A cold frame is a bottomless box with a clear or translucent lid. That’s it. The sun heats the air and soil inside; the lid traps that heat. On a clear March day when it’s 35°F outside, the interior of a cold frame reaches 50-60°F. On a January day that might be 25°F outside, the cold frame interior stays at 35-40°F if the sun is out. The difference between those temperatures and outdoor conditions is 4-6 weeks of additional growing season at each end of the year.
The cheapest cold frame in the world is a salvaged storm window set on a brick or lumber frame. It costs nothing if you have the window. It costs $5-15 if you find the lumber at a salvage yard. The most expensive cold frame is a commercial unit at $150-250. Both work. The commercial one looks nicer.
How Cold Frames Work
Passive solar heating is the principle. Glass or polycarbonate transmits shortwave solar radiation into the interior. The soil and plants inside absorb this radiation and re-emit it as longwave infrared, which the glazing traps. This is the greenhouse effect on a small scale, and it works reliably on sunny days even in deep winter.
The limiting factor is nighttime temperature. A cold frame doesn’t generate heat; it only traps daytime solar gain. On a clear, cold night, a cold frame interior drops to within 5-10°F of outdoor temperature. The practical effect is that cold frames provide protection against light frost but not hard freezes. They extend the season of frost-tolerant crops, not frost-tender ones.
Cold frames protect: spinach, arugula, kale, chard, mache, claytonia, Asian greens, radishes, carrots (for storage under cover), overwintering parsley and chives.
Cold frames do not protect: tomatoes, peppers, basil, squash, beans, corn, or any other frost-intolerant crop. If you want to extend the season for warm-season crops, you need floating row cover directly on the plants or a heated structure.
Zone equivalent: a cold frame in zone 6 gives you roughly zone 7 conditions - one USDA zone warmer. This is a commonly cited rule of thumb, not a precise calculation, but it’s directionally accurate. In zone 6, that means you’re operating with roughly the same frost protection as northern Virginia or Missouri rather than central New England.
Build Options and Costs
Option 1: Salvaged window + lumber frame (Cost: $0-40)
The classic cold frame is built around whatever transparent glazing is available - an old storm window, a sliding glass door panel, a polycarbonate greenhouse panel rescued from a job site. The frame can be simple: four 2x6 or 2x8 boards screwed together at the corners, with the back wall taller than the front to create a slope that maximizes sun angle and sheds rain.
Materials for a 4x3 foot frame using salvaged window:
- Salvaged storm window (3x4 ft): free to $15
- Four 2x8 boards, 4-8 ft lengths: $15-25 if not salvaged
- Screws and hardware: $3-5
- Total: $15-40
If you have wood scraps and a storm window, the cost is $0.
Option 2: New materials (Cost: $35-75)
If you’re buying materials new, use rot-resistant lumber (cedar or pressure-treated) for the frame and twin-wall polycarbonate for the lid. Polycarbonate is lighter than glass, doesn’t break if it falls, and transmits light almost as well.
Materials for a 4x4 foot frame:
- Cedar 2x6, 16 linear feet: $25-35
- 4x4 foot twin-wall polycarbonate sheet: $15-25
- Screws and hinges: $5-10
- Total: $45-70
At this size, you have 16 square feet of growing space.
Option 3: Commercial cold frame (Cost: $80-200)
Commercial cold frames range from simple aluminum-framed versions with polycarbonate lids ($80-120) to more substantial cedar units ($150-200). They’re easier to set up than building from scratch, often include venting mechanisms, and look more intentional if aesthetics matter.
For the purpose of payback math: the $40 DIY version and the $150 commercial version do the same horticultural job. The payback period is different.
What You Actually Grow and What It’s Worth
The best cold-frame crops are those that would otherwise be unavailable locally in late winter and early spring: baby salad greens, spinach, arugula, and radishes in March and April.
4x4 cold frame spinach example:
Spinach planted in a cold frame in early September in zone 6 continues to grow slowly through October. When the season shuts down in November, the plants go dormant under the cold frame but don’t die. In March, they resume growth 4-6 weeks before unprotected outdoor spinach would be plantable. A 16 square foot cold frame planted in spinach produces approximately:
- Spring harvest (March-April): 4-5 lb spinach
- Retail value at $4-6/lb: $16-30
- Seed cost for cold-frame spinach: $1.50
Net value from spring cold frame spinach alone: $14.50-28.50. From one season.
A $40 DIY cold frame pays for itself in the first spring harvest. A $150 commercial unit pays for itself in 2-4 seasons.
Fall extension value:
The same cold frame in fall extends the harvest of outdoor greens by 4-6 weeks. In zone 6, unprotected greens are done by mid-October in most years. Under a cold frame, they continue to early-to-mid November, sometimes later depending on the severity of the fall. That extension is worth another 3-4 lb of harvest, another $12-24 in grocery equivalent.
Combined annual value from a single 4x4 cold frame:
- Spring extension: $14-28
- Fall extension: $12-24
- Total: $26-52 per year
On a $40 build cost: payback in one season.
| Cold frame build cost | Annual value (conservative) | Payback period |
|---|---|---|
| $0-15 (fully salvaged) | $26-52 | First season |
| $40 (basic DIY) | $26-52 | First season |
| $75 (quality DIY) | $26-52 | 1-2 seasons |
| $150 (commercial) | $26-52 | 2-4 seasons |
Siting and Construction Details
South-facing is non-negotiable. A cold frame on the north side of a building receives limited direct sun and loses the majority of its thermal benefit. The structure should receive direct sun from mid-morning through mid-afternoon at minimum. A south-facing wall behind the cold frame reflects additional light into the interior and provides a small amount of thermal mass.
Slope the lid toward the south. The back wall (north side) should be 3-6 inches taller than the front wall (south side). This creates a sloped lid that presents a more perpendicular face to winter sun angles (which are low in the sky) and allows rain to run off rather than pooling.
Seal the frame. Cold frames work best when air infiltration is minimized. Caulk gaps between boards; fill large cracks. A loose, drafty frame loses most of its thermal benefit on windy nights.
Anchor it. An empty cold frame lid is essentially a kite. In wind, it will flip, fall, and break. Prop rods, hinges, or weights keep the lid in place on gusty days.
Venting: The Most Important Management Task
A cold frame left closed on a sunny 45°F February day will reach 80-100°F inside within 2-3 hours. Plants that survived a January freeze will wilt and die from heat in an hour. This is the most common cold frame failure mode, and it happens because the owner forgot to open the lid on a warm, sunny morning.
The rule: on any sunny day when outdoor temperatures are above 40°F, open the lid at least a crack. On days above 50°F, open it several inches. On days above 60°F, open it fully.
Automated vent openers (thermostatically controlled pistons that expand with heat and push the lid open) solve this problem. They’re available from garden supply companies for $20-40 per vent. They open the lid when internal temperature rises above the set point (usually 68-75°F) and close it as the temperature drops. If you’re not at home during the day, an automatic vent opener is worth the cost.
The Hotbed Variant
A cold frame with a heat source at the bottom becomes a hotbed - capable of starting warm-season seeds weeks before outdoor planting would be possible.
Electric heating cable: soil heating cable buried 2-4 inches under the growing medium maintains soil at 65-75°F regardless of outdoor temperature. A 48-watt heating cable suitable for a 4x4 frame costs $25-45. Electricity cost: 48 watts for 12 hours/day = 0.576 kWh/day at $0.12-0.16/kWh = $0.07-0.09/day. A 6-week seed starting season costs $3-4 in electricity.
Fresh manure: a 4-6 inch layer of fresh horse, chicken, or rabbit manure under the growing medium generates heat through decomposition - up to 140°F initially, settling to 80-90°F for 4-6 weeks. A traditional hotbed approach that works without electricity. The manure composted in place improves soil fertility after the heat is spent.
A hotbed shifts the economic equation: you’re now starting tomatoes, peppers, and other warm-season crops 6-8 weeks earlier indoors. The transplant-startup advantage compounds through the season in improved yield from longer-seasoned plants.
Comparing to Alternatives
A cold frame is one step in a ladder of season extension tools:
| Tool | Cost | Season extension | Best use |
|---|---|---|---|
| Row cover (spunbond, direct) | $0.05-0.15/sq ft | 2-4°F frost protection | Light frost protection |
| Cold frame (unheated) | $0-200 | 4-8 weeks; 1 zone equiv. | Cool-season crops, overwinter |
| Hotbed (cold frame + heat) | $40-250 | Season-independent for seeding | Warm-season seed starting |
| High tunnel (unheated) | $500-3,000 | 6-10 weeks | Large-scale; multiple beds |
| Heated greenhouse | $2,000+ | Year-round | Full year production |
The cold frame occupies the most cost-effective position in this ladder for home gardeners. It extends the season of the crops that benefit most from season extension (cool-season greens and root vegetables) at the lowest possible cost per square foot per season-week. The payback math at $40 investment and $26-52 annual value is faster than any other season-extension structure available.
Starting with a cold frame before investing in a high tunnel or greenhouse is rational sequencing: you learn the management discipline (venting, monitoring temperature, timing plantings), you discover how much you actually use season-extended growing space, and you verify the economics before committing to a $1,500+ structure. Many gardeners who build cold frames find they cover 80% of their season extension needs from a structure that cost $40 and a weekend afternoon to build. The ones who proceed to high tunnels do so with a clear understanding of the additional growing capacity they need and why - rather than building on speculation. See the arugula and spinach pages for specific variety selection and timing guidance for cold-frame production.