Cold Frames

Cold frames at Temple University Ambler Campus,
Ambler, Pennsylvania, U.S.
More Photos of these cold frames available here.

What is a Cold frame?
A shelter make specifically for plants in cold weather. It consists of four walls to trap heat and shelter plants, made of any sturdy material — plywood, concrete, even bales of hay and a transparent lid that admits light, typically an old window works perfectly as a lid, but Plexiglas or plastic sheeting tacked to a frame also works just as well.

But why use a cold frame for plants you ask?
Well, it's a great way to start plants earlier in the season (before the last frost of spring) and also a great way to acclimate your seedlings begun indoors to outdoor conditions.
A cold frame can extend you growing season by up to a month! In some climates, you can even grow straight through the winter!

Setting up a cold frame
According to Rodale's All-New Encyclopedia of Organic Gardening the best site to locate a cold frame is a south-facing, sunny spot with good drainage and some protection from the wind. Western facing Ideally, the site should get full sun from mid-morning to mid-afternoon. The glass lid should have at least a 10 percent angle for added sunlight exposure.

Cold frames can be permanent features in your garden where in the summer you can vent by opening the lid, of temporary structures you can put away when you're not using them. It will depend on the style you choose .

Before you set up a cold frame in a permanent spot, dig out the top 3 or 4 inches of soil inside the frame and replace it with a layer of coarse gravel; Then replace 6 inches of topsoil back ontop of the gravel - this is help with drainage! Mixing compost, processed manure, peat moss or other forms of organic humus with your existing soil to create a good fertile soil.

You can grow cold frame plants in pots, flats or right in the soil. Example: if you're growing just one type of plant, like salad greens, plant right in the soil.

Care Tips
Each cold frame will have specific requirements when it comes to the amount of care it needs.
Consider it you're own little experiment.

The watering requirements will vary from day to day and season to season. Generally, during the winter season the cold frame will only need to be watered once a week. Or you can let Mother Nature do the job by opening the top of your cold frame on a rainy day

If the soil is prepared properly, there should be little or no need for feeding (fertilizer) during the winter. The exception may be leaf crops, like lettuce, spinach and chard. A light feeding of an organic type 'Vegetable Garden' fertilizer two or three weeks after planting would be beneficial.

*Beware of slugs! The warmth will attract them and you will need to take appropriate actions to control them

Keys to using a cold frame successfully
You have to pay attention to the temperature!! The trick involved is keeping it COOL rather than warm.
Typically you want the temperature in your cold frame to stay BELOW 75 degree Fahrenheit if you're growing summer plants; and BELOW 60 degrees F for plants that normally grow in spring and fall.

A good rule of thumb is to prop the lid open 6inches when outdoor temperatures are above 40 degrees; when the outdoor temps clear 50 degrees F, remove the lid completely.

You can also purchased automatic venting systems from gardening catalogs. A crawlspace vent can something also work this way and can be purchased at any home improvement store.

On very, very frigid nights, a bit extra insulation may be required; Most heat escapes through the glass, so pile insulation on top. You can use old blankets, straw, newspaper or whatever is handy. And although snow insulates well too, you will want to brush heavy snow off the glass so it doesn't break!

Check out this video featuring many different designs of cold frames. Some are very easily assembled and good for small yards (if you live in a city). Others are large and good for big yards, but still easily put together and movable! At night you want to replace the lid to restore the warm environment for the night.


Information obtained from Organic Gardening and Ed Hume Seeds .