by Brandi McNeely, Master Gardener in Training

I’m sipping my coffee on the deck, overlooking my vegetable garden. Everything shows signs of fall. The onions and garlic have been harvested. The green beans are finished for the season and are ready for the compost pile. The zucchini and summer squash are suffering from powdery mildew, and the tomatoes are carrying the last few fruits of the season. Although summer is ending, the fall gardening season is just beginning – and I have a secret weapon: a cold frame.

Why Cold Frames Work

Several leafy green vegetables can survive and even thrive in cooler temperatures. Kale, lettuce, Swiss chard, and spinach produce sugars that lower the freezing point of water in their cells, protecting them from damaging ice crystals. These crops tend to bolt in the heat of summer but thrive in the crisp days of fall. Still, even hardy plants will eventually succumb to our harsh Canadian winters.

That’s where a cold frame comes in – you can significantly extend your growing season with one. Imagine brushing snow off your cold frame in November to harvest fresh kale for a salad!

What Is a Cold Frame?

A cold frame is a low, bottomless structure with a transparent lid that protects plants from wind and cold. The lid lets sunlight in, while the insulated base traps heat and creates a microclimate that can run several degrees warmer than the surrounding air. Ventilation is key – open the lid on sunny days to prevent overheating and ensure proper airflow.

Cold frames come in various styles depending on budget, materials, and space.

Types of Cold Frames

Traditional Cold Frame
Built from wood with an angled, hinged top – this could use glass, polycarbonate, or heavy plastic. They can be permanent or portable and built with reclaimed or new materials. An old window makes a perfect lid.

Straw Bale Cold Frame
 Stack straw bales to form a frame and cover with windows or plastic sheeting. Straw provides insulation and can be reused or composted in spring.

Hot Bed
 A sunken bed filled with compostable material such as horse manure, straw, or poultry bedding, covered with soil and topped with a cold frame. As the material decomposes, it generates heat, warming the soil below.

Need a visual? Watch DIY Cold Frame • Easy How‑to Tutorial with Niki Jabbour to see an easy cold frame build using recycled materials—great for Canadian gardeners. Or for a classic approach, check out How to Build a Cold Frame | This Old House, a clear demo using a salvaged window lid.

Tips for Success

• Timing is everything: Sow spinach, kale, or lettuce in late August or early September to harvest well into late fall – or even December, depending on snow cover.
  
• Ventilation matters: Even on sunny October days, closed frames can overheat. Keep lids open when needed.
  
• Spring bonus: In March or April, use your cold frame to start early crops like lettuce or radishes, or to harden off seedlings before planting.

The Quiet Magic of Cold Frames

No matter which style you choose, a cold frame is a valuable addition to any garden. It helps you garden later into the season and gives you a head start in spring.

With one simple structure, you don’t have to say goodbye to your garden when frost arrives. Picture a crisp November morning—brushing snow from the lid and harvesting fresh spinach for breakfast. That’s the quiet magic a cold frame brings to your backyard.

Banner photo “Large Cold Frame With Props” by Ofer El-Hashahar, CC BY-SA 2.0