MT: Hotbeds and Cold Frames


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Hotbeds and Cold Frames

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MT: Hotbeds and Cold Frames

  1. 1. Hotbeds and Cold Frames for Montana Gardeners by Ron Carlstrom, Gallatin County Extension Agent, Robert Gough, Professor of Horticulture, MSU, and Cheryl Moore-Gough, MSU Extension Horticulture Specialist Growing vegetables and flowers from seeds can be challenging in Montanas short growing season, but a full-size greenhouse isnt necessary to extend your MontGuide season. MT199803AG Reviewed 1/10THE USE OF COLD FRAMESAND hotbeds is an excellent way to 3 sashextend the Montana growing season, 3providing a substitute for greenhouses.The gardener with limited spaceinside the home can use cold frames 2" x 4" stake 2" x 4" cross tie 12"or hotbeds to start transplants fromseed. These simple structures also allowMontana gardeners to plant smallvegetable crops earlier in the spring 8" soiland enjoy fresh vegetables later into 6"the fall. Cold frames are heated entirely manure 18"by the sun’s rays; hotbeds use the 6sun’s rays and other sources of heatto maintain adequate temperatures.Consider the temperature and otherclimatic conditions of your location.When choosing between a cold frame FIGURE 1. A typical structure designand hotbed, consider the plants to begrown. Hardy plants such as cabbageneed only simple, inexpensive facilities,but heat-loving plants such as peppers and eggplants Most home gardeners use the materials they haveshould have more elaborate facilities for successful available. When selecting wood, consider its rot resistanceproduction in some of the colder areas of Montana. qualities. The heartwoods from cedar and redwood provide excellent rot resistance as does pressure-treated lumber.Structure and design Pine, fir and spruce need to be treated to increase rotFigure 1 illustrates a typical cold frame design. When resistance. Do not use creosote or pentachlorophenolselecting materials to construct your cold frame, keep in preservatives because they release vapors that can be toxic tomind that the inside of the structure will be warm and plants. Copper napthanate is non-toxic to plants when dry.humid. Wood is the primary structural component in Old storm windows make an excellent cover for yourmost cold frames. Some manufactured cold frames use structure. If you have none, there are many substitutesaluminum, plastic, and fiberglass. for glass. Heavy plastic film is an inexpensive covering but may last only one season. Plastic window pane, polycarbonate panels or ridged fiberglass panels will last for several years and hold up against wind. For More Online MontGuides, Visit
  2. 2. Make sure all joints are tight against theweather. The structure should have a top lid Cover at night duringthat can be propped open at different heights very cold weatherto allow for ventilation, watering and easyplanting. You must be able to secure the coveragainst wind gusts.LocationLocate hotbeds and cold frames on a well- Bank soil on sidesdrained sunny southern exposure. If you donot have a site with good soils, you may needto remove the soil inside the structure and FIGURE 2. Night insulation for cold frames and hotbedsreplace it with sandy loam to a depth of 30inches to provide good drainage. The back ofthe structure should face north and be built into a hillside, Radiant heat from electric light bulbs has also beenagainst a building, or banked to give protection. Use soil used with good success. Place eight 25-watt incandescentto bank the outside of your frame structure to increase bulbs along a board directly under the cover. This will beheat holding capacity. Protect your frame from prevailing enough to heat the standard 3 foot by 6-foot section of awinds. Locating the structure near water and electricity is hotbed. Take special precautions with this system to guardhelpful. against electrical shorts. A thermostat measuring soil temperature will also be necessary.Operation Fresh animal manure can be an economical sourceYou will need an easily-read thermometer for your hotbed of heat for a hotbed (Figure 4). Place fresh manure in aor cold frame. Locate the thermometer so you can read it compact pile near the hotbed. The manure will begin towithout removing the cover. compost and release heat. When heating begins, place Cool season plants grow best at 55° to 65°F while the manure giving off the most heat in the bottom of thewarm season plants grow well at 65° to 75°F. If air frame, the rest over it. After each 3 or 4 inch layer hastemperature inside the frame goes above 85°F, ventilate by been added, tramp it down so that the bed will be evenlyopening the top or install an automatic lifting device to packed. The amount of manure used will depend on airmaintain desired temperatures. Monitor the temperature temperature. Twenty-four to 30 inches of manure will beto insure the structure does not cool down too much. sufficient in most cold climates. When the temperatureWhile most gardeners worry about trying to heat the drops below 90°F in the frame, spread about 6 inches offrame, there is much danger from the sun overheating and good garden loam, sifted, over the manure. Do not sowkilling young plants in early spring. seed for one week, or until the first strong heat is over and You’ll need a method of retaining heat during the any weed seeds in the soil have had a chance to sprout.night. Use old quilts as an insulating cover (Figure 2). Remove these weed seedlings before planting.Rigid foam board insulation also works well. Rocks,bricks and water-filled bottles placed inside thestructure absorb heat during the day and give Cables to a weathertight Thermostat and remote sensing bulboff heat during the night. switch, fused and groundedHeating methods for hotbedsElectric heating cables placed directly underthe soil work well in a hotbed (Figure 3).These cables, equipped with a thermostat atthe soil surface, can provide gardeners with a 4" soilreliable source of heat. Follow manufacturerrecommendations when installing the cables. Hardware cloth Hotbeds can also be placed on the south side 24" soilof a house next to a basement window which Banked soilcan be cracked open to allow heat to enter the Heating cablehotbed. FIGURE 3. Hotbed with electric cable2
  3. 3. Ventilate to remove excess heat in early spring. Remove cover after danger of frost is past. Insulate Bank soil for insulation and drainage 6--12" topsoil approx. 3 approx. 3 24--30" composting manure FIGURE 4. Hotbed with composting manure The biggest drawback to using manure is that you Tomatoes and crucifers from six to eight weeks old arecannot regulate the temperature in the hotbed easily. This most desirable for transplanting into the garden. Vinemethod will also take some initial shovel work to dig a crops take up to four weeks; peppers about 10 weeks.pit inside the frame 24 to 36 inches deep. Change the As conditions within the frame are nearly optimal,manure yearly. transplants must become adapted to the more harsh field environment. Reduce moisture 7 to 14 days prior toPlanting suggestions for transplants transplant and gradually expose plants to field conditions.To grow transplants successfully, sow seeds in rows Set the transplants out into the garden on a cool, cloudyspaced about 4 inches apart when the soil temperature day at the recommended time. Hardened cabbage, lettuceat ½ inch is 70 to 75°F. Seeding depth should follow and broccoli will stand light frost. Tomatoes and peppersrecommendations on the seed package. After seeding, are frost tender.tamp down soil firmly, water gently so that the small seeds Plants will not suffer many setbacks in growth duringare not washed out, cover with soil and leave until the transplanting if they are moved with a block of moist soilseedlings break through the soil. around their roots. Set transplants a little deeper than When seedlings appear, give them plenty of sun and they grew in the flat or seedbed (Figure 5). Shade plantsgood ventilation. Excessively high temperatures make if possible to help newly set transplants get a better start,plants weak and leggy and more prone to disease. Water especially if planted on a hot day or in a windy spot.thoroughly when necessary, but only on bright mornings. FIGURE 5. Set transplants a little deeper than they grew in seedbed 3
  4. 4. Planting suggestions for extending theseasonThe home gardener wishing to harvest fresh vegetablesearlier in the spring may also harvest into the fall usingcold frames or hotbeds. The height of your structure willlimit the vegetables you choose to plant. Sow seeds inrows spaced 6 to 8 inches apart. Seeding depth shouldfollow recommendations on the seed package. Afterseeding, tamp down soil firmly, water gently so that thesmall seeds are not washed out, cover with soil and leaveuntil the seedlings break though the soil. Operate yourstructure as mentioned in the operations portion of thisbulletin. You will want to prop the cover wide openduring the heat of summer. Keep the soil moist, but donot over water. For more information on home gardening contact yourlocal MSU County Extension Agent. NLOAD OW D FREE E To order additional publications, please contact your county or reservation MSU Extension office, visit our online E W catalog at or e-mail orderpubs@montana.eduCopyright © 2010 MSU ExtensionWe encourage the use of this document for nonprofit educational purposes. This document may be reprinted for nonprofit educational purposes if no endorsement of a commercialproduct, service or company is stated or implied, and if appropriate credit is given to the author and MSU Extension. To use these documents in electronic formats, permissionmust be sought from the Extension Communications Coordinator, 115 Culbertson Hall, Montana State University, Bozeman MT 59717; E-mail: publications@montana.eduThe U.S. Department of Agriculture (USDA), Montana State University and Montana State University Extension prohibit discrimination in all of their programs and activities onthe basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital and family status. Issued in furtherance of cooperativeextension work in agriculture and home economics, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Douglas L. Steele, Vice Provostand Director, Montana State University Extension, Bozeman, MT 59717. File under: Yard and Garden (General) Reviewed January 2010 300-110SA