A cold frame greenhouse is a low, bottomless, enclosed structure with a transparent top that captures sunlight to protect plants from cold, wind, and frost, functioning like a small, unheated, passive greenhouse.
A cold frame is not a heated greenhouse. It has no artificial heat source and typically sits just 5 to 10 degrees above outside temperature. That modest temperature lift, combined with protection from wind and wet weather, makes it one of the most practical tools for extending your growing season without electricity, vents, or complex controls. Gardeners use cold frames for starting seeds earlier in spring, hardening off seedlings before transplanting, and sheltering cold-hardy plants through fall and into early winter.
If a heated greenhouse sounds like more than you need, but you want more control than direct outdoor planting offers, a cold frame is the middle ground. Our tested picks for the best cold frames and greenhouses can help you decide which size and material fits your garden.
How a Cold Frame Works
The transparent lid—glass, plastic, fiberglass, or polycarbonate—lets sunlight enter during the day. That energy warms the soil and air inside. At night, the lid traps some of that heat and blocks cold wind and frost from settling on the plants. The result is a protected microclimate that stays consistently a few degrees warmer than the outside air.
Venting is essential. On sunny days, even in cold weather, the interior can overheat quickly. Open the lid a few inches or prop it with a stick to release hot air, then close it before temperatures drop in the evening. Leaving it sealed on a warm day is the most common mistake and can cook plants in hours.
Common Uses and What It Can Handle
A cold frame reliably handles three jobs. It starts cool-season crops like lettuce, spinach, and kale two to four weeks earlier in spring. It hardens off seedlings you started indoors by giving them a protected transition zone. And it extends fall harvests of cold-tolerant vegetables like broccoli, carrots, and Swiss chard well past the first frost date.
Cold frames protect from frost but not from deep freezes. When temperatures drop into the teens or below, the interior will follow. For severe winter conditions, a cold frame is not a substitute for a heated greenhouse or indoor grow space. Its real strength is making the shoulder seasons productive with zero energy cost.
Design, Placement, and Common Mistakes
Most cold frames use a wood frame with a sloped lid that sheds rain and snow. Typical dimensions are 6 feet by 3 feet, with the back wall around 18 inches high and the front around 14 inches high, giving the lid a south-facing slope. The structure has no bottom—it sits directly on the ground or on a raised bed, so plants root into native soil.
| Material | Pros | Cons |
|---|---|---|
| Glass | Crisp clarity, good heat retention | Heavy, fragile, can break under snow or hail |
| Polycarbonate | Lightweight, durable, diffuses light | Scratches easier, slightly less heat gain |
| Fiberglass | Strong, lets in good light | Can yellow or degrade over years |
| Plastic film | Cheapest, easy to replace | Short lifespan, less insulation |
| Wood frame | Classic look, good insulation | Must be rot-resistant or treated |
| Aluminum frame | Rust-proof, lightweight | Conducts heat away, can get hot to touch |
Place the cold frame facing south to capture maximum winter sun in the Northern Hemisphere. Shelter it from prevailing winds with a fence, hedge, or the side of a building. The ground inside should have good drainage—soggy soil in a cold frame promotes rot faster than open ground because the cover traps moisture.
Untreated wood that contacts soil will rot within a season or two. Use rot-resistant cedar, pressure-treated lumber, or a metal frame if you want the structure to last. And never assume a cold frame is frost-proof just because it has a lid—on nights below about 26°F, add an insulating layer like old blankets or frost fabric over the lid for extra protection.
FAQs
Can I use a cold frame in winter?
Yes, for cold-hardy crops like kale, spinach, and mâche, a cold frame extends harvest into winter. But it is not a heated space—once temperatures drop into the teens, the inside will freeze. Add insulation on the coldest nights.
Do I need to water plants inside a cold frame?
Yes, but less often than open beds because the lid reduces evaporation. Check soil moisture weekly at first, then adjust. On warm days when the lid is open, water evaporates faster, so check more frequently.
How do I stop overheating on sunny days?
Vent the lid as soon as the interior temperature climbs above 75°F, even in midwinter. Prop it open a few inches with a stick or use an automatic vent opener that lifts the lid when it gets warm inside.
References & Sources
- Iowa State University Extension. “Yard and Garden: All About Cold Frames.” Explains cold frame function, placement, and venting practices.
- Royal Horticultural Society. “Cold Frames and Mini Greenhouses.” Covers typical uses, materials, and season extension timing.
- University of Missouri Extension. “Cold Frames.” Details construction, temperature management, and common mistakes.