Extend Your Growing Season With a Cold Frame in the Garden

My grandmother had a cold frame — a long rectangular box angled to catch sun with a sliding glass top against the garage's south wall. I didn't understand it then, but now I've built my own outside the kitchen: weather-resistant cedar, Plexiglas, and an automatic opener that lifts the top at ~50°F.

In Vermont, December is too cold for cold-frame propagation, but I organize seed orders as catalogs arrive. By late January, I'll test soil for arugula readiness. Warmer zones can sow earlier.

Gardening under glass isn't new — from biodomes to commercial greenhouses, heated facilities maximize solar energy year-round. A greenhouse lets you overwinter tender plants and start new ones for post-frost transplanting. But cold frames offer an affordable alternative: easy DIY with recycled bricks, lumber, old windows, or as kits/professionally installed.

Cold Frame Components

The gold standard (photographed at Chelsea Flower Show): powder-coated aluminum by Alitex, mortared brick base, built for generations. Ideal for hardening off seedlings before transplant if you have a greenhouse. Pots/trays inside, glass top propped by day, closed at night.

At Heligan in Cornwall, a gravel-based cold frame serves exclusively for transplanting; propagation happens in the sterile heated greenhouse. If you propagate indoors under lights, move seedling trays to a cold frame for ~a week before transplanting post-frost.

Another view shows a concrete base, painted wood frame, custom glass top. Young plants from heated greenhouses spend several days in cool weather, wind, and rain — a crucial layover for tender starts. Note the slanted top for drainage and optimal solar gain.

An elegant glass-and-metal conservatory type with removable lid for air circulation — charming, very British! Glass cloches (large domes over seedlings at night during spring chill) offer similar protection; find them at antique markets, upscale garden shops, or maybe your grandmother's tool shed.

Growing Edibles

Many edibles dislike transplanting — especially tap-rooted carrots and beets, best sown in situ. But antioxidant-rich greens like arugula, kale, and chard are excellent for sowing in batches every few weeks once soil hits ~50°F (use a soil thermometer).

I've ordered from John Scheepers and Renee's — excited to try arugula's delectable bite. Success depends on soil temperature, highly variable by location. Get organized in December, measure soil temp in January, test biweekly. Seed packets say "sow as early as soil can be worked" — ground not frozen.

Chard (Beta vulgaris) — cool-season annual ideal for cold frames. Follow packet directions, soak seeds briefly, sow in loose, compost-dressed soil. Germinates in ~10 days. Harvest outer leaves first; baby leaves are tender and sweet. A Gourmet Greens Braising Mix (Renee's) combines 'Silverado' and 'Eldorado' chards with two beets — thickly sown, harvested at 6 inches, lightly sautéed.

Red and green mesclun — easy health food; pull seedlings small for early salads, thin and transplant elsewhere.

Gardening Tips

Like any garden, cold frames need attention: soil amendment, even hydration, ventilation, labeling, re-seeding. If manually propped, don't forget to close at day's end — unopened frames can cook young plants on warm days.

In August/early September, sow more cool-season edibles to extend the season, keep top propped (close only for freezing temps). By Thanksgiving, you could be cutting baby spinach for a gourmet salad.

More: How to Grow a Cool Season Vegetable Garden