9 Tips for Growing Vegetables in a Cold Frame

Growing vegetables in a cold frame allows you to grow later into the fall and start crops earlier in the spring. And in some areas, veggies can survive all winter! However, this growing tool comes with a unique set of challenges. Learn nine tips for growing vegetables in a cold frame.

Choose a Sunny Location

Cold frames rely on the sun to trap and hold heat. Without sun exposure, the air and soil temperature inside will remain similar to those outside the frame. Therefore, selecting the proper location is crucial.

Place your frame in an area that receives full sun. If you live in the Northern Hemisphere, next to a south-facing wall is an excellent option. The location will provide full sun, and the wall will help act as a wind-break. Additionally, brick or stone walls can trap and hold heat, creating a warmer microclimate.

Keep Soil on the Drier Side

Any vegetable farmer who grows winter vegetables in a greenhouse or high tunnel will tell you high soil moisture is the enemy of healthy crops. Excessive soil moisture can spur the development of root rot and fungal foliar diseases like anthracnose and powdery mildew. The limited airflow in cold frames makes these diseases even more likely.

Since plants lose less water through evapotranspiration during cooler months, you can water less in the winter than in the summer. Once the day length drops below 10 hours, plan to water your crops only once or twice a week. If you notice signs of overwatering, decrease the frequency of your watering.

Watering your crops in the morning will allow any excess moisture to evaporate during the day. Maintaining dry crops can help prevent fungal diseases and keep your plants healthy. Drip irrigation is best since it keeps the foliage dry. However, overhead watering with a hose is also acceptable. Just vent your frame after watering to allow excess moisture to escape.

Vent for Airflow and Proper Temperature

One of the most common mistakes gardeners face is failing to vent. All types of cold frames allow you to open the top of the frame to allow excess heat to escape. If you fail to open your frame, the area inside can become too hot. Even on cold days, the sun can quickly cause the air temperature to soar above 100°F.

Large swings in temperatures can stress plants and even cause them to die. Therefore, you should keep the temperature relatively stable throughout the night and day. If the sun is out, vent or open the frame by noon to avoid high temperatures. Venting is appropriate on sunny days that remain below freezing, but you should fully open the frame on warm, sunny days.

Not only does venting allow you to control the temperature, but it also helps with airflow. Closed frames quickly trap moisture, leading to anthracnose, powdery mildew, and other fungal diseases. Opening the frame allows moisture to escape, leading to a less humid environment and less chance of disease.

Maintain Proper Plant Spacing

I’ve said it before and will say it again: proper airflow is essential for keeping crops healthy! Along with venting, you should space your plants far enough apart to allow air to pass between the crops. This will help allow excess moisture to evaporate, which will help prevent fungal diseases.

While you may be tempted to pack your veggies close together to make the most of your space, avoid doing so. Instead, follow the plant spacing recommendations listed on your seed packets. If there’s a range in spacing recommendations, err towards leaving a wider gap between plants.

For example, space kale, Swiss chard, and head lettuce at least 12″ apart. Leave at least six inches between rows of direct-seeded root crops like beets, carrots, and radishes.

Choose Cold-Hardy Crops

Although cold frames can trap heat, they won’t transform the winter months into summer. Therefore, choose cold-hardy crops if you use this method to extend the fall growing season into the winter. The following crops will grow well using this system:

Close Early Enough to Trap Heat

Gardeners often use cold frames to create a warmer environment and extend the growing season. But you must understand when to close it to create a warm environment.

As I wrote above, venting allows heat to escape. On the other hand, closing the frame traps heat. If you want the air and soil in the frame to remain warm during cool evening hours, close the frame a few hours before sunset. The frame will tap and hold heat like a closed car on a hot day.

The optimal time to close depends on the air temperature and sunlight. Close the frame later on sunny days and earlier on partly cloudy days. If you expect below-freezing temperatures following a cloudy day, place a piece of floating row cover over your crops to provide extra protection.

Take Note of Plant Height

There are many different cold frame designs, some only a few feet tall. Before you plant taller crops like broccoli, peas, and cauliflower, ensure the mature plants won’t outgrow your frame. If the plants grow taller than the top of your frame, you’ll lose the ability to close the top and trap heat.

If you’re working with a shorter design, you can still grow plenty of vegetables! Cold-hardy, low-growing crops like arugula, baby kale, lettuce, bok choy, cilantro, radishes, and hakurei turnips remain under a foot tall at maturity. Depending on your frame’s size, you may also be able to fit crops like mature kale, daikon radishes, and Swiss chard.

While cold frames are great for small gardens, they’re not the only equipment that allows you to extend the growing season. High tunnels and greenhouses can also trap heat, and their height allows you to grow taller crops. 

Keep an Eye Out for Pests

Sap-sucking pests like aphids and thrips often arrive in the winter, especially in controlled, closed environments. That means you should watch closely for these pests and treat them as soon as possible. If these little pests are unchecked, they can easily take over your vegetables, leading to discoloration and dropped leaves.

Since your plants are more closed off from the outside environment than open gardens, natural predators like ladybugs and green lacewings often have difficulty finding their way in. Without natural predators to keep pests in check, even a small number of aphids or thrips can multiply into hundreds or thousands of individuals.

You can treat these sap-sucking pests by removing infected leaves and releasing store-bought natural predators like ladybugs and parasitic wasps into the enclosed space. If the pests persist, spray infected plants with neem oil or insecticidal soap. Since these products work when they come into contact with the pests, aim to coat infected areas thoroughly.

Harden Off Spring Transplants with Care

You can also use your cold frame to harden off (acclimate) spring vegetable transplants before you plant them in your garden. It will be a nice buffer between a stable, warm indoor environment and a cold, windy outdoor spot.

If you’re using a frame to harden off frost-sensitive plants like tomatoes, basil, and peppers, ensure the danger of frost has passed before you plant. Although this method will trap heat and likely prevent these plants from succumbing to frost, the cold temperatures inside the frame will stress the plants.

I like to begin hardening off plants when nighttime temperatures are consistently above 40°F. Continue to vent the frame on sunny days and close during the late afternoon to trap in heat overnight.