A cold frame is a bottomless box with a clear or translucent top. It is set on the ground or over a planting bed to capture solar energy and heat the air, soil, and plants inside. A cold frame can extend the growing season by one to several months.
A cold frame can keep plants from 7° to 10ºF warmer than outdoors, sometimes as much as 20ºF warmer. Use a cold frame in spring to give seedlings a head start on the growing season and protect them from spring frosts. Use a cold frame in autumn to extend the summer and fall growing seasons into late autumn and winter.
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Cold frames can be made of heavy lumber, cinder blocks, concrete, or clear plastic. The transparent top or cover is tilted toward the south or southeast to catch the sun. The back side of the cold frame is commonly six inches or higher than the front to give a slope to the top cover (or sash) so that rain will run off and the sun’s rays will strike the inside of the frame more nearly at right angles.
Cold frames can vary in size: 6 feet long by 3 feet wide is the size of an average home garden cold frame. A small cold frame is commonly no wider than a person’s reach so that plants can be tended to without stepping into the frame. A typical cold frame is usually made eighteen to twenty-four inches high or deep at the back sloping to twelve to eighteen deep at the front. The deeper the frame the taller the plants it will accommodate.
For most home vegetable gardeners, nearly all the plants in the vegetable garden can be started successfully in a cold frame or hotbed (a hotbed is a heated cold frame–sometimes heated by electricity or solar power or by–in decades and centuries past–decomposing steer or horse manure placed below the inside planting bed).
A gardener who has had no experience with cold frames has usually an exaggerated idea of the skill or cost to start and manage one. The skill is not as much a matter of expert knowledge as of regular attention and care. Only a few minutes a day will give you a successful cold frame for growing your own crops and extending the season for fresh vegetables.
A cold frame can be a permanent or portable structure. The sides of a homemade cold frame can be made out of wood planks, cinder blocks, or brick. A temporary homemade cold frame can use bales of hay or straw for the sides. Some store-bought portable cold frames have aluminum frames and clear plastic or Plexiglass sides and covers. (Clear sides will not afford the same degree of insulation as solid-sided frames.)
A permanent cold frame set to the side of a garden plot can serve as a seedbed and starting place for crops year-round. A permanent cold frame can be heavily insulated to provide a growing place in the coldest winter regions. A portable cold frame–usually easily disassembled or small enough to be carried–can be moved around to protect small plantings or beds. A portable frame is highly suitable for mild winter regions and for small gardens–a portable cold frame can be removed from a garden bed, dismantled, and stored when the warm weather arrives.
Cold frames collect solar energy for heat. Because a cold frame is a passive solar collector the sloping top is best positioned to face south, southeast, or southwest in the northern hemisphere. This will allow maximum sunlight into the frame during winter when the sun tracks lower in the sky. The slope of the top can vary from 35 to 55 degrees. The higher the angle of slope up to 55 degrees the more sunlight will enter the frame during winter. One formula for setting the angle of the top or sash is to add 15 to 20 degrees to the latitude of your garden: thus if you live in Illinois or Pennsylvania on the 40th parallel, set the sash at 55 to 60 degrees.
The clear or translucent top or cover of the cold frame is called the sash, glazing, or light. The sash can be made of glass or rigid fiberglass, acrylic, Plexiglass, or UV-stabilized polyethylene plastic. Home-made cold frames often use old storm windows (window sashes) for covers. The cover or sash is usually hinged at the back of the frame for easy raising and lowering and ventilation, but a top can also slide off of the frame. A cold frame top should be easily opened to ventilate the frame–that is allow fresh, cool air into the frame when the outdoor temperature rises. (You can ventilate a frame by simply propping a stick or block under the sash. As well, automatic solar-powered frame openers attached to a thermostat can also raise the top when the inside temperature reaches above 70°F.)
Cold frames commonly open to the planting bed below and can be planted in directly; allow at least 4 to 6 inches of soil for salad crops and more for larger crops. Cold frames also can have sand or gravel bottoms on which pots, containers, and seed flats can be set. Soil, sand, or gravel can be laid over a weed-proof membrane attached around the base of the frame.
Cold frame basics
A cold frame is a low-profile open-bottomed box that is placed over crops to act as a small greenhouse. Cold frames are heated by the sun. (A hotbed is similar to a cold frame but is usually heated by electric cables placed under the growing bed.)
The sides of a cold frame are often sunk into the ground to make the structure airtight. The backside of the cold frame is usually several inches taller than the front. The frame has sloping sides fitted with a slanting, transparent roof. The sloping sides and transparent roof are positioned to face the warm southern tracking winter sun.
Cold frame advantages
A cold frame offers plants the advantage of good outdoor light while protecting them from the cold, rain, snow, wind, and hailstones. A cold frame can add 10 to 25 degrees to the outdoor temperature depending on the weather and the type of frame. Even a modest increase in soil and air temperature inside a cold frame can extend the growing season for most vegetables from 4 to 6 weeks both in spring and fall. That means an additional two to three months of edible gardening each year.
Here are things you can accomplish with a simple cold frame–a box with a glass top and the heat of the sunshine:
- Start seeds four to six weeks earlier in the spring than in the open garden (plants in the cold frame are likely to grow somewhat more slowly but will be stronger than those grown in a greenhouse as a result of the bright natural light).
- Protect seedlings and allow them to grow strong and sturdy until they are large enough to be transplanted into the open garden.
- Harden off or acclimatize plants started in a greenhouse or hotbed before transplanting or setting them out into the open garden.
- Grow certain crops to maturity; any crop can grow to maturity in a cold frame as long as the frame is large enough to accommodate the crop and the proper temperature and watering is maintained.
- Root cuttings of perennials.
- Grow cool- and cold-hardy crops such as lettuce, spinach, and other leaf crops late into the fall or through the winter.
- Protect semi-hardy crops planted in the autumn through the winter; for example, cabbage planted in the autumn can be protected by a portable cold frame through the winter and then allowed to mature once spring arrives.
Cold frames in cold winter regions
In very cold winter regions, cold frames can be fitted with insulated sides and set atop insulated foundations to retain more heat. A wooden, block, or cement cold frame can be insulated easily with the addition of 2 or more inches of Styrofoam around the inside of the frame and with a layer of 1½ inches on the bottom under the growing bed. Insulating foam can be easily cut and glued to the inside of the frame. As well foam shutters or mats can be placed atop the frame’s clear lid at night–this can increase night air temperature inside the frame by 8°F. A double-paned glass or plastic sash (top) or a sash fitted with an under-layer of 4 to 6 mil plastic sheeting will provide additional insulation of up to 6°F.
A simple way to outfit a cold frame for cold winter regions is to place hay bales or bags of leaves around the outside of the frame. Tarps, woven mats, and old blankets can use used to insulate the top; this additional protection can increase the temperature inside the frame by 6°F at night; which can mean a season extension of 6 to 12 weeks. The inside of a frame can be painted white to reflect sunlight or black to retain heat. Thermal collectors as simple as masonry blocks or jugs of water set against the north wall of a frame will retain solar heat during the day that is released back into the frame at night.
Permanent or portable cold frame? Depending upon the size of your garden, your needs, and the number of months you want to keep the harvest going, you can use a permanent cold frame or a portable cold frame.
A permanent cold frame can be built on a foundation of concrete or a block dug into the ground (which will provide increased insulation from winter temperatures) or constructed on the garden surface. A permanent frame can be constructed of heavy wooden planks–2 inches thick by 6 to 8 inches wide and can be fitted inside with additional insulating materials. A permanent frame is likely to be sturdier and longer lasting than a portable frame and can accommodate a steeper angled sash or light and be likely to withstand a heavier load of sleet or snow without sagging.
A portable frame may be all that is needed in mild winter regions where crops require less protection than in cold winter regions. In mild regions, the portable frame can be shifted from bed to bed or disassembled when not needed. Portable frames can be constructed to fit atop permanent raised beds. Many commercial cold frames are collapsible. Portability in a small garden will allow for greater growth opportunities with limited planting bed space. A portable frame, however, will likely have less insulating capacity than a permanent frame and be subject to wider temperature fluctuations.
To decide between a permanent frame and a portable frame consider:
- How and when the frame will be used?
- The climate and severity of winter.
Types of cold frames
As you consider a cold frame for your garden, you will learn about several types. Here are a few common types:
- Double-glass frames. A frame whose sash or light is double glass (or plastic) will offer maximum insulation being 40 to 50 degrees warmer during the day than outside and 25 to 30 degrees warmer at night. The double layer is separated by an insulating space of air.
- Single-span frame and lean-to frame. The single-span and lean-to frame allows the sash to rise at an angle of 40 to 45 degrees against a solid north-facing back wall or panel. The lean-to frame affords the most light to plants when it covers beds running east to west.
- Flat-topped frame. A flat-topped frame can be as simple as a raised bed with a storm window sash or framed plastic sheet set on top to protect seedlings. You can also make a flat-topped frame by placing a rigid clear top on beams set over cinder blocks, brick, or hay bales.
- Tent-style frame. A tent-style cold frame is steeply sloped from a ridge poll with two sash sides. The tent-style frame can be made from framed sheets of acrylic or fiberglass. With high-rising sides, the tent-style cold frame is suited for growing large vegetables.
- Double-sash cold frame. A double-sash cold frame offers access to the frame from two sides. The double-sash frame is a tent-like frame with a central hinge joining two sashes. The two sashes are joined at a T-shaped center support that runs the length of the frame. The double-sash frame allows for watering, care, and harvesting from both sides of the frame.
Cold frame site and construction
Cold frame site
Cold frames should be situated away from prevailing winds or protected by a fence or wall on the north side. They can be set on a slight slope so that water will drain away from them and they should be situated close to a hose bib for watering.
Cold frame construction
Cold frames are easily constructed: the sides can be made out of decay-resistant wood or concrete blocks and should be tall enough to clear the tallest plant you grow. The top or roof can be made out of glass, fiberglass, Plexiglas, or heavy clear plastic sheeting. The translucent top is usually framed and hinged to the larger box and fits snuggly to create a greenhouse effect. White paint on the inside of the frame will reflect additional light and heat into the box.
Cold frames can be large and permanent or easily assembled and disassembled and portable according to the season.
A portable cold frame can be set over an existing garden planting bed or a layer of 3 to 8 inches (7-20cm) of sand, garden soil, or planting mix can be placed in the frame for planting depending upon the crops.
Cold frame temperature basics
Keeping the frame warm. A cold frame covered with canvas tarps, blankets, mats, straw, or a mix of manure and straw will keep most crops warm through the coldest winter. In hot weather, solar radiation to the frame can be reduced by shading the sash.
One age-old method of adding heat to the cold frame is to place it over a trench of fresh horse manure covered with sand or straw and a foot of garden soil for planting. (This will raise the temperature in the cold frame substantially and might easily be called a hotbed.)
Cold frame temperature. Heat can build up inside a cold frame on warm and sunny days. The cold frame should be equipped with a thermometer and the sash or roof should be opened to permit air circulation when the temperatures rise. The ideal autumn daytime temperature for a frame is 65ºF and the nighttime temperature 60ºF; in winter when plants have matured the daytime temperatures can range from the high 30s to the 40s and can dip lower at night. To prevent plants from becoming overheated, remove or open the lid to let air in, then close it again at night or if a freeze is expected.
Crops for the cold frame
Autumn cold frame crops
Cold frame crops for growing in autumn for fall and winter consumption include arugula, chard, endive, escarole, Italian dandelion, lettuce, Mizuna, parsley, radish, scallion, and spinach.
Winter cold frame crops
Cold frame crops for winter and spring consumption include carrots, claytonia, Italian dandelion, kohlrabi, mache, parsley, radicchio, scallion, sorrel, and spinach.
Most cold-frame crops for winter eating should be started in late summer and early autumn. These plants should reach nearly full growth before the first winter freezes or snow. They will then sit dormant but protected inside the cold frame during the winter but can still be harvested. (Most crops do not grow when the temperatures are consistently below 40ºF. That’s why cold frame crops for harvest in winter should reach maturity before winter temperatures arrive.)
Mid-winter cold frame crops
Cold frame crops for sowing in mid-winter include arugula, broccoli raab, carrots, claytonia, cress, endive, escarole, lettuce, mâche, Mizuna, onions, orach, parsley, peas, purslane, radicchio, radishes, and spinach.
Season-by-season cold frame operations calendar
As the seasons progress there are several uses for a cold frame.
- Start cool-temperature spring vegetable seeds and seedlings in late winter.
- Start summer warm-temperature vegetable seeds and seedlings beginning in early- or mid-spring.
- Start fall and winter crops under shade cover (replace the frame’s glass or plastic sash with framed shade cloth).
- Protect warm-temperature crops from an early frost before harvest.
- Protect cool-temperature and cold-tolerant crops for harvest through the winter.
- Over-winter cold-tolerant crops started in the fall for renewed growth and a spring harvest.
- In early spring (February, March, and April), sow and germinate seeds of hardy and half-hardy early crops. Sow early crops in containers or sow in the soil under the frame as soon as the soil is workable.
- Hardy vegetables for cold frame growing include salad ingredients such as lettuce, spinach, kale, arugula, broccoli, cabbage, cauliflower, chard, radishes, and scallions.
- Cool-season root crops can be grown from seed to harvest in the soil beneath the frame; these include beets, carrots, radishes, and onions.
- Harden off or acclimatize seedlings to outdoor conditions in the frame after they have been started in the cold frame, hotbed, or greenhouse. Ventilate the frame to gradually expose seedlings to more and more outside air and light.
- Place the portable frame over a garden bed to raise the soil temperature and improve seed germination and accelerate the growth of seedlings.
- Late spring (April and May), after cool-weather seedlings are harvested or transplanted into the open garden, sow in the frame in paper pots tender, warm-season crops including beans, cantaloupe, celery, corn, eggplant, peppers, melons, pumpkins, squash, and tomatoes.
- Late spring, harden off in the frame tender seedlings started in the hotbed or greenhouse.
- As outside temperatures warm, do not allow the frame to become overheated this will result in weak and spindly plants. Harden off seedlings with ventilation and less watering; this will condition them to withstand the shock of transplanting
- Avoid growing cool-weather and warm-weather crops in the frame at the same time; they have different temperature requirements.
- Make first, second, and third sowings of quick-maturing, warm-weather crops for growing on before setting out in the garden. Raise or remove the top to keep the seedlings from becoming overheated.
- Remove the frame sash or lid and use the frame as a nursery bed to start fall crops or perennials from seed.
- Start root cuttings or herbs and perennials.
- Harden off tender plants started in a hotbed or greenhouse before setting them in the garden.
- Extend the harvest of warm-weather summer crops.
- Grow salad greens and other compact cool-weather crops.
- Place the portable frame over the garden bed to provide frost protection and added daytime warmth to keep late-summer and cool-season crops growing.
- Sow in fall for late fall use: lettuce, cabbage, cauliflower, spinach, radishes.
- Grow hardy vegetables for winter use: cauliflower, cabbage, kale, lettuce, and spinach. Sow seeds in August or September to allow the plants to grow to near maturity before the first fall frost. When days have less than 10 hours of sunlight per day, plants will stop growing but remain crisp and viable for many weeks.
- Extend the harvest of autumn crops. Hardy crops such as leeks, kale, parsley, arugula, and other greens can make some growth with cold frame protection.
- Winter-over vegetables such as kale, cabbage, Brussels sprouts, and cauliflower for new growth in the spring.
- Place dry leaves or straw in the cold frame to store in winter root vegetables: beets, carrots, parsnips, and potatoes. Do not allow the soil to freeze.
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