Cold frames (or hotbeds) are simple structures that have two main purposes, to act like miniature greenhouses to trap radiant heat and to provide protection and insulation from the elements. Cold frames traditionally have a sloped top that is positioned for maximum sun exposure, lift off or slide open sash (lids), insulated side walls that sit on the soil surface or are excavated below ground. Cold frames and hotbeds differ only in that one is heated and the other isn’t. Both types are useful in the garden — particularly from fall through spring to protect plants during cold or stormy weather. They are handy for extending the growing season and to provide a warm, sheltered area to ripen tomatoes longer into the fall or winter, to start cool weather crops (lettuce and leafy greens, radish, peas, cabbage, and more) earlier, or in some locations to overwinter forced bulbs, root vegetables, or hardwood cuttings. Many cold frames have the advantage of being temporary and portable too. Some gardeners profess that their cold frame extends their growing season by 4-6 weeks in spring and fall.
Cold frames differ from root cellars because they are designed to keep plants growing, not just for cold storage. They have no supplemental heat but rely on solar thermal energy for warmth. The design of the sloped wooden outer frame collects and traps heat from the sun. With a purpose to protect plants, most cold frames will keep plants one hardiness zone warmer (about 10 degrees F.) and work well to extend the season. Cold frames are great for getting an earlier start in the spring (especially when growing shorter, cool weather plants such as pansies, dusty miller, primroses, dianthus, dwarf snapdragons, cyclamen and ornamental cabbage), hardening off young plants in the spring or to keep root crops longer into the winter (instead of using a root cellar).
Hotbeds are cold frames that have the addition of a heat source (usually either mechanical or biological). Hotbeds are used for the same purposes as cold frames with the addition of being a good site to propagate woody cuttings which root faster with bottom heat. Hotbeds can have electric heating cables installed in the base substrate or use somewhat fresh manure for warmth.
Cold frames can be easily made from recycled materials and cost almost nothing for the materials, or an already manufactured version can be purchased for $100-$500. Do-it-yourselfers with access to 2 x 6s, 2 x 8s or 2 x 10s and window sash can easily construct a cold frame.
Cold frames and hotbeds can be located using the same principles. A southern exposure is beneficial so cold frames and plants can get the most benefit from sunlight. A west-facing direction is the second best orientation. A solid (or mostly solid) windbreak on the north side, which doesn’t cast any shade on it, will create an even warmer microclimate for the cold frame. If a close windbreak is not available, bales of straw can be used on the north side wall for temporary added insulation.
Site your frame near the house for easy access and attention. Pools of water in the vicinity of the frame should not sit for long periods during the fall, winter and spring. Good drainage is beneficial for plants. An accessible (summer and winter) water supply (and electrical outlet if needed) should be a consideration for locating a cold frame.
Most cold frames are a simple rectangular box, about 2-3 ft high that either sits on the soil surface or can be sunk into the ground. The sloped front faces south or west and is angled at about 1″ per foot (about a 6-8″ difference from top to bottom or 35 to 55 degrees). The angle will vary depending on whether the cold frame is to be used in the winter. The higher the angle, the more sun will enter the frame during the winter when the sun is lower in the sky. One simple formula is to add 15 to 20 degrees to your latitude. Another consideration is the equipment available to make the angled cuts. In some cases a standard 45 degrees will be perfectly adequate.
The frame height depends on what the cold frame is being used for — but to get the best angle for the sash, there isn’t much opportunity to go much beyond the standard 18″ back (north) and 12″ front (south) frame heights. Within reason, you can make your frame taller if you will be mostly hardening off older plants or shorter for starting flats of seedlings.
Many different materials, both recycled and new can be used for the sash (frame lid). These include such options as glass, fiberglass, poly film or wooden snow fence. In many cases the size of the sash will determine the size of the cold frame — ok it’s just much easier this way.
Double glazed windows make good durable sash that are heavier than other materials. Glass is generally looked upon as the best material to cover a cold frame. Other materials include discarded storm windows from screen doors or no longer needed patio doors or bathroom shower doors.
If polyethylene plastic is used, the film should be clear and at least 6 mil thick. Consider using a double layer for extra insulation. The poly is not very durable and will probably have to be replaced each year.
Fiberglass or polycarbonate materials make very good sash for cold frames, although they are quite a bit more expensive to purchase.
Comparing Cold Frame Sash Material [Sash Material, Pros and Cons]
Recycled windows can be used.
Good light transmission.
Good insulation value.
More hail-proof (weatherproof) than polyethylene.
Glass is heavy. The extra weight means the sash and side frame must be able to withstand the extra weight.
Opening and closing involves additional weight.
Broken glass is more difficult to replace and repair.
Expensive to purchase.
Inexpensive to purchase.
Probably must be replaced each year.
Easy to install and lightweight to handle.
A double layer will provide more insulation. Won’t withstand large hail stones or heavy snow or ice loads.
Sash must be secured so that it doesn’t take flight in a strong wind.
Strong and durable. Will last many years.
Better insulation value when using double polycarbonate layer panels. Expensive to purchase.
Nonstandard sizes involve extra cutting.
May lose good light transmission over time and become more opaque.
Expensive to purchase.
Nonstandard sizes involve extra cutting.
Easy to construct. Often used with polyethylene film to give added support.
Can easily be removed to ventilate. Not good insulation or thermal conducting without the addition of other materials.
Side Frame Materials
A wide range of materials can be used for the cold frame box. These include wood, brick, masonry, cinder, concrete blocks or metal pieces. The insulation value of each of these materials varies and should be a consideration when picking the best material as well as availability and handling ease. Straw bales can also be used to make a temporary cold frame using a couple of saw horses and some heavy plastic. Metal cold frames should only be used where minimal temperature protection is needed as they have very little insulation value and heat won’t be retained long.
Mostly cold frames have traditionally been made of wood. With this material they are easy to construct and can be dismantled, moved or expanded easily. Untreated wood is recommended (recycled or new) and home treated using a non-toxic wood preservative. If untreated wood is used, then the inside of the frame can be painted with a white latex-based paint to add more light reflection. Use a minimum of 1″ thick lumber (2″ will add greater durability, strength and insulation properties).
Seal all cold frame joints to maximize heat retention. Foamboard insulation can be used inside the frame on the above-ground, north-facing side for even more insulation.
Cold frames can be turned into hotbeds through the addition of a heat source. Hotbeds have the advantage over cold frames because they are more consistent in their temperature and can be used for an even greater number of overwintering or season extending purposes. If a constant and accurate temperature is required, electric heating cables should be used in the soil under the plants. If this is the case, position the hotbed near an electrical outlet.
For less demanding heating needs other materials such as light bulbs, manure, hot water or steam can be used to heat the hotbed. Many people find that hotbeds are not needed and simply use passive solar energy to heat their cold frames. When doing this, black painted barrels filled with water absorb heat during the day and slowly release it at night.
When readily available, manure is a cheap and convenient heat source. Hotbeds can be made by placing the frame on a manure pile and mounding the material around the frame. This works best to extend the spring and fall season. Alternatively, a layer of manure 18-30 inches deep below the hotbed will provide a source of heat. Let manure sit for at least 10 days before using it in a hotbed. Place a 6″ layer of soil over the manure inside the hotbed to protect plant roots. Monitor the soil temperature and move plants into the hotbed only when temperatures are below 85 F. Hotbeds using manure must also have excellent drainage. Manure will stop fermenting (and stop producing heat) if it becomes soaked with water.
On unusually cold nights, loose straw can be piled on top of the cold frame sash to provide extra insulation if needed. A thermal blanket (purchased from a nursery or a cast off flannel sheet) is also ideal to put over the plants. This will trap the heat that accumulated during the day and keep the plants warm into the night.
Soil temperatures between 65 -78 F. are ideal for starting many seeds. To prevent seedlings from stretching and growing soft, monitor the temperature once they germinate and vent the frame if air temperatures are above 75 F. Many cool-season crops (lettuce, peas, cabbage, cauliflower, onions, etc.) are ideal for cold frames and can be started much earlier in the spring. The frame though must be checked regularly and may need venting on bright, sunny days.
Venting a Cold Frame
Cold frames are designed to be quite versatile in their venting options. Depending on the amount of venting required and the direction of the wind, the sash can be propped up, slid up, pushed down or removed entirely to provide good air circulation. In many cases, when outside air temperatures reach 60 F. the interior temperatures are high enough to warrant venting – even if it is lifting the sash a few inches at the bottom to let the heat escape.
If other commitments don’t allow for frequent temperature checking of the cold frame so plants inside don’t get overheated or chilled, several models of automatic vent openers are available to purchase and install. These can be set to open and close at specific temperatures. Many have pneumatic cylinders that respond to temperature changes and gradually open the sash in increments as the temperatures rise. These start at about $50. No external power is needed for many of them and the automatic openers lift up to 12 pounds in weight up to 17 inches in height.
Root crops: beets, carrots, parsnip, rutabaga, onions, leeks, kohlrabi
Leaf crops: Brussels sprouts, cabbage, cauliflower, radish, Swiss chard, kale, collards, lettuce, mustard, spinach, mâche/corn salad, scallions, kale
Cold frame diagrams can be found at the following websites:
Originally published in the Acres U.S.A magazine, September 2010 issue.