas not a farmer.” Rob recommends thinking about high tunnels. For an apart, depending on the site’s windother farmers learn from his mistakes equivalent area under cover, they cost exposure. It is best not to construct theand “invest in the best.” “I’m sorry I less than a quarter of the price of a tunnel broadside to the wind, but ifdidn’t ask questions,” he concludes. more traditional high tunnel. With that is unavoidable, the tunnel will respect to environmental serve as an effective windbreak for modification, they are intermediateSingle-Bay Tunnels in the crops planted on its lea side. between traditional high tunnels andHaygrove Style the low tunnels commonly used byHaygrove’s newest product is the A 1/4 rope tied from hoop to hoop is vegetable growers. (See “Low used to form a ridge purlin. The purlin“Solo” single bay tunnel. It comes in Tunnels” on page 38.) is attached to heavy-duty groundsizes appropriate for the smallergrower. Typical dimensions are 24 x stakes at both gable ends. The Due to their segmented appearance, structure is quite “loose” when200 and 28 x 200, though various some growers call these structures uncovered; much of the tunnel’swidths and lengths are available. This “caterpillar tunnels” (see diagram). structural integrity comes from theQuonset-shaped structure is built of They have been used to grow14 gauge galvanized steel hoops. Its35 – High Tunnels
  9. 9. cover and the way it is secured to the temperatures. During the coldest Farm Tek, “the only source of veryground. periods of the year, sections of the long plastic” of which they are aware. sides (the cover) are propped up with Their other walk-ins are skinned inWalk-in tunnels may be covered with short “Y” shaped props or branches Typar.greenhouse plastics, heavy spun- cut for the purpose. Whenbonded fabrics such as Typar, or temperatures warm, the sides may be Caterpillars have become importantshade cloth. The cover should be rolled up along the entire length of the on Windflower Farm because they arematched to the intended use of the tunnel. Clamps or tall “Y” props can easy to construct and cover, and arestructure. The less expensive Typar help hold the rolled up plastic in inexpensive, while providing many ofmight be the better choice, for place. The sides must be rolled down the benefits of a multi-bay structureexample, if the goal is to give a boost when high winds threaten. like a Haygrove. Steel hoops and(or provide insect protection) to cool- plastic film for one of their 16 x 200season crops such as spring greens. These tunnels are highly portable. tunnels cost them about $1,500, aboutGreenhouse plastic would be the They may be erected and dismantled one-third the cost of a Haygrovebetter choice for an early planting of relatively quickly. For example, two multi-bay high tunnel. In addition, airtomatoes. And shade cloth might be 200 long units built to cover three flow in caterpillars is excellent—selected for rooting strawberry tip beds of lettuces were erected by Ted better than in a more expensive,cuttings. and Jan Blomgren and a co-worker conventional high tunnel—as over the course of a morning. caterpillars open fully like aThe covering is held fast by 1/4 ropes Haygrove. (But unlike a Haygrove,that are drawn over the top of the One way to reduce the annual costs of management of high winds in astructure (Canastoga wagon-style) and construction and dismantling is to caterpillar requires closing the sides.)are secured to stakes or earth anchors leave the caterpillar tunnel in placein the ground. These ropes give the from year to year, and to develop a list To get high quality fruit early fromstructure its segmented, caterpillar- of tunnel crops around which a crop their first planting, Ted and Janlike appearance. The edges of the rotation plan might be developed. transplant zucchinis and cucumbersplastic are left loose, but the covering into walk-in tunnels on May 1. Theseshould be sized so that there are at Walk-ins are highly adaptable tunnels span three beds that are sixleast two feet of extra material on structures. They may be built over feet on center. Early harvests of botheach side. In particularly windy existing crops, or over bare ground for of these vegetables are important inlocations, the covering may be a later planting. They may be built in meeting their goal of delivering asecured by placing rocks or small the fall, left uncovered during the diversity of “real vegetables” (assand bags on the edges of the plastic. winter, and then covered in the spring opposed to salad greens) to their NewAt the gables ends, the plastic is for an early planting. Or they may be York City-based CSA membership.bunched together using rope, and the used to cover tomatoes during therope is tied to a secure stake. The spring and summer, and then taken Cucumbers, planted two rows per bedtunnel’s dimensions should be down and reconstructed over an into black plastic (or six rows across aconfigured to fit commonly available existing fall spinach crop. tunnel), start yielding a substantialgreenhouse films or floating row harvest by the end of June. Thecovers. Caterpillar Tunnels at zucchinis go into bare ground, with Windflower Farm one row running in the middle of eachWhile they have many advantages, Ted and Jan Blomgren have found of the three raised beds. Each row ofstarting with their cost, walk-in many uses for walk-in tunnels to zucchini is sandwiched between rowstunnels are really three-season produce vegetables and cut flowers. of quick Asian greens. These greensstructures. The wide bow spacing that On Windflower Farm in Easton, New will be harvested by the middle ofkeeps them cost-effective greatly York, they construct some of their June just as the zucchinis are fillingreduces their snow load capacity, so caterpillars with metal bows spaced out the space. By the end of the firstthe covering should be removed 10 apart, and others using PVC bows week of July, the zucchini harvest isbefore winter. However, walk-in spaced 6 to 8 apart. At any given going strong.tunnels with a bow-to-bow spacing of time in the growing season, the4 and a width of 10 have reliably Blomgrens may have as many as On May 1, tomato transplants arewithstood snow. 9,000 to 12,000 ft2 of walk-in tunnels planted into three beds in walk-in on their farm, with the typical tunnel tunnels for first harvest in July. UsingWalk-in tunnels must be ventilated 200 long. Most are covered in 6 mil the hybrid variety ‘Mountain Spring’manually to avoid excessive greenhouse plastic obtained from as their mainstay, Windflower Farm36 – High Tunnels
  10. 10. gets a high quality fruit that bears well minimum of two people to install a pounds per plant on their 500 plants.over a long period of time. In 2005, plastic cover. (Production varies considerablythey harvested from their May 1 among the diverse heirloom varietiesplanting until late October. They have On walk-in tunnels built for fall grown at this farm.) At $3/lb. for #1also produced eggplants and bell lettuce and salad mix, they may still fruit, and $2/lb. for seconds, Chris andpeppers in walk-in tunnels. use Typar as the cover. Ted and Jan his wife, Tammara Van Ryn, grossed sometimes build tunnels over beds almost $16,000 on this crop, producedThe Blomgrens initially experimented where they have already set in one-eighth of an acre of caterpillarwith walk-in tunnels to protect China transplants. Using a marking rod, they tunnels.asters from aster yellows, a disease pound in stakes at 10 intervals andtransmitted by leafhoppers. The then lower hoops over the stakes. For Chris used 1-1/2" diameter Schedulealfalfa fields that surround an inside cover, they use a midweight 40 PVC pipe, rather than steel, forWindflower Farm are leafhopper fabric, such as 0.9 ounce Covertan, bows. The bows are spaced every 6habitat. Ted says they skinned these suspended over low wire hoops. They and placed on rebar stakes, with atunnels with Typar because “We harvest mature lettuce from these rope and another pair of stakes inthought we could get away without unheated tunnels as late as between. His tunnels are 10 wide,rolling up the sides,” something that Thanksgiving. Ted cautions that this which allows just two rows ofwouldn’t be possible for plastic- design has no capacity to bear snow. tomatoes per tunnel.covered tunnels which trap heat moreeffectively. “Now, tarnished plant bug Caterpillar Tunnels at New Ideally, he said, the tunnel would be 2is our only insect problem,” says Ted. Minglewood Farm wider, with a corresponding increase In 2004, Chris Lincoln grew half his in height to accommodate theBesides preventing aster yellows, the tomato crop at New Minglewood indeterminate varieties that he grows.tunnel environment produced China Farm in walk-in tunnels. Better yields As it is, when the plants fill out, theyasters with stems three feet long. and a higher percentage of #1s have are a bit crowded and grow into theShort-stemmed cut flowers are not convinced him to raise his entire sides and roof of the tunnel. Thedesirable and Ted and Jan realized tomato crop in these tunnels. This actual size is perfect for determinatethat the extended stem length was a year, he used four 100 long tunnels to tomatoes, which he does not grow.benefit that tunnels could provide grow 800 row feet of heirloomother cut flowers. The absence of tomatoes for retail sale at a farmers’ Wind is the biggest threat to the walk-wind and reduction of light in the market and to restaurants. in tunnel design, but built properly,walk-in tunnels are two factors they seem to hold up adequately.associated with longer stems. Chris notes higher yields, higher Unlike a more conventional high quality, a longer harvest season, and tunnel, where the plastic is attachedWindflower Farm currently grows cleaner fruit as benefits of growing directly to a steel frame, walk-instock, snapdragons, godetia, larkspur, fresh market tomatoes in these tunnel plastic is held down by ropesBells of Ireland, and lisianthus in tunnels. He also counts the low cost of secured to the earth by means of rebarwalk-in tunnels to achieve much the tunnels and the ease of moving stakes or earth screws. Like theearlier blooms and longer stems. They them to different sites on the farm as Blomgren’s structures, Chris useshave come to prefer walk-in tunnels to advantages to walk-in tunnels when ropes over top of the plastic andregular high tunnels. They are also compared to more permanent, secured to long rebar stakes drivenconvinced that plastic film is superior stationary high tunnels. deep into the ground. If these ropesas a tunnel cover to Typar except for are not secure, wind can causemid-summer production. In the Getting earlier tomatoes was not the damage. To prevent wind damage, itsummer, they use Typar to protect overriding factor in adopting this is critical that these anchors are longagainst insects, diffuse the brightness growing environment. New and stout enough to hold the ropes.of the sun, and shelter the flowers Minglewood’s typical tomatofrom wind. schedule (reported for 2005) is as Chris believes that walk-in tunnels follows: Seed on March 28, transplant have been more time-consuming forThough they are moving away from on May 16, begin picking on July 23, New Minglewood Farm than regularTypar, Ted stresses that it has two last harvest on October 18, first killing hoop houses would be. This is mainlyvirtues. It costs about half as much as frost on October 21. due to the labor involved with settingplastic, and its light weight makes it up and covering these tunnels beforeeasy to use. “I can cover a 200-foot In 2005, they sold 5,885 pounds of tomato planting and disassemblingtunnel myself,” he said. It takes a tomatoes with an average of 11.837 – High Tunnels
  11. 11. them later in the year after the plants easier field production. The O’Haras that these plants are winter annuals.have been removed. complement their low tunnels with They have no disease and insect pests two small 14 and 16 wide high when we grow them out of season.”Another regular task during the spring tunnels used for seedling production.and fall is opening and closing the The seeding dates for their winter andsides, depending on sun, rain, and The couple markets their mixed spring low tunnel crops aretemperature. To vent, Chris rolls up vegetables and maple syrup astonishing late. They continuethe plastic and clips it to a rope. Once cooperatively with Bryan’s father. planting through the beginning ofthe weather warms, the sides stay up Their stand always sells out at the two December and start up again inall summer. Saturday farmers’ markets their February. All these crops—which family attends from early May include lettuces, Asian greens,In 2006, Chris mulched with black through Thanksgiving. The nearby spinach, kales, Claytonia, scallions,landscape fabric. He used a weed Willimantic Food Co-op, which just beets, carrots, dandelions, parsley, andbarrier fabric to accelerate tomato tripled its store area, is their other arugula—are direct seeded with andevelopment, suppress weeds, and major outlet and has a standing order Earthway seeder. Normally, harvestprevent soil splash on the plants. He with them. continues until Christmas and thenchose a 6 wide fabric and covered the resumes in March. The mild winter of2 wide space between tunnels as well Several factors, including southern 2005-06 allowed them to harvest atas all the ground inside of the tunnels, exposure and wind protection, help intervals in January and February asenabling him to locate the tomato make low tunnels feasible for well.rows at the intersection of two pieces impressive season extension atof the fabric. This arrangement Tobacco Road Farm. The low tunnels At Tobacco Road Farm, beds areeliminated the need to burn or cut at the farm are situated on a sloping, standardized at 3 wide, separated byplant holes into the fabric. southwest facing field. Conifer 8" wheel tracks. For this bed width, windbreaks mitigate the prevailing the wire hoops must be at least 80" west wind. A third environmental long. The hoops are about 24 high at advantage to this site is a large center. The minimum hoop size isLow Tunnels: An damned river the size of a small lake located beyond a grove of trees next 3/16" round stock. Either steel or galvanized will do. Wire hoops areAlternative to High to the field. spaced 2 to 2.5 apart. The tunnels are covered with two layers of plastic atTunnels But this is not a snow-free environment. While most snowfall is frost, usually in early October. In March, the warming sun prompts theIn some climates, low tunnels can be light, the O’Haras may receive as removal of the plastic film and itsan inexpensive alternative to high much as 8" or 10" of snow at a time. replacement with Agribon. This spuntunnels for cold weather season And though snow usually melts in a polyester row cover breathes and letsextension. Low tunnels are simple, few days, their low tunnels have been water pass through. Both the plasticprotective structures consisting of covered by snow for a month, without and the fabric row cover are heldwire hoops covered with polyethylene harmful effects on the plants inside. down with up to a dozen 6 ml blackor fabric row covers. Though more However, when the tunnels are plastic sand bags.labor intensive, with well-planned covered in snow, harvest isbeds, they can be assembled with just impossible. Bryan buys 20 lengths of steel wire ata small investment. a local metal shop and has them chop The O’Haras have been using these it in thirds, precisely the length heOn Tobacco Road Farm in Lebanon, inexpensive tunnels for about 10 years needs. Bryan has used the same steelConnecticut (about an hour east of at an annual cost of about 7.5 cents wire for over 10 years. Chafing fromHartford), organic market growers per square foot ($3,000/acre) to surface rust only occasionally causesBryan and Anita O’Hara have over an extend their growing and harvest rips in the Agribon row cover, whileacre of low tunnels for extending fall season. At first, their goal was to push also helping to stick the cover onto thevegetable production and growing a fall production later. Driven by strong hoops. Galvanized steel is much morevariety of baby and braising greens customer demand for the good-tasting, expensive.and other vegetables throughout the cold weather-grown greens and theircolder months. The low tunnels are need for farm income, they just kept For ease of operation, the beds areremoved in late spring to allow for on expanding their off season covered in 40 sections. Wind build up production. Bryan said, “We realized makes longer sections unmanageable,38 – High Tunnels
  12. 12. and with the length of this low tunnel, For several years, Bryan has been • Low tunnels allow farmers to growtwo people working together can selecting Asian greens and arugula for without insect and diseaseuncover a whole field in little time. cold hardiness. This breeding has pressure.Bryan uses two layers of cheap 1.5 yielded quick results, boosting winter • Since low tunnels are closer to themil construction grade plastic. A 200 survival from 10% to 100% in arugula earth, they provide more heat to thelength of this film will suffice for two just by collecting seed from planted plants in the day. Bryan said they40 sections. To close the ends, 50 of that made it through the winter. For are 3° to 5°F warmer than ambientplastic is needed to skin 40 long low his first breeding experiments, Bryan temperature on cold nights, similartunnels. The ends are bunched up and seeded three adjacent beds of Mizuna, to their high tunnels.pulled tight to create tension over the Tatsoi, and Miruba. He replanted the • Cold growing conditions enhancetop of the hoops. next year from the seed he harvested the taste of the crops. from the Mizuna mother plants which • Low tunnels convert readily forBryan is an enthusiastic missionary successfully over-wintered. Each year regular summer production,for the environmental control he collects seed from subsequent allowing triple cropping.provided by low tunnels. They generations of plants interbred from • Soil preparation and other tractorprevent excess precipitation from the three cultivars that survive the operations can be carried out in thedamaging germination and ruining winter. The hybrid seed selected from field prior to quick installation ofcrops, and protect crops from wind a large gene pool produces very the low tunnels.and cold. Bryan has measured a 35°F vigorous plants with much greater • As they are easily removable, lowgain on sunny days. The low tunnels cold tolerance. He can stagger seed tunnels reduce soil salt build up.should be vented if the outside production by altering when hetemperature reaches about 70°F and harvests for salad mix. The O’Haras also recognize that lowopened to catch precipitation if tunnels have the followingexcessively dry. In 2005, Bryan kept Chickweed is the biggest weed disadvantages, some of which arethe beds uncovered until the problem in winter low tunnel common to high tunnels as well:extremely late first frost on October production. Before they grew year • The biggest issue is that low21, a lapse he came to regret since 20" round, the O’Haras used to tolerate tunnels are more labor intensive.of rain that month broke all records. If chickweed in their fields over the To hoe, harvest, or irrigate, thethe beds had been covered, the crops winter so now they are grappling with coverings must be removed.would not have been devastated and it a huge seed bank. Seeding crops very • Low tunnels cannot be automatedwould have been more possible to late in the fall allows late tillage, and require a lot of bending tocontrol chickweed with cultivation. reducing chickweed germination, and manage the cover, causing worker the Asian greens sown on December 2 discomfort.Irrigation is rarely required until later were their cleanest over-wintered • Low tunnels restrict accessin winter. The plants seem to like it crop. compared to high tunnels. Onedrier in winter, Bryan explained, can’t harvest on a cold, windy day.noting two mechanisms that plants use At Tobacco Road Farm low tunnelsto withstand cold temperatures: have a variety of advantages: • Cut lettuce grown under plastic film-covered low tunnels (or highdesiccation and mineralization of their • Low tunnels are very inexpensivesap to reduce their freezing point. tunnels) is very delicate and wilts and quick to install. BryanThese mechanisms make these crops quickly. This is one reason to estimated that two people couldextremely sweet-tasting in the winter. convert to a fabric covering like cover over an acre in two days. Agribon. • In the spring, low tunnels are moreIn the central Connecticut climate, • Low tunnels provide a perfect easily cooled than high tunnels tosnow build up has not been a problem environment for chickweed, the prevent bolting. ultimate winter weed.for the low tunnels. The hoops are • Low tunnels are easily convertedpushed down by snow load, but pop • The plastic cover on low tunnels from plastic to cloth covers. must be vented during warm sunnyup, pushing off the snow, as soon as • Low tunnels allow farmers tothe sun begins melting it. Snow late fall and winter days. manage water, protecting crops andaccumulation on the wheel tracks soil from excess rain and wind. Ainsulates the low tunnels, provides a (An earlier version of this profile slight slope allows water to run offwindbreak, and also holds down the appeared in “Growing for Market.”) the tunnels.row cover.39 – High Tunnels

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