Used to protect tender plants or rooted cuttings during the colder months, a cold frame is simply a box with a transparent lid or cover. It acts as a passive solar energy collector and reservoir.
the day, the sun's rays heat the air and soil in the frame; at night,
the heat absorbed by the soil radiates out, keeping the plants warm.
cold frame is useful at other times of year as well. In spring, it
provides an ideal environment for hardening off annual flower and
vegetable seedlings started indoors. Seeds of many plants can be sown
directly in the frame and grown there until it's time to transplant them
to the garden. In summer, you can replace the cover with shade cloth or
lath, creating a nursery for cuttings.
Set up your cold frame in a
site protected from harsh winds by trees, shrubs, a fence, or a wall.
To ensure that the frame will receive as much sunlight as possible,
orient it to face south or southwest.
Sinking the frame 8 to 10
inches into the ground increases heat retention significantly. Make sure
the location has good drainage, since you don't want water to collect
around the frame after every rain.
Building a Cold Frame
Start by selecting a cover, since its size will often determine the
dimensions of the frame. Good choices include an old window sash or
storm window; if you don't have one on hand, look for recycled windows
at garage sales. You can also make a cover out of clear acrylic or
fiberglass sheets sandwiched between narrow strips of wood and
reinforced at the corners with metal corner plates. Polyethylene film
stapled to a wooden frame is another option; it's quick and inexpensive,
though it lasts only a year or so. Make sure the cover isn't too heavy
to lift easily. Don't make it too wide, either, or you'll have a hard
time reaching the plants inside the frame; a width of 2-1/2 to 3 feet is
ideal. A length of at least 4 feet will allow you to grow a variety of
Build the frame from lumber, such as rot-resistant redwood
or cedar or less expensive plywood or scrap lumber. The frame should
slope from about 1-1/2 feet high at the back to a foot high at the
front; this traps the most heat and lets rainwater run off. For
strength, reinforce the corners of the box with vertical posts. Attach
the cover with galvanized steel hinges and apply weather stripping
around the top edges of the box.
is vital to prevent overheating. A minimum-maximum thermometer is useful
for keeping track of temperature fluctuations. Plan to prop open the
cover when the temperature inside reaches 70 degrees to 75 degrees F/21
degrees to 24 degrees C. Close the cover in late afternoon to trap heat.
(If you won't be around during the day, you can buy a nonelectric vent
controller that will automatically open and close the cover at a preset
temperature.) On very cold nights, drape the frame with an old blanket
or piece of carpet to provide extra insulation.