How to Make Cold Frames for Vegetable Gardens

Cold Frames CollageCold Frame made with Cinder blocks and old window frames

A simple coldframe can extend your growing season at either end by a month or even more , in some regions you can grow right through the winter with one. A coldframe is also an ideal place to harden and acclimate eggplant, tomato ,pepper and other seedlings that you’ve started indoors to conditions outside.

Basic Do it Yourself Cold Frames

A coldframe is basically four walls and a transparent roof that admits sunlight, shelters plants and traps heat. The walls can be made from any sturdy material such as cinder block, landscaping ties, plywood, concrete, a bale of hay works excellent also due to its insulating properties. An old window will suffice as a lid, as will plastic sheeting or Plexiglas fastened to a frame.

The back of the frame should be 4 to 6 inches higher than the front, creating a downward sloping to allow for runoff and to maximize the amount of light that reaches the plants inside . A southern exposure is best, to allow for maximum sunlight, ideally, the site should get full sun from midmorning to afternoon.

Dig out the top 3 to 4 inches of soil inside the area being framed and replace it with a layer of light coarse gravel to ensure good drainage, followed by about 6-8 inches of topsoil dependent on how tall your frame is going to be. You can also place coldframe plants that are in pots or flats on top of the soil when hardening them off for later transplant into the garden.

Monitoring the temperature is vital to successfully using a coldframe, the temperature should actually be somewhat cooler rather than warm. The temperature inside the coldframe should always stay below 75F for summer plants and 60 degrees F for plants that normally grow in spring and fall.

On excessively cold nights, the cold frame plants will need a little extra protection to prevent freezing. The majority of heat in a properly sealed cold frame escapes via the plastic or glass, piling insulation on top is advisable. Straw, blankets, newspaper, foam board or whatever is handy. Snow will actually insulate also. Be sure to brush excessive or heavy snow off the glass so it doesn’t break.

Vegetables that thrive in, or tolerate cool weather are best suited for a Coldframe. Lettuce, Brussel Sprouts, kale, spinach, collards, broccoli, Swiss chard, snap peas, shell peas, peas, onions such as Vidalia, Spanish sweet onions, cauliflower, mustard, turnips, are all cool weather resistant.

Other plants such as pepper, tomatoes, eggplant, okra , cucumber, pumpkins, melons, berries etc.. you can also use in the coldframe for the acclimation process. That is placing the seedlings , still in their pots or trays in the cold frame for a few hours daily – gradually increasing the the time spent outdoors till planting {transplanting} day arrives.

Cold Frame Tips

  1. Ventilate: Open the lid slightly, or full – dependent of the extent of heat, on warm days to provide air circulation.
  2. Temperature: Place a thermometer inside your cold frame . Open or close the lid to keep the temperature right for the particular plants . A soil thermometer if you’re starting seeds in the coldframe is advisable
  3. Frigid Weather: The cold frames generally only give 5-10 degrees of additional warmth. If temperatures are going to plunge into the 20s F, your plants will be in trouble. If this happens, simply pile some insulation on the cold frame.
  4. Excess Heat: Those 5-10 degrees can also mean trouble when temperatures start heating up. Don’t forget to open the lid of your cold frame on warm days to keep the plants from baking.
  5. Water: Plants in a cold frame will need extra attention to make sure they aren’t getting too much or too little water. Because you’ll have to water them by hand, you’ll soon get a feel for the needs of your plants.
  6. Harden Plants: To acclimate your seedlings to outdoor life, open the lid of your cold frame longer and longer each day, until you’ve finally removed it completely. Keep the lid handy in case of a cold snap!