When Cold Frames Really Help

Many gardeners plant cool-season vegetables like spinach, kale and winter lettuces in cold frames during the fall season to protect half-hardy perennials and overwinter bulbs safely.

Cold frames feature sloping sides fitted with transparent roofs that slant to capture winter sun’s warming rays, often made out of wood frames or using concrete blocks as insulation for extra insulation value.

They Keep Seedlings Healthy

Cold frames offer you a fantastic way to grow plants earlier and later in the season than would otherwise be possible. As they create an environment about 1.5 zones warmer than its surroundings, it makes cold frames ideal for sowing seeds that require additional warmth to germinate or holding on crops until temperatures warm up further.

Cold frames not only warm the soil and air, but they also protect tender plants from frost and garden pests. A cold frame can also be used to overwinter hardy salad greens like winter lettuce and mizuna as well as start seeds of cold-resistant herbs such as parsley, cilantro and chives.

Build a basic cold frame by covering an open ground area 3-by-6-feet in size with sturdy wood or brick structures and then adding a clear lid made from transparent material like glass (such as an upcycled window sash), acrylic sheet or plastic sheet – hinges may come in handy here as they allow opening up on warm days to provide ventilation; some people even choose to paint the interior white to reflect more light!

When in use, the lid must be opened gradually for longer on sunny days to prevent overheating and monitor the temperature with an attached thermometer. Ventilation is essential as moisture generated from water evaporating into the frame can quickly kill plants if left alone.

Once this has been accomplished, the gardener closes the lid of their cold frame to retain heat and safeguard their crop from frost. A cold frame makes it possible to start planting outdoor vegetables and flowers as early as mid-May in many locations, and is also an invaluable way to “harden off” seedlings and transplants grown indoors or at garden centers prior to being moved outside for long enough periods each day to adjust them to outdoor conditions – an often challenging process when done manually.

They Keep Leafy Greens Growing

Leafy greens thrive in cold frames, providing fresh salads throughout winter and providing much-needed vitamins when other crops have long since gone to seed. Lettuce, kale and spinach thrive well as do radishes and green onions; even hardier varieties like mizuna and winter purslane may survive; though their seeds must be regularly reseeded to keep up their production.

Before transplants can be placed outdoors permanently, most need a period of acclimatization known as hardening off to outdoor conditions – or “hardening off”. This process involves gradually placing plants outside for longer periods each day over several weeks or two and shutting them back indoors at night to protect from frost damage. Cold frames make hardening off easier as you simply open their lid gradually more often each day to gradually expose crops to cooler temperatures until they have fully adjusted to the weather conditions.

Cold frames can also be useful for starting warm-season vegetable seeds and rooting woody cuttings from shrubs and perennials, such as roses or lilacs. A similar structure known as a hot bed can give warm-season veggie plants like tomatoes and peppers an early headstart while rooting woody cuttings from roses and lilacs.

To build a cold frame, select an area that gets full sun throughout the day – ideally southern exposure – and construct a rectangular frame, commonly 3 by 6 feet, so rain or snow runs off easily. Use hinges to attach a clear cover that can be opened for ventilation on warmer days; fill your frame with nutrient-rich, well-draining soil amended with compost for increased drainage; add mulch or straw as extra insulation against temperature extremes (some temperatures can exceed 85F in mild winter days), using a thermometer to monitor temperatures inside and adjust sash accordingly to reduce fluctuations between these extremes.

They Keep Root Crops Growing

Add an extra zone of warmth to your garden for maximum planting success, harvesting crops well beyond peak gardening season and even protecting delicate vegetables from frost or pests, according to Eliot Coleman’s Winter Harvest Handbook. Cold frames also protect tender vegetation from freezing temperatures and pest infestation, protecting it from becoming targets for attack by Eliot Coleman himself!

Root crops like carrots, beets and radishes thrive in cold frames as do leafy greens such as spinach – one of the fastest-growing varieties! But you could also experiment with growing lettuce, kale collards turnips or other hardy varieties within your cold frame.

As with any greenhouse, cold frames require careful management in order to avoid overheating and damaging plants. Furthermore, water should be applied sparingly during cooler weather as evaporation occurs more slowly.

Visit your local library for resources on building and using cold frames, such as The Year-Round Vegetable Gardener by Niki Jabbour which contains expert advice for making the most out of cold frames and other hoop houses.

Prefabricated cold frames available at stores provide a convenient way for even busy gardeners to extend their growing season, and some even feature automatic vents which open and close depending on temperature fluctuations, to prevent overheating.

Frames can be placed atop existing soil or filled with soil for an elevated bed. Some come equipped with built-in heating cables, making them an excellent choice in cold climates. If you prefer DIY options, old milk jugs or soda bottles, plastic covered tomato cages, glass and metal cloches or transparent “houses” such as old milk jugs can add three or four weeks more growth to your garden.

They Keep Winter Harvests Coming

Cold frames help extend harvest seasons for leafy greens and root crops by protecting plants from frost in late autumn, enabling growers to harvest last of summer vegetables even as temperatures decline – this can bring extra income as consumers are willing to pay a premium price for out-of-season produce.

Cold frames offer many other advantages beyond offering extra winter harvest protection, including helping start seeds early in spring for cool-season crops like tomatoes and eggplants. Furthermore, cold frames can moderate soil temperatures – an advantage when dealing with heat-loving tomatoes and eggplants – and be used to acclimate winter pot plants and bulbs, which often take quite some time to adapt to outdoor temperatures.

Russ Libby shared his homemade wooden frame at the Unity Land Institute Small Farm Field Day to demonstrate how he uses it for starting and growing winter-hardy vegetables like spinach, kale, chard and bok choy – plus overwintering flowers such as fuchsias, geraniums and nasturtiums in it!

Cold frames can be built onto any structure, such as a greenhouse, hoop house or simply an insulated box. Wood construction tends to provide the best thermal barrier and help regulate temperatures within the frame; adding insulation can further enhance performance of this structure.

To keep crops in top condition during the winter season, Libby suggests venting the frame periodically to allow air circulation. For optimal results, do this during daytime when temperatures are warm before sundown – or use automatic lid openers which will open or close themselves depending on weather.

To maximize a cold frame’s effectiveness on your property, it is crucial that you understand it thoroughly. Take note of areas that naturally receive more warmth – for instance the south side of a house or fence or where snow melts first in spring – and build there. While ready-made frames may cost less and offer greater stability than DIY options, Libby suggests building one yourself if time and physical ability permit. Doing it yourself saves money and guarantees quality.