Autumn may be in the air, but the change in seasons doesn’t require a complete halt to your gardening. By installing a cold frame in your garden, you can extend your growing season and grow seasonal plants in cooler weather.

What Is a Cold Frame?

A cold frame consists of a bottomless box frame with a top made of glass or clear plastic that allows the sunlight in but protects plants within from cold air. According to Bronx Green-Up, you can build your cold frame to any dimensions you desire. However, their guide recommends a wooden frame that is three feet wide and four feet deep — using untreated lumber if you’re planning to grow food. It also suggests reusing an old window for the top. You can also build a cold frame using hay bales as insulating walls.

Year-Round Fresh Produce with Cold Frames

While many gardeners think of cold frames as simple season extenders, they’re actually capable of providing fresh produce year-round with proper management. Cold frames create a microclimate that can support slow plant growth even when outdoor temperatures drop significantly below freezing.

During winter months, cold frames act like passive solar collectors, capturing and storing heat during sunny days while providing crucial protection from harsh winds and extreme temperature fluctuations. The thermal energy absorbed by the soil and frame materials contribute to modulating rapid freeze-thaw cycles that can damage or kill plants. This stable environment allows cold-hardy vegetables like kale, spinach, and winter lettuce varieties to continue photosynthesis and growth even when snow covers the ground outside.

The magic happens through what gardeners call “four-season gardening.” By selecting vegetable varieties fitted for each season and practicing succession planting, you can harvest fresh greens, herbs, and root vegetables year-round.

Cool-season crops planted in late summer can provide harvests well into winter, until the garden is ready for early spring plantings. Some hardy perennial herbs like thyme, oregano, and chives can overwinter in cold frames, so you can enjoy fresh seasonings throughout the cold, dark months.

Strategic ventilation during mild winter days prevents overheating while maintaining the protective environment plants need. Many experienced cold frame gardeners report harvesting fresh salads on Christmas morning or picking tender baby greens during February snowstorms, which is virtually impossible with traditional outdoor gardening in colder climates.

Materials

Bronx Green-Up’s guide suggest using these materials:

• 1 2×8 board, 3 feet long
• 1 2×12 board, 12 feet long
• Box of 3½-inch outdoor screws
• Plastic, glass, or an old window for the top
• 1 or 2 small door hinges (to join the top to the frame)

You will also need:

• Measuring tape and pen for marking cuts and holes
• Saw
• Drill and drill bits

Instructions

The first board on the list of materials will serve as the front wall. The 2×12 board will need to be cut to create three segments — a 3-foot length for the back wall, and two 4-foot sections to serve as the sides of the frame. Next, cut the two side pieces so that the height measures 8 inches on the front end and 12 inches on the back end.

Pre-drill holes in the corners, then join one corner at a time on a flat surface to ensure accuracy. Once the frame has been assembled, attached the top using the hinges to secure the top piece.

Setting Up the Cold Frame

Bronx Green-Up’s guide suggests setting up your cold frame in an area that receives southern exposure, ensuring it will get the most sunlight. Depending on what you’re growing, the cold frame can be used to plant seeds earlier in the spring or later in the fall, then transport the seedlings into your garden in ideal weather. The plants become adapted to their environment better in a cold frame than if you start the seeds in an indoor garden.

Pre-drill holes in the corners, then join one corner at a time on a flat surface to ensure accuracy. Once the frame is assembled, attach the top using the hinges.

Setting Up the Cold Frame

Bronx Green-Up’s guide suggests setting up your cold frame in an area that receives southern exposure, ensuring it will get the most sunlight. Depending on what you’re growing, plant seeds in the cold frame earlier in the spring or later in the fall, then transport the seedlings into your garden in ideal weather. The plants become adapted to their environment better in a cold frame than if you start the seeds in an indoor garden.

Tips:

• The best winter plants to grow include greens, such as lettuce, arugula, parsley, radish, scallion, spinach, and Swiss chard. You will need to plant before a freeze, then harvest them as baby greens, but the cold frame keeps them at that growth stage for longer than during summer.
• Ensure that tender plants are packed and kept in pots or, if placed in the ground, pack them with soil, mulch, and leaves for insulation against cold.
• Keep the soil moist, but do not overwater your plants in the cold frame, or they could rot.
• On warmer days, ventilate your cold frame by opening the cover.

Keeping a thermometer on the surface of the soil can help you determine when it’s time to vent your cold frame. Ideally, in the spring, the temperature inside the frame should be about 70 degrees F. Keeping it at 65 degrees F is recommended during the fall months. If you’re unsure how much to vent, the Bronx Green-Up suggests erring on the side of venting too much, as it could allow the plants to build up a tolerance to the colder weather and grow hardier.

A cold frame can satisfy your green thumb’s desire to garden all year round. And by using fewer materials through reuse, building a cold frame is a green project, too!

Editor’s Note: Originally published on October 7, 2015, this article was updated in September 2025. Feature image courtesy of Ofer El-Hashahar.