How to Keep Greenhouse Warm in Winter

Updated September 2026 · Garden Advice Guide

Most advice on how to keep greenhouse warm in winter starts with a heater. Start with the building instead. A sealed, bubble-lined hobby greenhouse with a few barrels of water in it holds roughly 5 to 10°F above the outside air on a still night, and adding a layer of row cover on hoops inside adds another 4 to 8°F on top of that. Stack those and an unheated structure behaves like a garden one full zone warmer, which is enough to carry salad greens, hardy herbs, and dormant potted plants through a normal winter with no power at all. Heat is what you add after the building is doing its job, and only up to the specific minimum your crops need.

Know what you are defending against: run your ZIP through our free frost date lookup to see when your hard freezes normally start and end, because that window is the only part of the winter where any of this matters.

Updated September 2026

First, decide the number you are defending

Everything downstream depends on one decision: what is the coldest night temperature you are willing to let the inside of the greenhouse reach? There are really only four answers, and they are wildly different jobs.

• Keep it above about 20°F. Salad greens and hardy herbs. A well-built unheated greenhouse does this by itself in zones 6 and warmer, and with an inner layer down into zone 5.
• Keep it frost free, 35 to 40°F. Dormant tender perennials, citrus in dormancy, bulbs, cuttings holding over. This is the first target that usually needs some heat in a cold climate, and it is a modest step up.
• Keep it at 45 to 50°F. Active growth on cool-season crops, early brassica and onion seedlings, subtropicals that keep their leaves. Noticeably more demanding.
• Keep it at 55 to 60°F. Tomato and pepper seedlings, tropicals in leaf. This is the most demanding target of the four, and for most hobby growers it is the wrong one. Start those seeds indoors on a heat mat and move them out later.

Write your number on a card and tape it to the door. Every choice below is measured against it. The single most common mistake in a hobby greenhouse is heating the whole air volume to 55°F all winter in order to protect a tray of seedlings that would have been perfectly happy on a windowsill.

Minimum night temperatures by crop group

These are the temperatures below which you start losing plants, not the temperatures at which they grow well. Most of them stall out and simply sit still well above their minimum, which is fine; the point of a winter greenhouse is to hold plants alive and harvestable, not to push growth.

Two things to take from that table. First, the majority of what people want to overwinter sits in the 20 to 42°F band, and the building alone gets you most of the way there. Second, dormant hardy plants are actively harmed by warmth. A potted hydrangea kept at 50°F all winter leafs out in January and then has nowhere to go.

How to keep greenhouse warm in winter: the 9 fixes, in order

Do these in order. Each one makes the next one work better, and the last one on the list is the only one that uses power.

1. Seal the building before you add anything

Air leaks beat insulation every time. A hobby greenhouse loses more heat through gaps than through the glazing, and the gaps are all in the same places.

Walk the structure on a windy day with a stick of incense or a lit candle and watch where the smoke pulls. Then fix, in this order: the door, where the sweep and the latch never quite close; the roof and side vent flaps, which sit slightly open when the autovent arm cools and contracts; the laps between polycarbonate or glass panes; the joint between the frame and the base; and the corners of the end walls, which on cheap kits are simply not sealed at all.

Weatherstripping foam, clear silicone on the pane laps, and a threshold sweep on the door will do more for your night temperature than any amount of bubble wrap over a leaky shell. Do this in October. Sealing a building in January while standing in a cold draft is nobody's idea of a good afternoon.

2. Bubble insulation on the inside of the frame

Horticultural bubble insulation, the UV-stabilized kind with large bubbles, is the standard hobby greenhouse upgrade and it earns its place. Fixed to the inside of the frame it creates a still air gap against the glazing and cuts heat loss substantially, often on the order of a third to a half of what the glazing was losing.

The trade is light. Bubble insulation takes a real bite out of winter light levels, and winter light is already the limiting factor for anything trying to grow. So use it selectively:

• Line the north wall and the two end walls fully. The north side contributes almost nothing in light and loses heat all night.
• Line the roof and south side only if your minimum is 40°F or above. If you are just holding hardy greens, leave the south glazing clear and let the light in.
• Use large bubble, not packing bubble wrap. Small bubbles collapse in a season and the film yellows.
• Fix it to the frame, not to the glazing. You want a trapped air gap between the bubble and the pane. Clip it to the glazing bars with the little push-in fixings sold for the purpose.
• Overlap sheets by a few inches and tape the seams so warm moist air cannot circulate behind the layer.

3. Insulate the north wall properly

If you are willing to give up the north wall entirely, reflective foil-faced bubble board on that wall does two useful things at once: it stops heat loss on the side that never contributes light, and it bounces the low winter sun back onto your benches. In a small greenhouse that reflected light is worth having, because the sun in December sits low enough to come straight in the south glazing and hit the back wall.

Knee walls, the solid lower panels many kits have, are the other easy win. Rigid foam board cut to fit and pushed against the inside of a knee wall is invisible from the bench and stops a surprising amount of loss right at the level where your plants sit.

4. Thermal mass: water does the buffering

Thermal mass is what keeps the temperature from crashing the moment the sun goes down. Water is the best material available for it because it stores more heat per unit volume than soil, stone, or concrete, and it is easy to move.

The working rule is roughly 2 gallons of water for every square foot of south-facing glazing, which in a typical 8 by 10 hobby house means something like four to six 55-gallon drums, or a couple of dozen 5-gallon jugs if you cannot fit drums. Practical notes:

• Put it where the winter sun lands, which is against the north wall, since low sun comes through the south glazing and strikes the back. Mass in a shaded corner is just a heavy object.
• Dark containers absorb better. Black drums or dark blue jugs, not white ones.
• Fill barrels to about 90 percent and leave the lid loose, so a hard freeze has somewhere to expand into.
• Stacked jugs work fine and are easier to carry, arrange, and drain than a drum. They also double as watering cans that are already at greenhouse temperature, which is better for roots than a hose full of near-freezing water.
• Do not count the soil floor. A gravel or soil floor does contribute mass, but it is cold and slow and it is not a substitute.

Mass does not make the greenhouse warmer on average. It makes the swings smaller: cooler on a sunny afternoon, warmer at 4 a.m. Smaller swings are exactly what overwintering plants want.

5. A second layer inside: the greenhouse within a greenhouse

This is the highest return per hour of work on the whole list, and it is the one most hobby growers skip.

Set low hoops over your benches or ground beds and drape garden fleece or a medium-weight row cover over them at night. You have just built a second greenhouse inside the first one, and the plants underneath sit several degrees warmer than the greenhouse air. Two layers of light cover beat one heavy one, because it is the trapped air between layers doing the work.

The weights and the degrees each grade buys are laid out on our page for frost cloth for plants; the same product and the same numbers apply indoors, except that indoors it is protected from wind and rain so it lasts several seasons instead of one.

Two rules for the inner layer. Pull it back on sunny mornings so the plants get the light and the soil under them can absorb heat. And keep it off the leaves where you can, because any leaf pressed against a cold cover freezes right through it.

6. Insulate the perimeter and the floor edge

The ground under a greenhouse is a heat store, and it leaks sideways. In an uninsulated structure, warmth in the soil under the benches conducts out through the top foot of soil at the perimeter and disappears into the frozen ground outside.

Bury rigid foam board vertically, 12 to 24 inches deep, tight against the outside of the base all the way around. It is a dull, one-time job and it makes the floor mass actually work for you overnight. If burying is not possible, bank straw bales or bagged leaves against the outside of the base for the winter, which does a rougher version of the same thing and also blocks wind at the leakiest joint in the building.

7. Keep the air moving, all winter, all night

This sounds like it works against you and it does not. A small circulation fan running continuously does three jobs: it breaks up the cold pocket that forms at floor level, it evens out the temperature between the ridge and the bench so you are not heating the roof, and it keeps a film of moving air over the leaves, which is the single biggest control on winter disease.

A small oscillating fan on a shelf is enough for a hobby house. It uses a trivial amount of power compared to a heater and it will save you more plants than the heater does.

8. Compost heat and hot beds

A hot bed is the oldest trick in the book and it still works. Fresh horse manure mixed with straw, or a mix of fresh grass clippings, wood chips, and green waste, packed 18 to 24 inches deep in a frame with 4 to 6 inches of soil or potting mix on top, will run at 60 to 70°F in the root zone for six to eight weeks as it decomposes. Seeds germinate in it in February in a greenhouse that is barely above freezing at night, because it is heating the roots rather than the air.

The honest limitations: it is a burst, not a season. It peaks in the first two weeks and fades. It needs a real volume of fresh material to get going, so a bucket of kitchen scraps will do nothing. And an active compost pile inside a small closed greenhouse raises humidity noticeably, which you then have to manage.

A related and simpler version is bottom heat under a propagation tray. Root zone warmth at 65 to 70°F under a tray, with the greenhouse air sitting at 40°F, germinates almost everything and takes a fraction of the energy needed to lift the whole building. If your reason for heating is seedlings, this is almost always the right answer instead. When those seedlings do go outside, they still need the same gradual transition as anything else, which is what hardening off seedlings covers.

9. Thermostat controlled heat, sized to the target

If you still need heat after the eight fixes above, add it under thermostatic control and set the thermostat to the number on the card taped to your door, not to a temperature that feels pleasant when you walk in.

• Set the thermostat 2°F above your minimum, not 10. A frost-free house is set to 38 or 40°F, not 50°F.
• Put the sensor at plant height in the coldest corner, usually the north end at bench level, not up by the ridge where it is warmest and where it will switch the heat off while your plants freeze.
• Use a min/max thermometer for a week before you decide anything. Almost everyone is surprised. Many hobby greenhouses in zones 7 and 8 never actually drop below 32°F once sealed and mass-loaded, and the owners had been heating them for years.
• Heat the smallest volume you can. Partition off one end with a curtain of clear polyethylene and heat only that section. A third of the volume needs a third of the input.
• Never use an unvented combustion heater in a sealed greenhouse. Kerosene and propane heaters produce water vapor and combustion gases, and ethylene from an incompletely burning unit distorts and drops leaves on tomatoes and other sensitive plants. If you use one, it must be vented and the house must not be fully sealed.

Venting on sunny winter days

The counterintuitive part of winter greenhouse management: your biggest temperature problem in January is often overheating.

A sealed greenhouse on a clear 40°F day can reach 75 to 90°F inside by early afternoon. That heat wakes up dormant plants, dries out pots, and encourages soft growth that then gets frozen the same night. It also cooks anything sitting under an inner row cover you forgot to pull back.

The routine:

1. Open a vent or crack the door once the inside passes about 65°F, even if it is freezing outside. A small gap is enough; you are bleeding off heat, not flushing the building.
2. Close everything by 2 or 3 p.m., while the inside is still warmer than the outside. Closing early is how you bank heat in the mass for the night. Closing at dusk means you already lost it.
3. Fit an autovent if you are not home during the day. The wax cylinder units need no power and open at a temperature you set. In winter, adjust them up so they open later than they do in summer.
4. Watch the sunny days after a cold snap. Those are the days that damage plants, because the swing from 25°F overnight to 80°F at noon is bigger than anything the weather does outdoors.

Condensation and disease, the real winter killer

More overwintered plants die of botrytis than of cold. Gray mold thrives at exactly the conditions a closed winter greenhouse produces: cool, still, humid air with water sitting on leaf surfaces overnight.

What causes it: every plant and every square foot of damp soil transpires moisture into a sealed building all day. At night the air cools, hits its dew point, and that moisture condenses on the glazing and drips back onto the leaves.

What fixes it:

• Water in the morning only, never in the afternoon or evening, so foliage and the soil surface dry before the temperature drops.
• Water less than you think. A dormant plant at 40°F uses a fraction of what it uses at 70°F. Overwatering in winter kills more overwintered fuchsias and pelargoniums than frost does.
• Water the pot, not the plant. Keep the crown and the leaves dry.
• Run the circulation fan continuously. Moving air does not condense on leaves.
• Vent for ten minutes on mild days specifically to swap humid air for dry air, even when the temperature does not require it.
• Remove every dead leaf and fallen petal, weekly. Botrytis starts on dead tissue and moves into the live plant from there.
• Space plants apart. A packed bench with overlapping leaves has no air movement inside it at all.
• Keep the glazing clean. Winter light is scarce and a dirty pane can cut it noticeably, and weak plants get sick.

Is it worth heating? Answer it with a temperature

The useful version of this question is not about running a heater in the abstract. It is: what does each temperature target actually unlock, and does that match what you want to grow?

Read that table top down and stop at the first row that covers what you actually want to grow. In practice most hobby growers want row one or row two, and the whole apparatus of heaters and thermostats they were shopping for was aimed at row four.

There is a fifth option that is not on the table: do not heat at all, and shift what you sow. Hardy annuals and perennials sown in containers outdoors and left to freeze germinate perfectly in spring on their own schedule, no building required. Our page on winter sowing covers that method, and for a lot of plants it produces sturdier seedlings than a heated greenhouse does. A step up from a jug, with none of the running cost of a heated house, is a cold frame you build yourself, which holds winter greens and hardens off trays on the same unheated principle.

Worked example: an 8 by 10 hobby greenhouse in zone 6

Target: frost free, 38°F minimum, to hold four citrus in pots, a bench of pelargonium cuttings, a bin of dahlia tubers, and a ground bed of spinach and mache for winter salad.

October. Clear out summer crops, wash the glazing inside and out, and scrub the frame. Seal the door with weatherstripping and a threshold sweep, silicone the pane laps, and adjust the autovents so they hold shut. Bank straw bales along the outside of the north base.

Early November. Line the north wall and both end walls with large bubble insulation clipped to the frame, and leave the south roof and south side clear for light. Set four dark 55-gallon drums against the north wall and fill them to nine tenths. Install a min/max thermometer at bench height in the north corner and an oscillating fan on the shelf, running around the clock.

Mid November. Move the citrus and the cuttings in. Dahlia tubers go in a crate of dry wood shavings under the bench. Sow spinach and mache in the ground bed. Set hoops over the ground bed with fleece ready to drop.

Late November through February. Read the min/max every morning and reset it. The first week tells you what you actually have: in this example the house is running about 8°F above the outside low, so on a 25°F night the inside sits at 33°F. That is below the 38°F target, so a small thermostatic heater goes in at the north end, set to 40°F, and it runs for a few hours on the coldest nights only. Fleece goes over the salad bed at dusk and comes off in the morning. Vents crack whenever the inside passes 65°F and close by 2:30 p.m. Water only on mild mornings, only into the pots, and a lot less than in summer. Dead leaves come out every Saturday.

March. Heater comes out once the hard freezes stop. Bubble insulation comes off the end walls first to get light back in. Citrus goes back outside once nights hold above 45°F, transitioned gradually over a week rather than moved in one afternoon.

Frequently asked questions

How cold does it get in an unheated greenhouse?

A bare, unsealed hobby greenhouse runs only 2 to 4°F above the outside air on a still night, which is barely anything. Sealed, bubble-lined, and loaded with water barrels, it holds around 5 to 10°F above the outside low, and adding a fleece layer on hoops over the plants gives another 4 to 8°F at plant level. That total is roughly one USDA zone of improvement.

Does bubble wrap actually help a greenhouse?

Yes, if it is horticultural large-bubble film fixed to the frame with an air gap behind it, rather than packing bubble taped to the glass. It typically cuts heat loss through the covered area by a third to a half. The trade is winter light, so line the north and end walls first and only line the south glazing if you need to hold above 40°F.

How much water do I need for thermal mass in a greenhouse?

Plan on about 2 gallons for each square foot of south-facing glazing, in dark containers placed where the low winter sun strikes them, which is usually the north wall. In an 8 by 10 house that is roughly four to six 55-gallon drums. Mass does not raise the average temperature, it flattens the daily swing, which is what overwintering plants need.

Should I vent a greenhouse in winter?

Yes, on sunny days. A sealed greenhouse can hit 80°F inside on a clear 40°F day, which breaks dormancy and pushes soft growth that freezes the same night. Crack a vent once the inside passes about 65°F, and close everything by 2 or 3 p.m. while it is still warmer inside than out, so the thermal mass keeps the heat overnight.

What temperature should I set a greenhouse heater to in winter?

Set it 2°F above the minimum your coldest-tolerance plant needs, not to a comfortable working temperature. Frost free means 38 to 40°F, not 50°F. Put the sensor at plant height in the coldest corner rather than near the ridge, and run a min/max thermometer for a week first, because many sealed greenhouses in zones 7 and 8 never drop below freezing at all.

Why is my greenhouse so wet inside in winter?

Plants and damp soil release moisture all day, and when the air cools at night it condenses on the cold glazing and drips back down. The fixes are watering in the morning only, watering far less than in summer, running a circulation fan continuously, venting briefly on mild days to exchange humid air, and clearing dead leaves weekly before gray mold gets started on them.

Can I grow vegetables in an unheated greenhouse all winter?

You can harvest from one, which is not the same as growing in one. Spinach, mache, claytonia, kale, chard, arugula, scallions, and parsley sown in September size up in the fall, then hold through the dark months and restart growing in February. Do not expect anything sown in December to do much until the day length comes back.

Is a compost heap in the greenhouse a good heat source?

A properly built hot bed of fresh manure and straw, 18 to 24 inches deep under 4 to 6 inches of soil, gives 60 to 70°F in the root zone for six to eight weeks and is excellent for early germination. A general compost pile in the corner is not; it needs a large volume of fresh material to heat at all, it fades quickly, and it adds humidity to a building where humidity is already the main disease risk.