Cold Frames

cold frameMarch 5, 2015 – Kenosha News


Question: I’m so tired of winter and can’t wait to start gardening. I’ve thinking about buying a cold frame to grow lettuce this spring. Do you think it is worth purchasing and what can I grow in a cold frame? E.L.

Answer: A cold frame is a simple box with a lid that provides warmth from the sun and blocks the wind. The sun’s rays enter through a transparent cover creating a greenhouse effect that heats the interior of the cold frame.

The most common use of cold frames is to expand the growing season one to three months.  Many gardeners use cold frames to harden off transplants, but another good use is raising a few salad vegetables.  As well as the lettuce you mentioned, radishes, scallions and other leafy greens will grow to full size in a cold frame before their regular outdoor planting season.  In fall these same crops may be grown in the cold frame through November.

Furthermore, cold frames may be used in winter to force bulbs, store root vegetables, or propagate trees and shrubs by hardwood cuttings.

Permanent cold frames should be sturdy enough to withstand years of sun and weather.  You can purchase a cold frame or make one yourself using recycled materials. Most cold frames are made of wood with a hinged covering.  Salvaged wood and glass windows make a great covering, but they are heavy and breakable.  Alternative covers may be made of plexiglass or double layer of clear plastic. Cold frames built of lightweight materials allow gardeners to move the frame to different sun exposures as seasons and plants change. Lighter weight frames can also be set up on concrete blocks or bricks to add height for tall plants.

Cold frame lids should be hinged for easy opening.  On a sunny day, air in cold frames can get too hot for plants, therefore the lid should be propped open so cool air may enter the frame. Online garden suppliers offer temperature controlled cold frame hinges or ventilators that automatically open and close to vent the frame.

In general, cold frames should be located against a south or east wall near the building foundation to take advantage of its heat.

A cold frame may be made into a heat bed by burying waterproof thermostatically controlled heating cables in a layer of sand two inches beneath the plants. The bottom heat encourages seed germination and root growth in young plants and cuttings.


Barb Larson is horticulture educator for Kenosha County University of Wisconsin Extension. Barb has a Master’s of Science in horticulture from UW-Madison. If you have a plant or gardening question, contact Larson at  or 262-857-1945.