Larry Hodgson has published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings available to the public. This text was originally published in Gardens Central magazine, in November 2010.
There used to be a time when you could scarcely find a house that didn’t have a cold frame leaning against it, but by the 1970s, they were a thing of the past, a relic of the Victorian garden. Well, they say everything old is new again and a new generation of gardeners is discovering the advantages of this homemade season extender.
A Bit of History
From the mid 1800s to the 1940s, cold frames were a necessity for any gardener. After all, if you wanted to grow tomatoes, peppers, petunias or other plants that need a long growing season, you had to start your own. Seed catalogues were widely available, but local nurseries essentially offered only flowers, shrubs, trees and fruit trees.
After the Second World War, though, new types of stores, called garden centers, sprang up rapidly throughout the country, offering a range of garden products, including trays of annuals and vegetables.
Suddenly you didn’t need to start your own. You could simply buy them! Gardeners quickly learned to buy pre-started plants and yanked out the now empty cold frames.
Fast Forward to Today
Tired of growing the very limited vegetable and annual choices offered by garden centers and worried about chemical treatments greenhouse-grown plants may have undergone, gardeners are rediscovering the pleasures and advantages of starting their own plants from seed.
But they also realize there are only so many seed trays you can put in front of the average south-facing window. Suddenly, the cold frame, with all that extra space, begins to look good again.
Building a Cold Frame
The traditional cold frame is a fairly low, wooden-sided, bottomless structure with a glass roof.
Simply build a wooden frame that fits an old storm window, making the back of the frame higher than the front so the top is angled toward the sun, letting more light in to heat it while allowing rain water to drain off.
A couple of hinges fixing the window to the back of the frame will make it easier to open. You’ll also need a stake to prop the frame open for aeration and access. In most cases, you’ll find all these materials gratis in a basement, garage or attic. Even a cold frame built of new material rarely costs more than $25.
Not handy? Take a look on the Internet—there are plenty of easy-to-follow plans for making your own cold frame.
Traditionally the cold frame is placed up against the house, facing south. The house blocks the cold north wind, helping to maintain warmer temperatures.
You can also place the frame in the garden, insulating the north side with a mound of soil or styrofoam insulation. Of course, there are many variants on the cold frame theme. You can use plastic panelling or staple plastic film over a frame instead of using an old window. You can use water-resistant wood, cinder blocks or even bales of hay as walls.
And you can extend your cold frame to any length by adding more window frames. Many gardeners are finding a PVC pipe structure covered in plastic sheeting makes a great inexpensive cold frame.
Using Your Cold Frame
Traditionally the cold frame is not used to start seedlings or cuttings. That’s done indoors, in front of a warm window. That’s because most seedlings sprout best (and cuttings root best) under consistently high temperatures (21–24°C or 70–75°F), which are difficult to maintain in an unheated frame in early spring. However, once they’re growing, seedlings and cuttings actually do better with cool nights and warm days. This is where the cold frame comes in.
Move your plants into the cold frame once temperatures stay above 10°C (50°F) at night (many seedlings will take even less). The cool sunny conditions will produce dense dark green, sturdy plants—just like those you buy at the garden nursery, not those pale weaklings produced on a window sill.
Care of plants in the cold frame is simple. Just water them before they become drought-stressed (that can be more than once a week!) and fertilize weekly.
On sunny days, prop the window open to let excess heat out. On cold days, keep it shut. Watch your plants so they don’t overheat on sunny days. The opening and closing also harden off the plants so they’ll be ready to plant out as soon as there’s no longer danger of frost.
You can use your cold frame for much more than just the growing of vegetables and annuals.
Here are a few suggestions.
As soon as the snow melts, sow early vegetables such as lettuce, spinach and radishes in the frame. They’ll be ready to harvest before you need the frame for indoor-started seedlings.
In summer, use the frame to grow heat-loving vegetables such as melons and eggplants (Zones 5-8 USDA). In Zones 1-4 USDA, tomatoes and peppers may also need the “hot house” conditions of a cold frame to grow well. Make sure you open the window when the plants bloom so the bees can pollinate them.
In late August, sow a fall crop of lettuce, spinach and others in the frame—there’ll be plenty of time for a third harvest, as the frame will remain above freezing well into December.
In Zone 7 USDA and above, temperatures in your frame will probably never drop to freezing, so use it to grow cabbages and lettuce right through the winter. And you can force pots of bulbs (tulips, crocuses and daffodils) or overwinter half-hardy plants like New Zealand flax (Phormium spp.), Nile lily (Agapan- thus spp.) or cabbage palm (Cordyline australis) in it.
Most perennial seeds need prolonged but moderate cold before they will germinate, so sow them in seed trays in the fall, then place them in the cold frame for the winter. They’ll germinate all on their own in the spring.
And those are just a few examples. It’s really amazing how useful your cold frame will be!
Heating Things Up!
A hot frame is simply a heated cold frame. In the old days, gardeners dug a pit in the ground, filled it with fresh horse manure and covered it with 5 cm (2″) of sand, replacing the frame overtop.
The manure gave off heat, thus delivering frost-free conditions all winter. These days, you can place heating cables on the ground, covering them with 5 cm (2″) of sand. Hot frames can be used to start and grow seedlings and cuttings and will be warm enough to overwinter just about any plant. They’re essentially mini greenhouses.