For gardeners who crave fresh greens in early spring or wish to keep vegetables growing late into fall, a cold frame is one of the best tools you can add to your backyard. A cold frame is simple, effective, and budget-friendly. It acts like a mini greenhouse, protecting tender plants from frost and extending the gardening season by weeks or even months.
Whether you’re growing lettuce in March, hardening off seedlings in April, or keeping carrots crisp in December, a cold frame provides a microclimate that keeps soil warm and shields plants from harsh weather.
This guide covers everything you need to know about cold frames—from why they’re valuable to how you can build one yourself using basic tools and inexpensive materials.
What Is a Cold Frame?
A cold frame is essentially a low, box-like structure with a transparent lid that traps solar heat. It is often placed directly over soil beds or containers, allowing sunlight to warm the enclosed space while keeping wind and frost out.
Key Features of a Cold Frame:
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Frame: Usually wood, plastic, or even straw bales.
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Cover: Transparent material like glass, acrylic, or polycarbonate panels.
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Size: Small enough to manage easily but large enough to cover multiple plants.
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Placement: Typically positioned with the lid sloping toward the south to maximize sunlight exposure.
Benefits of Using a Cold Frame
1. Extend the Growing Season
The primary reason gardeners build cold frames is to grow plants earlier in spring and later into fall, sometimes even overwintering hardy crops.
2. Protect Plants from Weather Extremes
Cold frames shield plants from frost, wind, and heavy rain while providing a stable microclimate.
3. Hardening Off Seedlings
Seedlings started indoors can be gradually acclimated to outdoor conditions by placing them in a cold frame before transplanting into the garden.
4. Inexpensive and Low Maintenance
Unlike full greenhouses, cold frames are inexpensive to build and require little upkeep.
Best Uses for a Cold Frame
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Cool-Season Crops: Lettuce, spinach, kale, and radishes thrive in cold frames during chilly months.
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Root Vegetables: Carrots, beets, and turnips store well in the soil under the protection of a cold frame.
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Seed Starting: Start vegetables like cabbage and broccoli weeks earlier than outdoor planting dates.
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Overwintering Herbs: Hardy herbs such as parsley and chives continue producing under protection.
Planning Your Cold Frame
Before you start building, consider these factors:
1. Location
Place your cold frame in a sunny location with southern exposure. Avoid shaded spots from trees or buildings.
2. Size
A common size is 3 to 4 feet wide by 6 feet long, but it can be built to fit your space. Width is important because you need to reach across easily without stepping inside.
3. Angle
The lid should slope so that the back of the cold frame is higher than the front. This helps with sunlight penetration and allows rain or snow to slide off.
4. Ventilation
Without proper ventilation, a cold frame can overheat on sunny days. Design it so the lid can be propped open easily.
Materials Needed
You don’t need expensive materials. In fact, many gardeners build cold frames using recycled items.
Common Materials:
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Frame: Pressure-treated lumber, cedar, or even old bricks.
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Cover:
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Old window frames with glass panes (a favorite for DIY gardeners)
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Polycarbonate panels
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Heavy-duty acrylic sheets
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Hinges: To attach the cover to the frame.
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Screws and Nails: For assembly.
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Insulation (optional): Straw bales or rigid foam boards can be used for extreme winter climates.
Tools Required:
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Measuring tape
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Saw
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Drill or screwdriver
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Level
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Safety gloves and goggles
Step-by-Step Instructions: Building a Wooden Cold Frame
Here’s how to build a simple cold frame using wood and an old window:
Step 1: Choose the Size and Dimensions
For most backyard gardeners, a cold frame about 3 feet by 6 feet works well. Keep in mind:
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Height at Back: 12 to 16 inches.
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Height at Front: 8 to 12 inches.
This slope allows optimal sun exposure and drainage.
Step 2: Cut the Wood
Using 1-inch by 6-inch boards (or similar), cut:
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Two side pieces (with angled cuts to create the slope).
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A front and back piece.
Step 3: Assemble the Box
Use screws to attach the sides to the front and back, forming a rectangular box with sloping sides.
Step 4: Attach the Lid
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Use hinges to attach your window or clear panel to the back of the frame.
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Ensure it opens easily for ventilation and plant access.
Step 5: Position Your Cold Frame
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Place the cold frame on level ground with the front facing south.
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For best results, slightly dig the frame into the ground or place it on top of an existing raised bed.
Step 6: Seal Gaps
Seal any small gaps with weather stripping or caulking to prevent cold drafts from entering.
Using Straw Bales as a Quick Cold Frame
If you’re looking for a temporary or inexpensive option, consider using straw bales:
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Arrange four straw bales into a rectangle.
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Place an old window or clear panel across the top.
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This setup provides excellent insulation and can be assembled in minutes.
Managing Temperature in a Cold Frame
While cold frames protect against frost, they can overheat easily when the sun comes out.
Tips for Temperature Control:
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Use a Thermometer: Monitor internal temperatures, especially during sunny days.
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Ventilation: Prop open the lid when temperatures exceed 50–60°F inside.
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Insulation for Winter: Add straw or leaves around the outside for extra warmth in cold climates.
When to Use a Cold Frame
Spring
Start cool-weather crops several weeks before your last frost date. Use the cold frame for seed starting or early transplants.
Summer
Remove the lid entirely and use the cold frame as a raised bed or for heat-loving seedlings that need protection from pests.
Fall
Plant cool-season crops like spinach, arugula, and lettuce in late summer to harvest well into fall.
Winter
In mild climates, you can overwinter hardy crops. In colder climates, use the cold frame to store root crops like carrots and parsnips directly in the soil.
Tips for Success
1. Choose the Right Crops
Cold frames work best for cool-season vegetables and hardy greens. Warm-season crops like tomatoes need more consistent heat than a cold frame can provide in early spring.
2. Rotate Crops
To avoid soil disease, rotate plant types annually within your cold frame.
3. Keep It Clean
Remove dead plant matter and keep the inside tidy to prevent pests and diseases.
4. Monitor Moisture
Cold frames can dry out quickly, especially on sunny days. Water when the top inch of soil feels dry.
5. Upgrade Over Time
As your gardening needs grow, you can add automatic vent openers, improve insulation, or even connect multiple cold frames together.
Common Cold Frame Mistakes (and How to Avoid Them)
Mistake 1: Overheating
Solution: Open the lid for ventilation on sunny days. Install an automatic vent opener if you can’t monitor temperatures regularly.
Mistake 2: Choosing the Wrong Location
Solution: Place your cold frame in full sun, ideally with a southern orientation. Avoid shaded or low-lying areas prone to frost pockets.
Mistake 3: Using Thin or Weak Materials
Solution: Build with durable wood or sturdy recycled materials, especially if you live in a windy area.
Mistake 4: Forgetting to Check Moisture
Solution: Check soil moisture at least every other day. Cold frames can be surprisingly dry even in cool weather.
Estimated Cost to Build a Cold Frame
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DIY Wood & Window Cold Frame: $20–$50 if using reclaimed materials.
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Store-Bought Panels: $50–$100, depending on size and material quality.
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Straw Bale Version: $10–$20 for straw bales plus the cost of a window or cover.
Building your own cold frame is generally more cost-effective than purchasing one pre-made.
Sustainability and Cold Frames
Cold frames are environmentally friendly because they:
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Use passive solar energy (no electricity needed).
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Often repurpose materials like old windows and scrap wood.
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Reduce food waste by extending your harvest season.
Final Thoughts
A cold frame is a gardener’s best friend for season extension, seed starting, and overwintering hardy crops. It’s inexpensive to build, easy to maintain, and makes it possible to enjoy fresh greens and vegetables when your neighbors have already packed away their garden tools.
By following the steps outlined here, you can build your own cold frame using simple tools and recycled materials, giving you a valuable addition to your garden that pays off year after year.
Whether you’re a beginner gardener or an experienced grower, investing a little time in building a cold frame can transform the way you grow food, making your garden more productive and efficient.