It's late winter and it’s the time of year when gardeners want to start planting something. 

Although vegetable and flower seeds can be started indoors, that process requires a fair amount of space, supplemental lighting and constant management of soil moisture, which can be tricky to master. It can also be messy, as I found out last year when I spilled potting mix all over the basement carpet!

For gardeners wishing to get a jump on spring, a simple cold frame or hotbed structure allows for late winter seeding of cool-season crops and earlier outdoor planting of warm-season crops in late spring. These small structures can also be used later in the spring to harden-off or acclimate warm-season seedlings grown indoors or in a greenhouse to outdoor conditions.

A cold frame is a simple, low-tech, box-like structure with a glass or plastic cover and is designed to capture the passive energy of the sun, providing a favorable environment for growing cool-season crops in late winter and early spring. These small structures collect heat as the sun’s rays shine through the glass or plastic covering providing heat for the interior of the cold frame similar to the heat generated inside a greenhouse. Cold frames can be placed directly on the ground or on top of raised beds.  For added insulation, cold frames can be installed several inches below the soil level or surrounded on all sides by straw bales.

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Cold frames and cold frame kits can be purchased online or at larger garden centers, but gardeners who are handy with tools can easily construct a cold frame with some lumber and glass or polycarbonate plastic. Old windows salvaged from home window replacement projects make excellent covers for cold frames and hotbeds.

Hotbeds are the same structures as cold frames but they include supplemental heating provided by soil-heating electrical cables or tapes placed beneath the soil in the structure. As an alternative to using electricity to provide supplemental heat, straw-based manure can be placed under the soil in a hotbed, and as the manure decomposes, heat is produced, raising the temperature of both the soil and the air in the structure. 

To construct a manure-based hotbed, remove about 10 inches of soil beneath the structure, add a small amount of gravel for drainage, add 18 inches of straw-based horse manure and cover with about 6 inches of soil.

In central Ohio, cold frames and hotbeds will typically allow gardeners to plant cool-season crops three to four weeks earlier than seeds planted in unprotected soil. Depending upon the weather, that means that these crops can be planted as early as late February and early March in cold frames and hotbeds. 

The most cold-tolerant cool-season vegetables that can be planted earliest in cold frames and hotbeds include kale, chard, spinach, mesclun, green onions and even peas. Lettuce, collards, greens, cabbage, radishes and perhaps beets, carrots and broccoli should be planted two weeks later. When selecting vegetable varieties to start in a cold frame or hotbed, be sure to select the most cold-tolerant varieties available.

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Warm-season crops such as peppers, tomato, squashes, eggplant, melons, cucumbers and beans shouldn’t be planted in cold frames or hotbeds until early to mid-April. It is extremely difficult to grow both cool-season and warm-season vegetables in the same cold frame or hotbed simultaneously because of the differing temperature requirements.

It is also possible to start cold-tolerant annual flowers and herbaceous perennials in late winter in a cold frame or hotbed. Annual flowers that tend to do best in a cold frame or hotbed include snapdragon, calendula, ageratum, celosia, cosmos, sunflower, cornflower, alyssum and marigold. Herbaceous perennials that can be started in a cold frame or hotbed include aster, astilbe, liatris, coreopsis, penstemon, phlox, yarrow, carnation, coneflower, salvia, coralbells and hollyhock.

Ventilation will be needed in cold frames and hotbeds in early spring or anytime when daily outdoor temperatures reach 45 degrees Fahrenheit. Propping open the top of these structures will prevent the buildup of extremely warm temperatures, which can result in weak and spindly plants. 

Although soil inside a cold frame or hotbed rarely requires supplemental watering during periods of cold temperatures, periods of warmer temperatures typically experienced in early spring may cause soil to dry out, requiring supplemental irrigation.

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So if you are itching to get a jump on spring planting, head to your garage or workshop and build a cold frame or hotbed so you can start planting cool-season crops in the next few weeks.