How to Build a Simple Cold Frame from Scrap
A cold frame is one of the most useful structures a gardener can add to a yard. It costs almost nothing if you use salvaged materials, requires no power, and extends the growing season by 4--6 weeks on each end. In zone 7a, a cold frame allows spinach and lettuce planting in early March -- 6 weeks.
—- title: "How to Build a Simple Cold Frame from Scrap" slug: how-to-build-cold-frame hub: care category: "Season Extension" description: "A cold frame built from salvaged lumber and an old window extends your growing season by 4—6 weeks on each end. This guide covers dimensions, siting, materials, and ventilation — the most commonly missed step." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-
A cold frame is one of the most useful structures a gardener can add to a yard. It costs almost nothing if you use salvaged materials, requires no power, and extends the growing season by 4—6 weeks on each end. In zone 7a, a cold frame allows spinach and lettuce planting in early March — 6 weeks before the last frost date — and extends fall harvests into December with minimal effort.
The principle is simple: a transparent lid traps solar heat, and insulated walls retain it overnight. The difference between a cold frame and a small greenhouse is that a cold frame is unheated — it relies entirely on captured solar energy and thermal mass.
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What a Cold Frame Can Do
Per Penn State Extension, a cold frame with a glass or double-wall polycarbonate lid typically maintains temperatures 10—15°F warmer than outdoor air at night in most of the mid-Atlantic. In zone 7a (average January minimum 10—15°F), this means:
- Spring: Starting cool-season crops (lettuce, spinach, kale) 4—6 weeks before the last frost
- Fall: Extending harvest of cool-season crops through November—December
- Hardening off: The standard use — acclimatizing indoor-started seedlings to outdoor conditions before transplanting
- Overwintering: Protecting marginally hardy plants (borderline zone hardiness) through winter cold
Per Cornell Cooperative Extension, cold frames cannot protect plants through prolonged periods below 10°F without additional insulation (old blankets or bubble wrap laid on top of the lid on the coldest nights).
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Siting
Per Penn State Extension:
- South-facing slope or wall: Maximum solar gain. A south-facing slope of 10—15 degrees is ideal. A cold frame placed against a south-facing brick or masonry wall benefits from the thermal mass of the wall, which releases stored daytime heat overnight
- Not north-facing: A cold frame on the north side of a structure receives minimal winter sun and provides little temperature benefit
- Wind protection: Prevailing winds reduce thermal efficiency. Positioning near a fence, hedge, or building on the north side reduces heat loss
In my Melville yard, I've used a cold frame against the south-facing brick foundation of the house for starting spinach in late February. The thermal mass of the brick makes a 5—7°F difference on cold nights compared to a freestanding frame.
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Dimensions
Per NC State Extension:
- Width: 3—4 feet — deep enough to use efficiently, narrow enough to reach the center without stepping inside. Most salvaged storm windows are 2—4 feet wide
- Length: Any length. 4, 6, or 8 feet are practical for a single window or two windows side by side
- Back wall height: 12—18 inches
- Front wall height: 8—10 inches
- The sloped top — higher in the back, lower in the front — angles the lid toward the sun for maximum light interception
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Materials
Frame (walls): Per Clemson HGIC, options in rough order of durability and cost:
- Cedar or redwood: Rot-resistant, lasts 10—15+ years, higher cost
- Construction-grade lumber (2x10 or 2x12) treated with water-sealant: Adequate, lower cost
- Concrete blocks: Very durable, excellent thermal mass, zero cost if salvaged. Heavy to move
- Old railroad ties or landscape timbers: Durable, but some older ties contain creosote; not recommended for food production areas
- Recycled wood pallets: Free, but variable quality and often not thick enough for good insulation
Avoid pressure-treated lumber labeled CCA (chromated copper arsenate) in contact with soil where food is grown. Per Penn State Extension, modern ACQ-treated lumber is considered acceptable by most extension sources, but untreated cedar or redwood is preferred for food-production frames.
Lid: Per NC State Extension, options from best to acceptable:
- Old storm window (glass): Free to cheap, heavy, durable, high light transmission
- Double-wall polycarbonate (6mm): Lightweight, excellent insulation (R-value ~1.5), good light transmission, cuts with a circular saw
- Single corrugated polycarbonate: Cheap, light, but scratches easily and yellows over time
- 6-mil greenhouse poly film stretched over a wood frame: Cheapest, least durable (replace every 2—3 years), light transmission good initially
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Construction: Basic Box
Per Penn State Extension, a standard single-window cold frame:
Cut list (for a 4×4 foot frame using a 4-foot salvaged window):
- Two side boards (4 feet long, back taller than front — e.g., back 18 inches, front 10 inches; cut at an angle to create the slope)
- Two horizontal boards: back board 4 feet long × 18 inches; front board 4 feet long × 10 inches
- The side boards are cut to a parallelogram to create the slope
Assembly:
- Cut the side boards to shape (parallelogram or stepped) to create the sloped top
- Assemble with 3-inch exterior screws or galvanized nails at the corners
- For 2×10 or 2×12 lumber, a single board on each side is sufficient; for thinner boards, double up
- Check for square before the glue sets — a racked frame will let wind under the lid
Lid attachment: Per Clemson HGIC, the lid should hinge at the back (easiest) or be completely removable (for harvesting). A simple hinge at the back allows easy propping for ventilation. A screw-eye and hook on each side keeps the lid closed in wind.
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Ventilation — The Most Critical Step
Per Penn State Extension, temperatures inside a closed cold frame on a clear sunny day can exceed 100°F within 1—2 hours of sunrise, even when outdoor temperatures are below freezing. This will kill seedlings.
Ventilation method: Prop the lid open 2—6 inches depending on outdoor temperature. A notched stick or cut-to-length piece of wood works well as a prop. On warm days (outdoor temps above 50°F), open the lid fully.
Thermometer: Place a min/max thermometer inside the frame and check it twice daily. Per NC State Extension, the target temperature range for most cool-season crops inside the frame is 40—75°F. Above 80°F, begin ventilating.
Automatic vent openers: Per Clemson HGIC, solar-powered automatic vent openers (wax-piston type, $20—40) open the lid when the interior reaches a set temperature and close as it cools. These are valuable if the frame is used when you're away during the day. The hydraulic action of wax expansion is reliable and requires no electricity.
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Watering
Per Penn State Extension, plants inside a cold frame receive no rainfall and can dry out faster than outdoor plants due to heat buildup. Check soil moisture every 2—3 days; water when the top inch is dry. Overwatering in a cold frame creates conditions favorable for damping off.
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Common Cold Frame Problems
| Problem | Cause | Fix |
|---|---|---|
| Seedlings collapse and die | Overheating from no ventilation | Install thermometer; vent daily on sunny days |
| Frost damage inside frame | Lid not sealed; extreme cold snap | Seal gaps; add insulating blanket on top of lid on coldest nights |
| Condensation and disease | Poor drainage; overwatering | Improve soil drainage; water in morning; vent to reduce humidity |
| Lid blows off in wind | No latching mechanism | Add screw-eye hooks on both sides |
| Weeds proliferate inside | Soil brought in from garden | Use weed-free potting mix or solarized soil for frame bed |
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FAQ
How much does it cost to build a cold frame from scratch? Very little if you scavenge materials. Per Penn State Extension, a frame using salvaged lumber and a found storm window can cost under $10. A new frame with cedar boards and a polycarbonate lid costs $50—100 in materials.
Do I need to anchor the cold frame to the ground? In exposed locations, yes. Per Clemson HGIC, wind can flip an unanchored frame. Drive 12-inch stakes through the inside corners into the soil, or place concrete blocks on the inside of the perimeter boards.
Can I overwinter perennials in a cold frame? Yes, for marginally hardy plants. Per NC State Extension, a cold frame provides roughly 1.5 USDA zone of protection — meaning zone 7 plants have a reasonable chance of surviving zone 5 winters inside a well-sited cold frame. Add additional insulation on the lid during the coldest nights.
Is a hot bed different from a cold frame? Yes. A hot bed is a cold frame with a heat source — traditionally composting manure beneath the soil, now typically electric soil heating cables. Per Penn State Extension, hot beds maintain temperatures 20—30°F above ambient and allow seed starting much earlier than cold frames. They require a power source or manure and more maintenance.
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Recommended gear: Best lettuce varieties for heat tolerance and bolt resistance — our buyer's guide covering picks for every budget, ranked by Extension publication consensus and personal use.
Sources
- Penn State Extension — <a href="https://extension.psu.edu/cold-frames">Cold Frames and Hot Beds</a>
- Cornell Cooperative Extension — <a href="https://cce.cornell.edu">Season Extension</a>
- NC State Extension — <a href="https://plants.ces.ncsu.edu">Cold Frame Construction</a>
- Clemson HGIC — <a href="https://hgic.clemson.edu/factsheet/cold-frames/">Cold Frames</a>