How to Keep a Greenhouse Warm at Night in Fall and Winter

The coldest hour in a greenhouse comes just before sunrise, and by then the warmth of the afternoon is long gone. Step inside at 6 a.m. on a clear November morning and the air can feel barely different from the yard. Plenty of gardeners get caught off guard by this, because a greenhouse looks like it should make its own warmth.

It doesn't. It collects heat while the sun shines and gives it back, steadily, after dark. Keeping plants safe overnight comes down to three jobs: slow the loss, store some daytime heat, and add a heat source only for the gap that remains. This guide follows that order, which is also the order of cost, cheapest first.

Why a Greenhouse Gets Cold at Night

Heat leaves a greenhouse in three ways, and each one has its own fix.

Conduction is heat moving straight through the covering and frame. Glass, plastic film, and polycarbonate (a rigid, lightweight plastic panel) all conduct, but at very different rates. Metal frames conduct well, which is why frost and condensation often show up along the framing first.

Infiltration means warm air escaping through gaps while cold air slips in to replace it. Door edges, vent seals, panel joints, and the gap along the base all count. On a windy night, infiltration can be the biggest leak of all.

Radiant heat loss is heat leaving as invisible infrared energy, the same thing you feel on your face under a clear night sky. The covering and everything inside it radiate toward the cold sky overhead. Clouds send part of that energy back down, so a clear, still night can run colder inside than the forecast low suggests. A plastic-covered house can occasionally read below the outside air in those conditions.

Decide How Warm You Actually Need to Be

"Warm" depends entirely on what's growing. A house full of spinach and kale asks for far less than one holding tomato transplants or a potted lemon tree.

• Frost-hardy greens and roots (spinach, kale, mâche, claytonia, carrots): many can ride out nights in the 20s under a layer of row cover, though leaves may wilt until the sun warms them.
• Tender cool-season crops (lettuce, chard, cilantro): happiest when the house stays above freezing, with only brief dips below.
• Warm-season crops (tomatoes, peppers, cucumbers, basil): growth slows well before frost, and nights below roughly 50°F can stress them. Basil is usually the first to complain.

The real decision is whether you want to grow through winter or simply keep plants alive through it. Holding a house at frost-free (around 35 to 40°F) costs far less than holding it at 60°F, and the gap in heater runtime is large.

Seal the Leaks First (the Cheapest Degrees You'll Gain)

Before you buy anything, find the drafts. On a windy evening, light a stick of incense and walk the inside perimeter slowly. Smoke that streams sideways marks a leak. At dusk, you can also have a helper stand outside while you run a flashlight along seams from inside, since light that escapes shows where air can too.

The usual suspects:

• Doors: weatherstripping (a compressible seal along the door edge) and a door sweep along the bottom close most of the gap.
• The base: a gap between frame and ground lets cold air flow in at plant level. Mounded soil, a lumber skirt, or rigid foam board along the outside of the base all help.
• Vents and fans: louvers (angled slats) and exhaust fan shutters rarely seal tightly. A snug insulated panel over an unused fan opening stops the draft.
• Panel joints and tears: glazing tape or greenhouse-grade sealant on joints, patch tape on torn film.

One caution before you go airtight. A completely sealed house traps humidity, and damp, still air invites botrytis (gray mold, a fuzzy gray rot that attacks stems and leaves). Close the leaks you can find, keep a small controlled vent, and plan on running a circulation fan, which comes up in the nightly routine below. Sealing buys the first few degrees. Storing heat buys the next few.

Store Daytime Heat With Water and Stone

Thermal mass is any dense material that soaks up heat while the greenhouse is warm and releases it as the air cools. Water does this better than almost anything a home gardener can buy by the cubic foot. A 55-gallon barrel holds roughly 450 pounds of water, and every pound gives off about one BTU (British thermal unit, a standard measure of heat) for each degree it cools.

Use dark containers or paint them flat black, fill them completely, cap them to limit evaporation, and set them where winter sun reaches them. The north wall is a good home because the barrels won't shade your beds. Stone, brick, and concrete pavers work too, and so does moist soil, which holds more heat than dry soil.

Set realistic expectations. Mass flattens the overnight dip rather than preventing it. It rescues a night that would have bottomed out a few degrees too low, but it can't carry a house through a hard freeze alone, and it works best when sunny days recharge it. A string of gray days leaves the barrels cold.

Build a Greenhouse Inside the Greenhouse

Heating a whole house is expensive, since most of that air isn't touching a plant. Smaller volumes are cheaper to protect, which is the logic behind wrapping just the crops.

Row cover (lightweight spun fabric, also sold as frost cloth) catches heat rising from the soil and keeps frost from settling on leaves. Supported on wire or PVC hoops, it becomes a low tunnel (a short covered arch over a bed). Fabric weight matters. Light cover adds a little protection and passes plenty of light, while heavy cover adds more warmth and blocks more sun, so save the heavy weight for the coldest nights. A few degrees of gain is a fair expectation.

Insulating the structure comes next. Horticultural bubble film (large-bubble plastic made for greenhouses, not packing wrap) can be fixed to inside walls with clips or, on glass, a light mist of water. It cuts both heat loss and light. Winter light is already scarce, so start with the north wall and the lower side walls, and leave the south-facing roof clear. A white or reflective panel on the north wall bounces light back onto the plants while it blocks loss.

Choosing and Sizing a Heater

Sometimes the layers aren't enough. Heating is where mistakes get expensive, so size the heater before you shop.

The estimate takes three numbers. Start with the total area of the covering in square feet, walls and roof together. Multiply by the temperature difference you need to cover (your target minus the coldest expected outside night). Then multiply by the covering's U-value, a rating of how fast heat passes through a material, where lower is better. Rough values: single-pane glass about 1.1, double-layer inflated poly film about 0.7, twin-wall polycarbonate roughly 0.6. Thickness changes the figure, so check the maker's specs for your panels.

Here's a made-up example. Take 400 square feet of twin-wall polycarbonate at 0.6, holding 45°F on a 15°F night. That's 400 × 30 × 0.6 = 7,200 BTU per hour. Heaters are rated in BTU per hour, and electric ones in watts, with one watt equal to about 3.4 BTU per hour, so 7,200 BTU works out to about 2,100 watts. Add a cushion for wind and leaks, since the formula assumes still air and a tight structure. Lower the target to 35°F and the same house needs 400 × 20 × 0.6 = 4,800 BTU per hour, a third less.

Then pick the type that suits the house:

• Electric fan heater with a thermostat: the simplest choice for small and mid-size houses. It produces no combustion gases and needs no exhaust. Run it through a GFCI outlet (ground-fault circuit interrupter, which cuts power when current leaks, as it can in damp spaces) with outdoor-rated cords of adequate gauge, and keep it clear of plastic, fabric, and water spray.
• Vented propane or natural gas heater: higher output for larger houses and colder regions. Choose a model vented to the outdoors, supply combustion air the way the manufacturer specifies, and install a carbon monoxide detector. Running costs follow local fuel prices.
• Heat mats and cables: these warm only the root zone. They suit seedlings and cuttings, and paired with a row cover they can carry a seed-starting bench through cold nights. They do nothing for tall plants.

Placement matters as much as output. Set the heater low, away from walls and plastic, with space for air to move around it. Put the thermostat probe at plant height, out of the direct blast, so it reads what the plants feel. A thermometer that texts you when the temperature falls below a set point is cheap insurance, and it's the only thing that will tell you the power went out at 2 a.m.

The Late-Afternoon Routine That Holds Heat

Equipment helps, but timing does a surprising amount of the work. Close vents in mid-afternoon on a cold day, while the sun is still on the panels, so the house seals in warmth instead of venting it just as the light fades.

Water in the morning. Wet leaves going into a cold night invite disease, and a damp bed holds a little more heat than a dry one, so the morning watering helps on both counts. Then set up a small horizontal airflow fan (a low-powered fan that gently stirs the air in a circle) and run it on low overnight. It keeps cold air from pooling around plants and cuts condensation, which protects against botrytis.

On a cold-forecast afternoon, run through this:

1. Check the forecast low and the cloud cover. Clear, calm nights run colder.
2. Close vents while the sun is still on the panels.
3. Set row cover over the beds, tucking the edges.
4. Confirm the heater's set point and that the probe sits at plant height.
5. Switch the circulation fan to low.
6. Next morning, read the min-max thermometer, write the number down, and reset it.

Signs Your Plants Got Too Cold

Check the plants at mid-morning, once the house has warmed. Frost damage looks dark and water-soaked at first, then the leaves go limp, turn black, or dry to paper within a day. Tender plants like basil, tomatoes, and peppers show it first.

Chilling injury (damage from cold above freezing, which hits warm-season crops) is quieter. Look for pale or spotted leaves, cupped growth, and plants that simply stop growing. Purple tinting on tomato leaves often shows up when roots sit in cold soil and can't take up enough phosphorus. It's a sign to warm the root zone, not to add fertilizer.

Resist the urge to prune right away. Damaged growth can shield the tissue below, and the full extent of the injury takes several days to show. Your overnight log from the min-max thermometer will tell you what temperature caused it, which is more useful than any guess.

How Much Help You Need by Region

The same greenhouse needs very different help depending on where it stands. USDA Hardiness Zones (the government map that divides the country by average coldest winter temperature) give a rough guide, though local microclimates matter too.

In mild-winter regions such as the Gulf Coast, much of the Southeast, and Southern California (roughly zone 9 and warmer), sealing and a layer of row cover handle most nights, and a small thermostat heater covers the occasional cold snap. Overheating on sunny days often becomes the bigger problem, so vents matter as much as heat.

The Pacific Northwest rarely sees deep cold in the lowlands, but long damp, gray stretches stress plants through low light and high humidity. Ventilation, airflow, and dry foliage deserve more attention there than heater output.

Gardeners in the Midwest, Upper Midwest, and Northeast (zone 6 and colder, as a rough guide) face weeks of hard freezes. Plan on layers and supplemental heat together, or run the house cold and grow only hardy crops like spinach and kale.

Where to Start This Weekend

Buy a digital min-max thermometer with a remote probe and put it at plant height. Leave it for two weeks of ordinary fall weather, writing down the overnight lows next to the forecast lows. That log shows whether you need better sealing, a few barrels, or a heater, and it keeps you from buying equipment for a problem you may not have.

If the gap is a couple of degrees, fix the drafts and add a row cover. If the log shows readings near freezing with tender crops inside, that's when the sizing math above earns its keep. Either way you'll be working from your own greenhouse's numbers instead of a forecast and a guess.