Building and Using a Cold Frame (Plus Some Alternatives)

See previous post about cold frames here.

Six bales of straw make up the walls of this cold frame

 We helped a customer construct a simple, straw bale cold frame a few weeks ago.  As seen above, an enclosure is made around the bed with the bales.  We pushed them together tightly and stuffed straw into any cracks to prevent drafts.  For the top, they covered two old screens from a sliding glass door with plastic.

Sliding screen doors covered with plastic serve as a cover

When an overnight freeze is expected, the tops are put in place a few hours before sundown to gather the sun's warmth.  In the picture above, only one screen is in place.  A second identical plastic covered screen will completely seal off the interior of the cold frame.  Swiss chard is visible growing in the lower right quadrant.

Young kale growing in the cold frame

Kale is quite cold hardy and with this cold frame in place, they should be able to grow it all winter long.  The green grass on the left are sprouts from wheat seed left in the straw bales.

Sweet basil growing next to kale

 Even after getting down to 22 degrees, this volunteer sweet basil plant is doing just fine.  Basil is notoriously intolerant of freezing temperatures, so it is a good sign that the cold frame has been effective.

How to use a cold frame:


Cold frames are not a "set it and forget it" tool.  They require the gardener to monitor the forecasted temperatures and the conditions of the plants inside.  If the temperature during the day is in the mid 60s, the temperature inside could easily climb into the 90s or higher, roasting the plants inside.  If it gets too hot inside, lettuces and other greens will think it's summer time and will bolt (send up flower stalks), which makes them bitter and inedible.  Control the temperature inside by propping open the top or removing it all together on warm days and sealing it up tightly on cold nights.

Don't let me discourage you though.  This time of year, the top stays off most most nights.  As an example, looking at the forecast ahead, our customers won't need to cover their cold frame again until Saturday night.  Today is Monday, so that's a pretty good stretch without having to do anything.  Early in the season, the cold frame is there to get you through the cold snaps here and there and allow your garden to enjoy the great growing weather in between.

Jugs of water inside the cold frame will absorb heat during the day and radiate it back out at night.  This can help keep the cold frame warm as we get farther into fall and winter.  When temperatures dip into the teens and lower, a heat source may be needed at night.  A 100 watt light bulb is simple and effective.  A small, electric heater with a thermostat can also be used.  Please exercise all due caution with electrical devices.

Here's a previous post on cold frames with more information.

Alternative uses of household items:

A cracked aquarium makes a great miniature greenhouse

The lettuce in the aquarium above had been harvested for last night's dinner.  Note that the plants were cut 1-2" off of the ground.  This allows it to regrow again and again without requiring replanting.

A fish bowl, a one gallon pickle jar, and a juice bottle with the bottom cut out

Many things around the house can be used to keep the cold out.  Ask around for old, unused fish aquariums.  If you cannot find any, Wal-Mart typically has small fish aquariums at a low cost and a few of those can keep fresh salads on your table well into the winter.  Treat these like cold frames and remove them on warm days.  Put them in place a few hours before dark when a freeze is expected.

An unused fish bowl keeps the cold at bay for this lettuce

Note the increased growth of the previously covered lettuce

Random thoughts:

• The straw bales will absorb water and begin to break down over the winter.  They should make a nice addition to the compost pile in the spring.
• I am contemplating replacing the straw bales on the south side with some transparent material to allow more sun into the cold frame.  As winter progresses and the sun sinks lower into the southern horizon, that southern wall will cast a longer and longer shadow into the growing area.  Perhaps a sheet of transparent corrugated siding used in green houses?
• Remember that with the top in place, rain fall is excluded.  Water as neccesary.
• The artificially warm environment can attract/house damaging insects, voles, and mice.  Keep an eye out.

UPDATE:  See updated results and experiences with cold frames and glass enclosures here.