Greenhouses and ❂ 9Season ExtendersC ertain crops grow just fine in Alaska’s short growing season, even when direct seeded (planted outside in the spring), while other crops benefit greatly fromseason extension techniques and/or greenhouses. Cool ❂ Topics in this chapterseason crops that do well without any help include potatoes, ❂❂ Selecting a siteturnips, radishes, beets, lettuce and other greens, peas and ❂❂ Season extenderscarrots, which can all be direct seeded. Crops in the Brassi- ❂❂ Greenhousescaceae family (broccoli, cabbage, cauliflour, kale, etc.), are ❂❂ Greenhouse benches andalso cold hardy, although seeds should be started indoors to bedsensure they reach maturity by the end of the growing sea-son. Many warm season crops that grow well with a little ❂❂ Greenhouse utilitieshelp from season extension techniques include basil, snap ❂❂ Popular greenhouse cropsbeans, strawberries, zucchini and other types of squash, to in Alaskaname a few. Tomatoes and cucumbers are ubiquitous green- ❂❂ Glazing and covershouse crops. Although season extension techniques require ❂❂ Watering systemsadditional time and money, for crops such as strawberries ❂❂ Natural ventilationit’s usually worth the additional effort and expense. In gen- ❂❂ Pollinationeral, more expensive techniques afford a greater degree ofcontrol over and ability to use an outside heat or ventilationsource. With the additional costs associated with structuressuch as a greenhouse, it makes sense to grow higher valuecrops. Any season extension technique will require someadditional expense and labor and so two questions could beasked: Is the crop valuable enough (to you or at the market)to warrant the additional expense? Can the crop be grown By Pat Patterson, Extension Program Assistant,successfully without additional help? Lane County, Oregon State University. Edited by Heidi Rader, Extension Faculty, Agriculture and Horticulture Agent, Cooperative Extension Service, University of Alaska Fairbanks. Adapted from Greenhouses for Home Gardeners: Structures and Equipment, HGA-00337, University of Alaska Cooperative Extension Service.
170 • Greenhouses and Season Extenders—Chapter 9 the soil. A special type of polyethelyene Selecting a site mulch called Infra Red Transmitting mulch The most important factor in selecting (IRT-76®) was tested at the UAF Agricul- a location for a cold frame, hoop house, ture and Forestry Experiment Station and high tunnel or greenhouse is sunshine. was shown to warm the soil almost as much Protection from strong winds is also very as clear plastic mulch, while also suppress- important since they are more susceptible ing weeds. Equal, if not greater, yields were to wind damage than many other types of achieved with clear plastic mulch, but only construction. Other considerations include when herbicides were used. factors important in site selection for any type of building: level ground, well-drained Cold frames and hotbeds soil and a location that does not collect Cold frames and hotbeds are inexpensive water from the surrounding landscape. It structures for growing cool-weather crops is often recommended that high tunnels or in early spring and fall. Recycled shower greenhouses in northern climates should be doors or windows work particularly well oriented with the long side running in an for a cold frame. They range from simple to east-west direction to maximize sunlight, elaborate. Cold frames rely on the sun for but for home gardeners with a limited num- their sole source of heat. Heat is collected ber of site options, other location factors when the sun’s rays penetrate clear plastic, often dictate the orientation. glass or fiberglass (Figure 1). Hotbeds are heated by soil-heating cables, steam-car- Season extenders rying pipes or fresh, strawy manure buried beneath the plant rooting zone. To get the most from a garden, extend the In early spring, a cold frame is useful for growing season by sheltering plants from hardening off seedlings that were started cold weather in both early spring and fall. indoors or in a greenhouse. Hardening off is Row covers, cloches, plastic mulches, hoop important because seedlings can be set back houses or a combination of these techniques seriously if they are moved directly from can save your crops from unexpected frosts a protected location to a garden. A cold in late spring or early fall and can allow you frame provides a place for gradual adjust- to confidently transplant earlier than you otherwise would. Plastic mulches Plastic mulches are useful for warming the soil and suppressing weeds in Alaska. They should be placed as tightly as possible on the soil; plants such as zucchini, corn, peppers and pumpkins can be grown in the plastic. Clear polyethylene mulch warms up the soil, but also acts as a perfect environ- ment for weeds to grow. Black polyethelyne mulch blocks the sunlight required for weed growth, but is not very useful for warming Figure 1.—Cold frame.
Chapter 9—Greenhouses and Season Extenders • 171 Figure 3.—Solar pod.Figure 2.—Dutch light. New cold frame designs include passive solar energy storage. For example, blackment to outdoor weather. You also can start containers filled with water absorb heat dur-cool-weather crops right in the cold frame ing the day and release it at night. The solarto transplant later to the garden, or you can pod is a design that provides this type ofgrow them to maturity in the frame. In the heat storage (Figure 3). Other designs havesummer, cold frames are useful for growing a very high back and a steep glass slope andwarm season crops. Annuals can be started are well insulated. They may include mov-in a frame weeks before they can be started able insulation that is folded up in the dayin the open. You can replace the soil in a and down at night or during extremely coldportion of the cold frame bed with a me- weather.dium suitable for rooting cuttings. Frames can be built from many materials; The ideal location for a cold frame is a wood and cinder block are most common. Ifsouthern or southeastern exposure with a you use wood, choose a species that resistsslight slope to ensure good drainage and decay or one of the new plastic woods.maximum light exposure. A sheltered spot Never use creosote- or pentachlorophenol-with a wall to the north provides protection treated wood, because these substancesagainst winds. Sinking the frame into the are harmful to plants and humans. Kits areground a bit also provides earth insulation. available, some of which contain automatic To simplify use of the frame, consider ventilation equipment.a walkway to the front, adequate space There is no standard size for a coldbehind the frame for removing the sash and frame. Size depends on the amount of avail-perhaps a counterbalance to make it easier able space, desired crops, and size of theto raise and lower the sash. Some gardeners available covering. Do not make the framemake their cold frame lightweight enough too wide for weeding and plant care; 4 toto be moved from one section of the garden 5 feet usually is the maximum width. Theto another. Another possibility is the Dutch sash should be sloped to the south to allowlight (Figure 2), which is a large, but porta- maximum exposure to sun.ble, greenhouse-like structure that is moved A simple method of insulation is to placearound the garden. burlap sacks filled with leaves over the
172 • Greenhouses and Season Extenders—Chapter 9 frame at night to protect against freezing. Cloches and row covers Another method is to stack bales of straw against the frame. Cloches originally were bell-shaped Ventilation is most critical on sunny days glass jars set over delicate plants to protect when temperatures rise above 55°F. Raise them from the elements. The definition has the sash to prevent heat buildup inside the expanded, however, to include many types frame. Lower it early enough to conserve of portable structures that shelter plants heat for the night. An automatic nonelectric from drying winds and cold air. The idea lifter is a good investment. is to provide a greenhouse-like atmosphere A cold frame can be converted to a hot- for seeds and small plants in order to get an bed. For a manure-heated bed: early start on the season or to extend the fall 1. Dig out an area 8 or 9 inches deep garden as long as possible. (deeper if you need to add gravel for Cloches are set over individual plants or increased drainage). made into tunnels for whole rows. They trap 2. Add an 8-inch layer of strawy horse solar radiation and moisture evaporating manure. from the soil and plants. Cloches generally 3. Cover with 6 inches of good soil. are lightweight, portable and reusable. The best designs can be closed completely at For an electric-heated bed: night to prevent frost damage and opened 1. Dig out an area 8 or 9 inches deep. or completely removed during the day for 2. Lay down thermostatically controlled good air circulation. Make sure cloches electric cable in 6- to 8-inch loops, are anchored or are heavy enough that they evenly spacing the cable but never al- don’t blow away. lowing it to cross. The hotcap and the ubiquitous cut-off 3. Cover with 2 inches of sand or soil. plastic jug are simple forms (Figure 4). 4. Lay out hardware cloth to protect the More elaborate ones include fiberglass cable. tunnels, special plastic cloches, row cov- 5. Cover with 4 to 6 inches of good soil. ers with slits to allow aeration and panes of glass connected by specially designed hinges to form a tent (Figure 5). You can quickly assemble an excellent Quonset-type cloche by covering a 4- to 5-foot piece of concrete reinforcing wire with row cover or plastic. The heavy-duty wire comes in 5- and 7-foot lengths. Al- ternatively, willows or PVC pipe can also be used to build this hoop structure. If you have a raised bed garden, the hoop house can be permanent. If you are planting directly in the ground, rebar can be used to anchor PVC pipe. Spun-bonded polypropylene is also aFigure 4.—Cloches made from a cutoff plastic soda bottle and a popular row cover. It permits passage of airmilk jug. and water, but is substantial enough to trap
Chapter 9—Greenhouses and Season Extenders • 173Figure 5.—Season extenders and plant protection methods include plastic and glass.heat, shield young plants from wind and in- work on a rainy day, are inexpensive andsects and provide a modified atmosphere for simple to build, offer increased reliabil-early vegetable crops. Lay these row covers ity, can decrease disease, weeds, and pestloosely over a planted row in early spring. incidence, and increase yield and quality ofAs the plants grow, they raise the light- warm season crops. However, in contrast toweight cover. You can leave these covers on greenhouses, these light-weight structuresmost vegetable plants until warmer weather. can be easily damaged by snow or wind, soThese covers also protect turnips from root they must be well anchored. They can alsomaggots. Spun-bonded polypropylene fab- be more labor intensive because they oftenrics come in different weights, with heavier lack automated ventilation or heating sys-weights affording greater protection from tems. An advantage for commercial growersfrost but allowing less light through. These is that since high tunnels are not consideredfabrics generally offer between 3 and 5 de- permanent structures, they are not taxable.grees of protection from frost. They should Hoop houses can easily be built frombe anchored with soil, stones or wood PVC pipe, white spruce poles or willows.(stakes are not ideal as they create a hole in An easy, inexpensive way to make a hoopthe cloth). With care, this cloth can be used house is by converting carport structures.year after year. Hoop house or high tunnel kits, usually made from bent metal pipe (galvanizedHoop houses or high tunnels steel), can also be ordered. Special high The next step on the continuum of sea- tunnel kits come with a fancy wire calledson extension techniques is a hoop house, “wiggle wire.” This wire is designed to holdwhich you can walk through, or a high tun- the visqueen on without tearing holes in it.nel, which you can drive a tractor through. A double-walled high tunnel, with a fan toCrops are almost always planted directly in maintain air space in between the layers,the ground in these structures. Hoop houses is an inexpensive way to insulate a hoopor high tunnels can be a pleasant place to house; however, this decreases light pen- etration to some degree.
174 • Greenhouses and Season Extenders—Chapter 9 Greenhouses Freestanding greenhouses Greenhouses, although generally more Freestanding greenhouses isolate green- expensive than other season extension house activity from the home and offer techniques, provide the greatest degree more flexibility in greenhouse size, shape of control. Because they are permanent, and configuration. They can be constructed greenhouses can more easily be insulated in the best location on the property for light and heated, if desired. Automatic ventilation interception, drainage and wind protection. systems can also provide increased reli- Freestanding greenhouses are also easier to ability with relatively less labor. In Alaska, expand than attached greenhouses. greenhouses are used to start seeds in the Access to freestanding greenhouses may spring, grow crops such as tomatoes and be more difficult, especially if they are used cucumbers and provide the perfect environ- in the winter. They require separate utility ment for beautiful hanging baskets. Some services or lines from the house for water, growers use greenhouses year round to electricity and, for some greenhouses, heat. grow crops hydroponically using geother- A freestanding house usually has higher mal heat. Greenhouses are broadly catego- heating costs than the attached house. It has rized as attached or free-standing. more exposed surface, leading to greater heat loss. The freestanding greenhouse also Attached greenhouses has a higher initial construction cost than the attached greenhouse since it requires an An advantage of attached greenhouses is extra wall and service line installation. that they usually have easy access, which makes it more convenient to attend to A-frame plants. They can create an extension of your indoor living space, and they may provide A-frames have sloping sides all the way solar heating and add an insulation layer to to the foundation, which makes them the the house. They usually have lower heating best style for passive snow removal and costs and lower initial construction costs limiting snow buildup (Figure 6). Like hoop (because there is one less wall to build) houses, they are relatively inexpensive and and it is easier to tap into the home water, simple to build. Frames are often made electrical and heating systems. Size limita- from two-by-four lumber and are generally tions and location restrictions are potential covered with a single layer of polyethylene disadvantages of attached greenhouses. film. People sometimes attach a second Both are determined by the availability of layer of film to the inside if they use the suitable exterior wall space. Compared to a house during cooler weather. The biggest freestanding greenhouse, site location may drawback of the A-frame style is that it not be the sunniest space in the yard. Anoth- limits both house width (and therefore size) er disadvantage is that greenhouse moisture, and headspace within the structure. summer heat, dirt, insects and chemical It is more awkward to work in an A- odors may enter the home. frame house than in other styles of green- houses, and flats are often grown on the ground due to the lack of head space. Soil temperatures may remain cold in a green- house and lag well behind air temperatures,
Chapter 9—Greenhouses and Season Extenders • 175 gable style pro- vides considerably more headspace than an A-frame or Gothic arch and has much more room for growing hang- ing plants aboveFigure 6.—A-frame and slant-leg greenhouses. benches. Gable greenhouses can be covered with poly-so if the roots of plants placed directly on ethylene film, but generally a rigid type ofthe ground are cool, growth may be restrict- material is used such as polycarbonate, glass,ed from reduced water and nutrient uptake acrylic or fiberglass.even though air temperatures are conduciveto good plant growth. One way to limit this Slant-legproblem is to put flats and pots on a layer of The slant-leg style is similar to the gableinsulation placed over the soil. style, except that sidewalls are slanted slightly outward (Figure 6). This makesGothic arch it wider than the vertical-leg gable style, Gothic arch greenhouses have an arched providing additional floor space. Like theroofline and walls that form a continuous gable style, it has a flat, sloping roof thatshape. Snow slides off them better than from sheds snow well, depending on the pitch.a hoop house, but not as well as from an The slanting sidewalls require a reinforcedA-frame. They have more headspace than an frame, with the result that slant-leg housesA-frame, but not as much as a hoop house of are very sturdy and suited for locations withsimilar height. Frames are constructed from heavy snowfall and thunderstorms.both pipe and wood. Roof supports must beflexible enough to make the arch curvature Pit(glued plywood strips are one type of ma- Pit greenhouses are built partially belowterial used). Gothic arch greenhouses are ground. They are most commonly builtfrequently covered with polyethylene film, where topography is suitable, such as thebut they can also be covered with flexible side of a hill with good sun exposure. Pitfiberglass and polycarbonate panels. greenhouses can have lower heating and cooling costs, since the earth acts as insula-Gable tion to keep them warmer in the winter and Gable-style greenhouses have sloping, flat cooler in the summer. They are relativelyroofs connected to vertical sidewalls. The expensive to construct, because reinforcedangle determines how well snow slides off, walls are required in the pit and both interi-as well as the total height of the structure. or and exterior drainage systems are usuallyThe well-defined roofline is adaptable to ef- needed to keep the pit dry.ficient roof-ventilation systems. Frames areconstructed from both metal and wood. The
176 • Greenhouses and Season Extenders—Chapter 9 type of bench is commonly used for plant Greenhouse benches and propagation, where cables can be buried beds for bottom heat and mist nozzles installed The layout and design of greenhouse above. benches determine the amount of usable Plants can be grown in solid bottom greenhouse space and the ease of working benches to maturity, depending on their within that space. Experiment on paper with size and rooting depth. They are frequently different bench sizes and different patterns used commercially for growing cut flow- of benches and aisles to help you decide ers. In addition to efficient bench layout, the on the best overall design. Benches must creative use of racks, shelves and overhead be strong enough to support the weight of hanging plants can take advantage of all plants and soil (or growth media), as well potential growing spaces in a greenhouse. as the weight of the water in a fully watered Growing conditions vary, both vertically pot. They should be a comfortable working and horizontally, within even a small green- height (usually about 30 to 36 inches) and house, so effectively maximizing grow- an arm’s reach in width. When a bench is ing space requires recognition of different accessible from both sides it can be twice as greenhouse microclimates and knowing wide. which plants will do well in each of them. Benches can be constructed from a va- riety of materials, including wood, gal- Ground beds vanized pipe, concrete block and plastic. Tall plants, such as trellised tomatoes, Wood, because of the moist atmosphere cucumbers and corn are frequently grown in in greenhouses, should be pressure treated ground beds in greenhouses and hoop hous- with a preservative that is nontoxic to es. Beds are often raised and about 3 feet plants. Bed construction should facilitate wide; the soil must be 8 to 12 inches deep. good airflow through plants, including Ground beds may be loosely mounded or air circulation from below the bench top. enclosed within a wooden frame. Plant cul- Greenhouse bench tops are made from ture is similar to raised-bed gardening out slatted wood, wire mesh or expanded of doors. metal sheets for this reason. Slatted wood is relatively inexpensive and easy to work with but doesn’t permit as much airflow as Greenhouse utilities wire mesh or expanded metal. Wire mesh is All greenhouses require a water supply, much less expensive than expanded metal most need an electrical supply and some sheets, but over time it begins to sag and its require a fuel supply for heat. Home green- useful life is much shorter than expanded houses are normally connected to utility metal. Greenhouse bench materials should sources within the home. Before greenhouse be light-colored (or painted white or alu- construction, make sure the home capacity minum) to reflect light and maximize light is adequate for the added connections and interception by plants. peak-use rates. Plan the installation and Plants can also be grown directly in solid connections from the very beginning of the bottom benches with sides that are filled greenhouse construction process, paying at- with 4–6 inches of growing medium. This tention to where underground lines must be run to freestanding greenhouses.
Chapter 9—Greenhouses and Season Extenders • 177Ventilation venting the heater to the outside is necessary to remove potentially harmful gases. Ventilation systems promote airflowthrough a greenhouse and help control bothtemperature and humidity. The importance Popular greenhouse crops inof managing temperature and humidity in Alaskagreenhouses is discussed in more detail inthe UAF Cooperative Extension Servicepublication, Controlling the Greenhouse TomatoesEnvironment, HGA-00136. Because tomato pollination and fruit set depend on a greater degree of temperatureHeating control than many crops, it’s best to grow Many home greenhouses are operated them in a greenhouse. Optimal tempera-seasonally and shut down during the colder tures for tomatoes range from a minimumparts of the year. They do not require a per- of 60°F during the night to no more thanmanent heating system, but many gardeners 80°F during the day. Humidity levels of 70provide supplemental heat during spring or percent are optimal for tomatoes.fall cold spells with portable gas and elec-tric heaters. Small, portable heaters work Cucumberswell to protect plants on unexpectedly cold Cucumbers are sensitive to temperature,nights, but gas heaters that are not vented humidity, light, carbon dioxide, moistureto the outside can create problems if they and fertilizer salts. Optimal temperaturesare not maintained and burning cleanly. for cucumbers range from a minimum ofIncomplete combustion can create ethylene 65°F and 70° to 75°F. Humidity levels of 70gas, which is a plant hormone that causes percent are optimal.abnormal, distorted growth in excessive For greater detail on growing theseconcentrations. Tomatoes and fuchsias are popular crops in greenhouses, refer to UAFvery sensitive to ethylene. Cooperative Extension Service publications Several types of permanent heating units Cucumber Production in Greenhouses,can be installed in greenhouses that are HGA-00434, and Greenhouse Tomato Pro-operated during the colder parts of the year. duction, HGA-00435.The choice of a heater often depends onlocal availability and the cost of different fu-els. A heater must be properly sized so that it Glazing and covershas adequate capacity to heat the greenhouse Glazing is the light transmitting materialon the coldest night of operation. Required used to cover season extenders or green-heater output is determined by calculating houses. Many high-quality glazing mate-potential heat loss, which depends on the rials are available, but they vary in theirsurface area of the greenhouse, the insulat- strengths and weaknesses. Important prop-ing value of the glazing, wind conditions erties to consider when selecting a glazingand the temperature difference between are light transmission, heat retention, dura-inside and outside air. As discussed above, bility, longevity, maintenance requirements and cost.
178 • Greenhouses and Season Extenders—Chapter 9 Plastic film Polycarbonate Plastic film glazing is lightweight, very Polycarbonate is a rigid plastic glazing, flexible and the least expensive type of but the sheets have enough flexibility to greenhouse covering. A single layer has bend around gently curved surfaces such high light transmission, but also high heat as the Gothic-arch greenhouse. It has high loss in cold weather. Many hoop houses light transmission, low maintenance, is have a double layer of plastic with an very strong yet lighter than glass and is a insulating layer of air between them. This fire retardant. Polycarbonate is available reduces light transmission by about 10 in single- and double-layer sheets. As with percent, but reduces heat loss by 35 per- other glazing materials, the double sheets cent. Use only greenhouse-grade plastics. lose some light transmission but reduce heat They have additives that slow deterioration loss to a greater extent. Double sheets also from ultraviolet light, are stronger and have have greater longevity than single sheets. infrared inhibitors that reduce heat loss. Polycarbonate sheets must be installed to Greenhouse-grade plastics will last 4 to 10 allow for some contraction and expansion years, compared to other plastics that will of the material with temperature changes. only last one growing season. Some materi- als also have anti-drip surfaces that reduce Acrylic droplet formation from water condensing on Acrylic is a rigid plastic glazing that is the interior of the plastic film. clearer than polycarbonate, although it may slowly yellow with age. It is available only Glass as a double-layered sheet that lacks flex- The introduction of synthetic glazing ibility and must be installed on flat sur- materials has greatly reduced the number faces. Light transmission and heat loss are of glass-covered greenhouses, but in many equivalent to double-layer polycarbonate. ways glass is still the standard of compari- Acrylic has the longest life expectancy of son. It has high light transmission and good any glazing except glass, but compared to durability. Glass has the greatest longevity polycarbonate it attracts more dust and dirt, of any glazing material (25+ years). Use scratches more readily and is more flam- low iron glass for the highest light trans- mable. mission and tempered glass for the greatest strength. Double-pane glass reduces the Fiberglass reinforced plastic relatively high heat loss of glass, although Fiberglass for greenhouse glazing is a some loss in light transmission also results. rigid plastic that will bend around gently Glass must be installed and maintained to curved surfaces. It has high light transmis- prevent air and water leakage between the sion, but comes only in single sheets (flat panes. Older greenhouses had overlapping or corrugated); it also has a high heat loss. panes with a bed of glazing compound and Fiberglass reinforced plastic is lower in cost sealant. Modern glass greenhouses use than other rigid plastic glazing, but it does rubber or vinyl gasket material below the require more maintenance since it must be edge-to-edge glass and bar caps over the top recoated every few years. of the seam.
Chapter 9—Greenhouses and Season Extenders • 179 a faucet and a hose. The proportioner has aWatering systems tube that is placed in a bucket of fertilizerSeason extenders and greenhouses concentrate, and when water moves throughHand watering the proportioner it siphons some concen- Most home greenhouses are watered by trate and mixes it with the water flowinghand with a hose and suitable nozzle. One to the irrigation hose. Fertilizer injectorsadvantage of hand watering is that is gives pump fertilizer concentrate into the irriga-the gardener time to look at every plant tion stream. They are more accurate thanand evaluate its condition. It is also easy to proportioners and useful as the numbers ofadjust watering to be able to apply differ- plants, and thus volumes of water, increase.ent amounts of water to different plants. They can also be plumbed into automatedHowever, as greenhouse size and number irrigation systems. When using fertigation,of plants increase, hand watering becomes a backflow preventer must be used to keeptime-consuming compared with other meth- fertilizers from flowing backwards into wa-ods. ter sources such as wells and ponds.Drip systems Overhead sprinklers Drip irrigation systems or soaker hoses Sprinkler systems are relatively simpleare efficient for watering when using season to install and convenient to operate. Oneextenders. In greenhouses, drip systems can drawback is that excess water must bebe individual capillary tubes for each pot or applied; this makes up for water deflecteddrip tape for ground beds. Drip irrigation by plant leaves and water that falls on theis very efficient in water use and keeps the non-growing areas. Also, wet foliage andnon-plant areas dry so that you can work higher humidity levels increase the potentialaround plants while they are being watered. for plant disease. Overhead sprinklers areDrip systems require close observation to sometimes used in high tunnels, but morebe sure they are working correctly and need frequently in greenhouses.maintenance to keep the water flowing from Greenhouses onlyemitters. Drip systems are relatively easy to In addition to the previously mentionedassemble, but they do take more time to set watering systems, the following techniquesup than most of the other watering systems. work well for greenhouses.This is especially true when individual dripemitters are installed to a large number of Capillary matspots. Capillary mats water plants from below. They are water-absorbing materials placedFertigation on solid bench tops with a few drainage Healthy plant nutrition is frequently holes. Pots or flats are put on top of the mat,maintained in greenhouses by using soluble the mat is watered and water is wicked upfertilizers that are delivered to plants in from the mat into and through the growingtheir irrigation water. A number of devices medium. Periodic top watering is recom-are available to help accomplish this. The mended to provide leaching and preventsimplest and probably the most useful and potential salt buildup.economical for a small home grower is aproportioner that can be attached between
180 • Greenhouses and Season Extenders—Chapter 9 Ebb and flow benches exchange. Ridge vents along the peak of the Ebb and flow benches are similar to greenhouse are the most efficient in remov- capillary mats in that plants are watered ing air, and a general rule of thumb is that from below. They differ in how the water is the total area of the vent openings (ridge delivered. The bench top of an ebb and flow plus side) should equal 20–30 percent of bench is periodically flooded with a pool of greenhouse floor area. Solar-powered levers flowing water, the water is drained and the that sense the temperature can be invalu- cycle is repeated at regular intervals that able in cutting down the stress of manually are appropriate for the water needs of the ventilating a hoop house or greenhouse. plants. Greenhouses only Although natural ventilation is effective, Hydroponics it sometimes benefits from the assistance Water is the central part of the entire of a small fan to speed the flow of stagnant growing system for plants grown hydropon- air. The air also moves passively along ically. The water supply is built into other pathways of least resistance, and a properly aspects of plant culture. There are a variety placed fan can introduce some turbulence of hydroponic systems, ranging from plants that reduces channeling and results in more anchored by rock in a trough of water to uniform overall airflow. Sometimes such plants grown in Styrofoam trays that float in fans are placed in the ceiling and blow verti- a pool of water. In all types of hydroponic cally downward across the bulk flow of air. culture, water must be well aerated and free of disease and provide a steady supply of nutrients to plants. Hydroponic systems Pollination require more monitoring time, more atten- When covering tion to their operation and more mixing and crops that flower adjustment of nutrient solutions, thereby and fruit, it’s impor- adding to the system complexity. tant to consider how they will be pollinated. For Natural ventilation crops such as strawberries, Season extenders and greenhouses opening and closing row covers Natural ventilation generally provides with clothes pins or another mechanism is adequate air exchange for season extenders one way to provide access to pollinators. and small greenhouses. Natural ventilation This is also easily done with hoop houses or depends on differences in temperature in- high tunnels. Planting flowers on the ends side and outside the greenhouse. Ventilation of hoop houses or high tunnels or even in- effectiveness increases as the temperature side can be a way to draw in pollinators. For difference increases. Warmer, more humid more information on encouraging beneficial greenhouse air rises out of vents in the top insects in your garden and season extend- of the greenhouse and is replaced by cooler, ers or greenhouses, see UAF Cooperative dryer air from outside that enters through Extension Service publication, Beneficial lower side vents. Insects and Spiders of Alaska, PMC-10075. Natural ventilation requires adequate Pollination for tomatoes in greenhouses vent area to provide enough airflow and air is often simulated by shaking the plant or
Chapter 9—Greenhouses and Season Extenders • 181by buzzing them with an electric buzzer UAF School of Natural Resources andintended for such use. Seedless cucumbers Agricultural Sciencesdo not require pollination although seededvarieties do. In a greenhouse, female flow- Growing Fresh Vegetables; Midnight Sun-ers should be manually pollinated with a light and the Earth’s Warmth, SNRAS/male flower. AFES Misc. Pub. No. MP 09-10. Matheke, G.E.M., P.S. Holloway, and P.J. Wagner. April 1991. IRT-76® Polyeth-For more information ylene mulch film and growth of sweet corn in Fairbanks, Alaska. Number 25.UAF Cooperative Extension Research Progress Report.publications Other publicationsGrowing Everbearing Strawberries as An- nuals in Alaska, HGA-00235. Bartok, J. 2000. Greenhouses for Home-Beneficial Insects and Spiders of Alaska, owners and Gardeners, NRAES-00137. PMC-10075. Ithaca, New York: Cornell University.Controlling the Greenhouse Environment, Coleman, E. 1999. Four-Season Harvest: HGA-00336. Organic Vegetables from Your HomeCucumber Production in Greenhouses, Garden All Year Long. Chelsea Green HGA-00434. Publishing Company.Fluorescent Lights for Plant Growth, HGA- Ortho’s All About Greenhouses. 2001. Des 00432. Moines, Iowa: Meredith Books.Greenhouse Tomato Production, HGA- Mohyuddin. M., Darby, D., Chang, J., and 00435. Betty Vladicka. 2004. Hobby Green-Greenhouses for Home Gardeners: Struc- houses in Alberta. Alberta Agriculture, tures and Equipment, HGA-00337. Food and Rural Development.Plastic Mulch and Row Covers for Vegeta- ble Production in Alaska. FGV-00647. WebsitesPlastic Mulch and Row Covers for Veg- Greenhouse Plans etable Production in Alaska (DVD), eHouseplans: www.ehouseplans.com/ HGA-01268. project_plans/greenhouseplans.html8The Attached Solar Greenhouse, EEM- High Tunnels 01259. High Tunnels.org and www.hightunnels. org Planning and Building a Greenhouse National Sustainable Agriculture Infor- mation Service: http://attra.ncat.org
182 • Greenhouses and Season Extenders—Chapter 9