My grandma had a cold frame. It was a long rectangular box angled to catch the sun with a sliding glass shirt, situated along the south facing wall of the garage. At the time I didn’t understand what it was, but now I have assembled one of my own, just outside my kitchen, made of weather-resistant cedar, Plexiglas along with an automatic opener apparatus which levers up the top when temperatures reach about 50 degrees.
In Vermont, it’s too cold and the days are too short in December to think about cold frame propagation, but I could still organize my shopping list as seed catalogs have begun to arrive in my mailbox. Sometime in late January I’ll begin to check the soil to see whether it’s ready for some arugula. If you are gardening in a warmer zone, then you are going to be able to sow seeds a little sooner as weather permits.
Gardening under glass is nothing new. From bio-domes to conservatories to commercial greenhouses, large-scale plant propagation occurs yearlong in heated centers created to maximize the sunlight’s energy. The lucky homeowner with a greenhouse can overwinter tender crops and begin new plants for setting out after frost has passed. But cold frames offer a reasonable alternative to a greenhouse. They’re simple to construct as a DIY job with recycled bricks, lumber and old windows, or cobbled together with other repurposed materials, and are available as kits or even bought from manufacturers and professionally installed.
Cold Frame Components
Here is the golden standard of cold frames (photographed in London at the Chelsea Flower Show). Made of powder-coated aluminum by Alitex, among England’s premier greenhouse manufacturers, it’s designed to last for generations with a mortared brick base.
If you’ve got a greenhouse, a cold frame sited to take advantage of sunlight onto a south-facing wall is excellent for hardening off seedlings prior to transplant. Pots or trays can be placed indoors, with the glass surface supported by metal rods throughout the day and closed at night for protection.
While touring estates and public gardens in England, I had the opportunity to visit Heligan in Cornwall and took this photo before crews transferred the healthy skillet begins into prepared beds in the walled kitchen garden.
This cold frame has a gravel base so that it’s used almost exclusively for transplanting purposes. Propagation is done from the heated greenhouse, where conditions could be kept sterile.
If you spread with a light rack inside, trays full of seedlings could be moved into a cold frame for about a week prior to transplanting when the threat of frost has passed.
Here’s another view of this structure. You can see it has a concrete base and painted wood frame with a custom glass top. The young plants were pulled in the warm comfort of their heated greenhouses from the horticultural team on a mild spring afternoon, to be subjected to many days of cool weather, rain and wind prior to being transferred.
In this case, the cold frame plays a crucial function in the garden scheme for a layover for tender starts.
Notice the slanted top, which permits water to drain off and optimizes solar profit.
This is another type of cold frame, an elegant small conservatory of types, made of metal and glass with a quaint removable lid which allows air to circulate around transplants or new seedlings when weather stays unpredictable — absolutely magical, very British!
Glass cloches provide similar protection — they are big domes that are placed over seedlings at night throughout the spring chill. You can get these in antique markets and Luxurious garden stores — or maybe your grandma has some lying around the tool shed.
If your cold frame will be used for active propagation, bear in mind that lots of edibles dislike transplanting — especially tap-rooted plants like carrots and beets, which are best grown in situ and sown directly into garden beds.
Greens full of antioxidants, vitamins and minerals — like arugula, kale and chard — are great candidates for sowing in batches every few weeks after soil temperatures reach about 50 degrees Fahrenheit (use a soil thermometer to test).
Below are some I have purchased through John Scheepers and Renee’s, which I’m excited to try, especially the arugula with its delectable bite.
Success with growing edibles in a cold frame is dependent upon soil temperature, which is widely variable depending on where you reside. Get arranged in December and begin to quantify your soil temperature in January, testing every two weeks. Seed packs have planting directions, usually saying “sow as soon as soil can be worked” in early spring, meaning that the ground is not frozen.
Don’t have a soil thermometer? Get one at your garden centre or order online.
Chard (Beta vulgaris) is a cool-season annual that is excellent for growing in a cold frame. Follow germination directions on your own seed packet and then soak in warm water temporarily before shaking out from loosely worked soil dressed with mulch. Seeds take approximately ten days to germinate.
One of the super foods full of minerals and antioxidants, chard could be sown successively through the growing season, its leaves delicious chopped in soups or added to omelets. When harvesting, cut on the leaves. Baby chard Collars are sweet and tender.
A Gourmet Greens Braising Mix from Renee’s Garden contains ‘Silverado’ and ‘Eldorado’ chards with two types of beets — all supposed to be thickly sown and harvested when 6 inches tall, then lightly sautéed.
Red and green mesclun is just another easy-to-grow wellness food; pull the seedlings when little and add to early spring salads. Plants could be thinned and then poured everywhere in the garden.
Just like a greenhouse or regular garden bed, a cold frame needs focus. Soil amendment, even hydration, proper ventilation, maintaining plants labeled and re-seeding are part of the fun.
If you’ve got a frame that needs to be propped open and shut in the end of day, be cautious to not forget as young plants left in an unopened cold frame could be killed by excessive warmth onto a warm day.
Back in August or early September, sow more cool-season edibles into your cold frame to expand the growing season, and keep the top propped open. (Close it is only required when freezing temperatures strike.) Who knows, by Thanksgiving you could be cutting infant spinach to get a gourmet salad.
More: How To Grow a Cool Season Vegetable Garden