These mini-greenhouses can be useful for serious gardeners. Though often used for hardening off seedlings, they can also be helpful in extending the growing season in the fall for cool-season vegetables such as lettuce, kale, green onions, and radishes. You may also want to start pansies in the fall, overwinter them in the cold frame, and set out large plants that give immediate color in the spring. Cold frames also can be used to overwinter nursery plants or give the cold treatment needed to force bulbs. In these last two cases, the cold frame is covered with a tarp or something similar late in the fall just before the ground freezes so that the temperature hovers just above freezing. During the summer, you can remove the top and use the structure as a nursery. A hotbed can function like a coldframe if not heated. Adding heat allows plants to be started earlier.
Cold frames and hotbeds used to require almost constant attention during the day due to venting requirements. Venting is absolutely necessary on bright, sunshiny days, even if the outside temperature is relatively cool. If the frames are not vented in a timely manner, the plants can easily overheat. Venting in normally done by having the sash hinged to the main structure. This sash is propped open to let excess heat escape whenever temperatures demand. Though sashes can be propped open by hand, today we have automatic ventilators that use a temperature-sensitive compressed gas to automatically open sashes. These are often called solar vents as they rely on the sun to heat up the interior of the coldframe or hotbed to activate the vent. Solar vents do not require an external power source and can be set to open at different temperatures. An important feature that many solar vents do not have is the ability to release quickly so the lid on the coldframe or hotbed can be opened completely. This makes watering much easier. As mentioned, many solar vents do not have the quick release feature. Two that do are the Univent Standard and Agetc Automatic Vent Opener. If you know of any others, let me know and we can list them in a later newsletter.
Basic Design of Cold frames and Hotbeds
The structure of both cold frames and hotbeds is the same. Basically it's a box covered with glass, plastic or clear fiberglass sash. The box size varies but is often 5 to 6 feet deep and 6 to 12 feet wide. Height also varies but is often about 18 inches in the back and 12 inches in the front. The slope should face south so that rays from the winter sun can be captured more easily.
The only difference between a cold frame and a hotbed is that hotbeds contain a heat source. In the early part of last century, that heat source was often 12 to 24 inches of fresh, straw-laced horse manure placed in a pit under the structure. Today, electric heating cables are often used. Hotbeds are more versatile than cold frames and allow young, tender plants to be started earlier in the year.
Cold frames and hotbeds can be purchased, or you may want to build your own. Plans for constructing either structure can be found at: https://extension.missouri.edu/media/wysiwyg/Extensiondata/Pub/pdf/agguides/hort/g06965.pdf (Ward Upham)