I have used a cold frame for years to grow and harden off my plants before placing into the garden whenever the temperature cooperates. It started off as a 2 X 12 box with a fiberglass top. I have tried plastic greenhouse but stop using these after the second greenhouse blew down the road. This year I purchased a rigid plastic Jewel cold frame made in Germany with automatic vent opener (I figured I deserve it)!
So, what is a cold frame?
A cold frame is nothing more than a box with a clear lid that to trap heat and shelter plants from low temperatures and battering weather. Typically, bottomless for good drainage, cold frames sit low to the ground and have no artificial heat source. A transparent lid absorbs sunlight and can be lifted for air circulation (or kept shut to keep out the elements).
Historically, cold frames were built as greenhouse extensions tucked against the outer walls with southern exposure as seen (with their glass lids removed) outside Victorian glass houses. They offered a place to harden off seedlings on their journey from the cozy confines of the greenhouse to outdoor planting beds.
What are the benefits of cold frames?
Provide a frost-free haven for tender plants that won’t survive freezing conditions. Move plants into a cold frame until the weather is warm enough to transplant them into the garden.
Have ideal conditions to gradually acclimate seedlings grown indoors to conditions outside without having to carry them in at night.
Are easy and affordable to make (or purchase).
You can place in the garden to raise cool season vegetables like lettuce, radishes, and spinach thru November or get an early jump in March.
Where is the best place to put a cold frame?
To maximize warmth, light exposure, and weather protection for plants, cold frames should be sited in a south-facing position. Other site considerations are drainage and protection from wind.
Take a cue from history and put your cold frame adjacent to another outbuilding for added insulation and to buffer it from weather on one side.
Can I make my own cold frame?
Cold frames are easy and affordable to make, requiring little more than a few boards, an old window (or piece of glass or plastic), some hinges, screws, and a bit of muscle. I have use straw bales in the past with a window placed on top.
Ventilation is key: when outdoor temperatures are above 40 degrees Fahrenheit, prop open the lid 6 inches; when the outdoor temps clear 50 degrees Fahrenheit, open or remove the lid. Be sure to restore the lid in late afternoon to trap the heat inside for the cool night. Consider a non-electric vent controller to automatically open and close the cover at a preset temperature.
