What a Cold Frame Does (and Why Every Gardener Needs One)
A cold frame is the simplest season-extending tool you can build. It is a bottomless box with a transparent lid, angled to catch the sun. Inside, temperatures run 10–30°F warmer than the outside air. That margin is enough to grow salad greens through a zone 6 winter, start seedlings six weeks early in spring, and harden off transplants without babysitting them.
We built our first cold frame eight years ago from scrap cedar and a $5 window from Habitat for Humanity ReStore. It is still in service. Total build time was two hours. It has earned back its cost many times over in early lettuce, spinach, and arugula that we would have otherwise bought at the grocery store at $4 per bag.
Materials: Old Window vs Polycarbonate Panel
Salvaged Window
The cheapest and most sustainable option. Any single- or double-pane window works. Check ReStore, Craigslist, Facebook Marketplace, or your own garage. An ideal window is 30–36 inches wide and 24–36 inches deep — that gives you about 6 square feet of growing space. Avoid windows with cracked glass (obvious) or rotting frames (they will disintegrate in one season). If the window has lead paint (pre-1978 homes), skip it or strip and repaint with exterior latex.
Polycarbonate Panel
Twin-wall polycarbonate (the stuff greenhouses are made from) is lighter, nearly unbreakable, and insulates better than single-pane glass. A 24×48-inch panel costs $15–$25 at Home Depot or Lowe's. You will need to build a simple frame around it from 1×2 lumber. The insulation value (R-1.5 vs R-0.9 for single glass) means 5–8°F warmer interior on cold nights — a meaningful difference in zone 5 and below.
Which to Choose
If you have a window, use it. Free beats everything. If you are buying, go with polycarbonate — it lasts longer, weighs less, and performs better in cold climates.
Materials List
This plan fits a standard 32×24-inch window. Adjust dimensions for your window or panel.
Lumber
- 2 pieces 1×12 cedar or white oak, 32 inches long (front and back)
- 2 pieces 1×12 cedar or white oak, 24 inches long (sides)
- 1 piece 1×6 cedar or white oak, 32 inches long (back riser — creates the angle)
- 4 corner blocks: 2×2 × 12 inches (interior corner reinforcement)
Hardware
- 16–20 exterior wood screws, #8 × 2 inches (stainless steel or ceramic coated)
- 2 hinges, 3-inch strap or T-style
- 1 chain, hook, or notched stick (for propping the lid open)
- Optional: automatic vent opener (thermatic piston type, $25–$35 — more on this below)
Glazing
- 1 salvaged window (sized to fit) OR
- 1 sheet twin-wall polycarbonate, cut to 32×24 inches, with a 1×2 frame
Angle Calculation: Match Your Latitude
The lid should be angled so the glazing faces the winter sun as directly as possible. The ideal angle equals your latitude minus about 15 degrees (because you are optimizing for the low winter sun, not the summer sun). Here is a quick reference:
| Your Latitude | Region Examples | Ideal Lid Angle | Back Riser Height |
|---|---|---|---|
| 30°N | Houston, Jacksonville | 15° | 4 inches |
| 35°N | Raleigh, Memphis, Albuquerque | 20° | 5.5 inches |
| 40°N | Philadelphia, Denver, Indianapolis | 25° | 6.5 inches |
| 45°N | Minneapolis, Portland OR, Montreal | 30° | 8 inches |
The back riser height is the extra piece of lumber attached to the back wall that creates the slope. For most gardeners in zones 5–7, a 6-inch riser works well. Don't overthink this — even a few degrees off optimal makes very little practical difference. The sun is forgiving.
Step-by-Step Build Instructions
Step 1: Cut the Side Pieces at an Angle
Each side piece needs to be tapered so it is taller at the back and shorter at the front. The front height is 12 inches (the full 1×12 board width). The back height is 12 inches plus your riser height. If your riser is 6 inches, the back of the side piece is 18 inches tall.
Mark the front height (12 inches) and back height (18 inches) on each side board and draw a line between them. Cut along that line with a circular saw or jig saw. You now have two trapezoidal side pieces.
Step 2: Assemble the Box
Stand the front board (12 inches tall, 32 inches wide) on edge. Butt one side piece against it, flush at the bottom. Pre-drill and drive two screws through the side into the front board's end grain. Repeat for the other side.
Now attach the back board. The back is the 1×12 (12 inches) plus the 1×6 riser stacked on top. Screw the 1×6 to the top of the 1×12 first, then attach the combined back piece to both sides. You should now have a four-sided box, taller at the back and shorter at the front.
Screw the four corner blocks into the interior corners for reinforcement. These prevent the sides from bowing outward under soil pressure if you are using the cold frame directly on a bed.
Step 3: Attach the Lid
Lay the window or polycarbonate panel on top. The hinges go on the back (tall) edge. Screw the hinge leaves into the back riser and the window frame. The lid should open fully and rest against something — a wall, a stake, or a prop chain.
Step 4: Add a Prop Stick or Vent Chain
You need a way to hold the lid open at various heights for ventilation. The simplest method is a notched stick — a 1×2 with notches cut at 2-inch, 4-inch, and 6-inch heights. Wedge it under the lid at the desired opening.
The better method is an automatic vent opener. These use a wax-filled piston that expands when the interior temperature reaches a set point (usually 55–60°F) and pushes the lid open. When it cools, the piston contracts and the lid closes on its own. We use the Univent brand — about $30, no electricity required, and ours has lasted five seasons with zero maintenance.
Venting: The Number One Mistake New Cold Frame Growers Make
On a sunny day in March, the interior of a closed cold frame can exceed 100°F by noon — even when the outside air is 40°F. That kind of heat shock will kill cool-season crops in hours. Lettuce bolts. Spinach wilts. Seedlings cook.
Here are the temperature rules we follow:
- Below 40°F outside: Keep lid closed. The frame is doing its job keeping plants above freezing.
- 40–50°F outside, sunny: Prop the lid open 2–3 inches. This vents excess heat while keeping the wind off your crops.
- 50–60°F outside: Open the lid halfway or fully. Your plants do not need the cold frame on days this warm — they just need the protection overnight.
- Above 60°F: Remove the lid or prop it wide open. The growing season has arrived.
If you can't be home to vent manually, the automatic vent opener is worth every penny. Set it and forget it. Our cold frames vent themselves from March through May while we are at work.
What to Grow in a Cold Frame
Cold frames are best for cold-hardy crops that tolerate light frost. Here is what we grow in ours, by season:
Late Fall / Winter (November–February, Zone 6–7)
- Spinach (Bloomsdale Long Standing, Space)
- Mâche (Vit)
- Claytonia (Miner's Lettuce)
- Winter lettuce (Winter Density, North Pole)
- Arugula (Astro, Esmee)
Early Spring (February–April)
- Lettuce starts — sow inside the frame and transplant out when weather warms
- Radishes (Cherry Belle, French Breakfast)
- Peas (Sugar Snap — start them in the cold frame 4 weeks before last frost)
- Brassica transplants — start broccoli, kale, and cabbage seeds in the frame in February
Fall (September–November)
- Second planting of spinach, lettuce, and arugula
- Cover cropping — sow crimson clover or winter rye in the frame as a seed nursery
For a full year-round growing schedule, read our guide on growing salad greens year-round. And if you are timing your plantings around frost dates, our spring planting guide by USDA zone has the dates you need.
Placement Tips
- Face the glazing south (in the Northern Hemisphere). This maximizes solar gain.
- Place the frame against a south-facing wall or fence if possible. The wall absorbs heat during the day and radiates it back at night.
- Avoid low spots where cold air pools. Cold air flows downhill like water — the bottom of a slope is the worst location.
- Make sure the lid does not open into a walkway where someone will hit their head on it (speaking from experience).
Building the cold frame is the easy part. If you need the raised bed to go with it, our pre-cut raised bed kits are sized to work as a cold frame base — just add the lid.
Frequently Asked Questions
How cold is too cold for a cold frame?
A single-glazed cold frame protects down to about 20°F outside temperature (keeping the interior above freezing). A double-glazed polycarbonate frame can handle 15°F. Below that, add a row cover or blanket over the frame at night for an extra 5–10°F of protection. In zone 5 and colder, you may need to combine the cold frame with a layer of straw mulch around the plants for deep winter growing.
Can I use a cold frame on top of a raised bed?
Yes, and it is the best way to use one. Set the frame directly on top of a raised bed (or build it to the same dimensions). The raised bed soil absorbs heat during the day and releases it at night, creating a thermal mass effect that keeps temperatures more stable than a frame sitting on bare ground.
How long does it take to build a cold frame?
About 2 hours for a first-timer with basic tools — drill, saw, tape measure, and a square. If you have the materials prepped, you can assemble one in under an hour. It is the easiest and most rewarding DIY garden project you can build.
Do I need to water plants inside a cold frame?
Yes. Rain cannot reach plants under the closed lid. Check soil moisture weekly in winter and every few days in spring as plants grow faster. The enclosed environment reduces evaporation, so you water less often than outdoor beds, but you still need to water.
Old window vs polycarbonate — which lasts longer?
Polycarbonate wins on longevity (15–20 years vs variable lifespan for a salvaged window). But a salvaged window costs nothing and works perfectly. Our oldest cold frame uses a window from a 1990s house renovation and it is still going strong after eight years. Use what you have.
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