Cold Frame It and Keep Growing

Cold frames are an invaluable tool for season extension in a vegetable garden. Cold frames allow us to jump-start the spring harvest months before most gardeners begin to sow seeds and stretch our harvest throughout winter.  Read on for tips on how to garden with a cold frame.

By definition, a cold frame is an outdoor structure consisting of a bottomless frame and a glass/plastic top, which is heated through sunlight and is designed to protect plants from cold and adverse weather conditions. They are simple to construct and can add weeks to your growing season.  With a plan and a few basic supplies, anyone can enjoy fresh greens and tender vegetables throughout the winter months.  There are many styles and plans available to the home gardener via the internet, garden centers, or the local county extension office.


Decide what type of frame structure will work best on your site.  If you are new to this idea, it is best to start with a smaller frame.  For the Top of the frame, almost any transparent material-glass, fiberglass, polyethylene-will work.  Whatever top is used, it must be able to create a draft-free seal.  For the Walls, use a rectangle of straw bales, stacks of cinder blocks, bricks or the most common are frames made out of wood. You want to be able to access the middle of the cold frame so keep the width to no more than four feet. The length of the unit  is never an issue, but must be sized to fit the Top being used to cover the frame.


Assemble the frame in a sunny spot and angle the top 25 to 30 degrees toward the south. Make certain the angle will allow for snow melt or rain to run off.  Heavy snows left on top can break or collapse glass/plastic.  Put a thermometer inside the frame and track the temperature relative to outdoor conditions.  Temperature control is very important to successfully growing plants inside your cold frame. **A well constructed frame can overheat very quickly and it is best to keep it on the cool side, especially in the spring and fall. In general, if the outside temperature is below 30 degrees leave the top on. If it is above 40 degrees, prop open a few inches in order to vent out excess heat. If you have a day with the temperature over 50 degrees, you should remove the top completely. Replace the top late in the day in order to trap some heat for protection from plunging night temperatures. It is also a good idea to cover the top at night with some sort of insulation, like a blanket or layered newspapers (snow is an insulator, but the top needs to be angled, not flat to avoid breakage/collapse). In the morning, remove the insulating layer and let the sun shine in.


The planting surface should be free from weeds and old plant debris.  Gently, hoe the soil and smooth out level. Choose clean seed (can be unused see from earlier in the spring) broccoli, beets, chard, green onions, kale, various lettuces, radish and spinach all are excellent cold frame crops.  Plant the seeds in rows at the appropriate level per seed type, label the rows, cover lightly with soil, and water in.  Replace the Top of the cold frame.

If time does not allow for tending a winter garden, consider building a cold frame to use in the spring garden season. They are perfect for “hardening off” your seedlings. Plants started from seed indoors and under grow lights need several days of part time exposure before fully transitioning to their new life outside in the garden. Trays of seedlings can be placed in the cold frame for a half an hour on day one, and increased by another 30 minutes everyday after that. **Remember,  monitor the temperature on sunny days as temperatures can get hot enough in the enclosure to “cook” the seedlings.  The seedlings can withstand temperatures as low as 50 degrees, but any lower will delay  their growth.