Winter Greenhouse Guide: Heating, Insulation and What to Grow

We may earn a commission if you buy through links on this page, at no extra cost to you. Picks are based on specs, retailer data and owner reports, not paid placement. Disclosure.

Most "greenhouse in winter" advice is either a one-line tip or a promise of tomatoes in January. The reality sits between: a backyard greenhouse can feed you greens all winter for little or no heating cost, and it can keep tender plants alive if you size the heat properly. This guide covers what to grow, how to heat a greenhouse in winter (with the sizing math), insulation, thermal mass, ventilation, snow and safety. It draws on University of Alaska Fairbanks (UAF), Penn State and South Dakota State extension publications, the RHS and manufacturer manuals. How we research.

Unheated vs heated winter greenhouse

The RHS draws the same line: an unheated greenhouse can shelter half-hardy (H3) plants, while heating to frost-free, 4°C (39°F), lets you grow tender (H2) plants all winter. UAF's heating tables assume 60°F inside at night, which is a commercial growing temperature, not what most backyard growers need.

What to grow in a greenhouse in winter

South Dakota State University Extension groups vegetables by how much frost they take:

A bonus from SDSU: root crops get sweeter when the soil is below 40°F. Kentucky State University lists lettuce, spinach, kale, radishes and carrots as its cool-season fall and winter high-tunnel crops.

Light is the real limit: the Persephone period

Maine grower Eliot Coleman coined the "Persephone period" for the weeks when day length is under 10 hours and plant growth practically stops. The dates depend on your latitude; in East Tennessee it runs roughly 26 November to 16 January, and further north it is longer. Penn State Extension says that until about mid-February to early March there is too little sunlight for crop growth, however warm the structure is.

The practical rule: plant winter crops in late summer and early fall so they're full-size before the Persephone period, then harvest from them through winter. A heated greenhouse won't make December seedlings grow fast; it just keeps them alive.

Tunnel within a tunnel

Penn State says high-tunnel growers keep spinach and greens producing through winter with inner floating row covers or low tunnels, which trap heat in the crop canopy. In a backyard greenhouse, that means hoops of wire or conduit over the beds with frost cloth, put on in the evening and pulled back on sunny days. A cold frame inside the greenhouse does the same job for a tray of seedlings.

How to heat a greenhouse in winter: heater sizing

UAF Extension gives the formula (credited to Kramer, 2013), which it calls more accurate than per-floor-area shortcuts:

Heat loss (Btu/h) = surface area of the covering (sq ft) × U-value × temperature difference (°F)

Surface area means walls plus roof, not the floor. The U-value is the covering's heat-loss factor. UAF's figures (from NRAES-137):

Palram's own data for its SUNLITE sheet converts (our arithmetic) to about 0.67 for 4 mm twin-wall, 0.62 for 6 mm, 0.58 for 8 mm and 0.51 for 10 mm, so thicker panels help, but going from single to double glazing is the big step.

Worked example: an 8x10 greenhouse

1. Surface area. An 8x10 with 5 ft sidewalls and a 7.5 ft peak: sides 100 sq ft + ends 80 + gable triangles 20 + roof about 94 = about 294 sq ft.
2. Temperature gap. You want 40°F inside; your coldest typical night is 0°F. TD = 40°F.
3. Heat loss, twin-wall. 294 × 0.6 × 40 = 7,056 Btu/h.
4. Convert to watts. 1 kWh = 3,412 Btu, so 7,056 ÷ 3.412 = about 2,070 W. A 1,500 W heater (about 5,118 Btu/h) falls short on that night; a 3,000 W, 240 V heater (about 10,236 Btu/h) covers it with margin.
5. Same house, single glazing. 294 × 1.2 × 40 = 14,112 Btu/h, about 4,140 W: double the heater and double the bill.
6. Warm house check. Holding 60°F instead (TD 60): 294 × 0.6 × 60 = 10,584 Btu/h. UAF's per-floor-area shortcut for a freestanding double-glazed house at 0°F outside, 60°F inside, is 150 Btu per sq ft, or 12,000 Btu/h for 80 sq ft; UAF notes the shortcut tends to overestimate.

These are our calculations using UAF's method; your numbers will differ with the shape of your greenhouse and your local design low. UAF advises that a slightly oversized heater is always better than an undersized one, and that a backup heater is wise for cold snaps or if the main one fails. Size on the heater's output rating. For running costs per night with EIA electricity prices, see how much a greenhouse costs; for heater picks by size, see the best greenhouse heaters.

For a bigger load, the Dr. Infrared DR-218 3000 W (240 V, IPX4) gives about 10,236 Btu/h but needs a NEMA 6-20 receptacle. If you add an external thermostat to a 1,500 W heater, note that the popular Inkbird ITC-308 is limited to 10 A (about 1,100-1,200 W); the bayite BTC211 is rated 1,650 W / 15 A.

Insulating a greenhouse for winter

Every unit of heat loss you prevent is heater capacity you don't have to buy and power you don't pay for. In order of payoff:

1. Seal the gaps. Fit door seals, tape panel gaps and close vents fully at night. Drafts can matter as much as the glazing.
2. Add horticultural bubble insulation. Fix it to the inside of the glazing, especially the roof and north wall. RHS notes a plastic layer cuts out about 10% of light, usually a worthwhile trade, except for crops that need maximum light such as winter lettuce.
3. Insulate the north wall solidly. If little sun comes through it, a foam board or a reflective panel loses less heat than glazing.
4. Insulate the perimeter. UAF halves the perimeter's heat-loss factor (0.8 to 0.4) with 2 in foam board 24 in deep. See our greenhouse foundation guide.
5. Use row covers at night. The tunnel-within-a-tunnel approach keeps the crop warm without heating all the air.

Thermal mass: water barrels and more

Thermal mass soaks up daytime sun and gives it back at night. UAF's table shows a 55-gallon water barrel stores 4,590 Btu when it cools from 60°F to the greenhouse minimum of 50°F, and 18,340 Btu from 90°F. Concrete stores 224-896 Btu per cubic foot and brick 271-813 over the same range. A well-built solar greenhouse can capture enough on a sunny March day to heat it for two more days (UAF, citing Bartok).

Put that in context with our 8x10 example. On a typical night losing about 3,500 Btu/h, one barrel cooling by 10°F covers a bit over an hour (our arithmetic). Water barrels smooth out temperature swings and help on marginal nights; they don't replace a heater on a hard freeze. Paint them dark, place them where winter sun hits, and keep them under benches so they don't steal growing space. UAF also says solar greenhouses should face within 20° of true south with the ridge running east-west.

Some four-season designs build this in: the Growing Spaces 15 ft dome includes an above-ground pond as thermal mass. More in our geodesic dome greenhouse guide.

Ventilation and humidity in winter

Closed-up greenhouses in winter swing hot on sunny days and stay damp at night, which invites gray mold and mildew. NGMA's guidance is about 1.5-2 CFM of ventilation per sq ft of floor in winter, which holds the house within 15-20°F of the outside under average conditions; for an 8x10, that's 120-160 CFM (our arithmetic). Practical steps:

• Leave automatic vent openers in place; set them to open on sunny days.
• Run a small circulation fan; UConn IPM sizes horizontal airflow at about 2 CFM per sq ft of floor.
• Water in the morning so foliage dries before night.
• Watch humidity with a sensor, not guesswork.

No WiFi in the garden? The TempPro TP65B shows the greenhouse reading on an indoor display up to 500 ft away, with max/min records. See greenhouse fans and vent openers for exhaust options.

Snow load: keeping the roof up

Snow is how many winter greenhouses fail. Check the published snow rating of your kit and clear the roof before the load reaches it:

• Mainstream Palram kits (Mythos, Hybrid, Balance) are rated about 15 lb/sq ft; the Palram Glory 8x12 lists 20.
• Heavy-duty kits go higher: the Riverstone MONT is rated 24 lb/sq ft and the Janco Palmetto 50 lb/sq ft.
• Shape helps: steep or arched roofs such as the Exaco RIGA and the bell-shaped Palram Bella shed snow better than shallow gables.
• Brush snow off from outside with a soft broom, working down the slope; never knock it off with a hard tool that can crack panels. Gently tapping the roof from inside helps loosen a crust.
• Heated greenhouses shed snow better, because the warm glazing melts the bottom layer.

In real snow country, start with our picks in polycarbonate greenhouse kits and the heavy-duty options in the 8x10 greenhouse guide.

What makes a 4 season greenhouse?

• Insulating glazing: twin-wall polycarbonate at minimum; 8-10 mm (Riverstone MONT 8 mm, Palram Glory 10 mm) or 16 mm triple-wall (Exaco RIGA XL) for cold climates.
• A real snow rating and a roof shape that sheds snow.
• An insulated, anchored base with perimeter foam.
• Power for a heater, fans and heat mats, run by an electrician with GFCI protection.
• Thermal mass and a south-facing orientation.
• Automatic ventilation for sunny winter days and summer heat.

For seed starting in a cool greenhouse, a heat mat is far cheaper than heating the air: Bootstrap Farmer recommends soil at 65-80°F for germination, and Hydrofarm says its Jump Start mats warm the root zone 10-20°F above the air. A Jump Start 17 W single-flat mat is enough for one tray.

Propane and gas heaters: CO and ethylene safety

Propane heaters are popular because greenhouses rarely have power, but they come with rules:

• Vent the greenhouse. The Mr. Heater Big Buddy manual says to use it only in an enclosure with a window or roof vent, with at least 18 sq in of open vent area (for example a 4¼ x 4¼ in opening), and not to run other fuel-burning appliances in the same space.
• Combustion gases harm people and plants. RHS says burning gas or paraffin releases water, ethylene and carbon monoxide, so the greenhouse must be vented, which reduces the heating effect. Ethylene can damage sensitive plants, so these heaters suit hardy plants and emergency use best.
• Use a heater with safety cut-offs. The Big Buddy has an oxygen depletion sensor and tip-over shut-off; never use a heater meant only for outdoor use.
• Fit a battery CO alarm and never sleep or work for long in a greenhouse with a burning heater.
• Follow the manual on cylinders and clearances, including the distance to plants, film and panels.

Propane use is modest: Mr. Heater rates the smaller Portable Buddy at 0.044 gallons per hour on its 4,000 Btu setting. Treat propane as backup heat or for short frosts, and electricity as the main heat if you can get power to the greenhouse.