Do Greenhouses Work in the Winter? What to Expect by Zone

Updated August 2026

A greenhouse is very good at capturing heat and only moderately good at holding it after dark. That single fact explains most of what happens between November and March.

On a sunny January afternoon you can stand in a greenhouse in shirtsleeves while it's 25°F outside. Come back at six the next morning and it's 27°F in there. Same building, one night apart. What you can do with that depends on where you live and whether you decide to add heat.

Below we cover how a greenhouse behaves through a winter day and night, what your glazing does after dark, what you can grow unheated in your zone, whether you need to heat it at all, the mistake to avoid when the temperature drops, and how snow load actually works.

Do Greenhouses Work in the Winter?

How a Greenhouse Works in Winter

The mechanism is the same in January as it is in June. Sunlight passes through the glazing, warms the surfaces inside, and the structure holds that warmth in. Winter simply gives you less of it to work with, and longer nights to lose it through.

On a sunny day, a well-ventilated greenhouse typically runs 20 to 30°F warmer than the outside air. Close it up and that gap can reach 40 to 50°F, which is why a sealed greenhouse still needs ventilation in January. A bright winter day with the vents shut will overheat seedlings.

Overnight is the part that surprises people. Once the sun goes down there's no more heat coming in, and the greenhouse gives back what it gathered. If it's 20°F outside overnight, a typical unheated greenhouse holds somewhere around 22 to 28°F, depending on insulation and thermal mass. That's a meaningful margin, and it isn't the tropical room people sometimes picture.

Thermal mass is the lever most people overlook. Water, stone, and soil all absorb heat during sunny hours and release it slowly after dark. A few black water jugs along the back wall will hold warmth noticeably longer overnight than an empty greenhouse, and the same mass moderates the afternoon temperature spike on the way up. It costs nothing and it's the first thing to try.

What Greenhouse Glazing Does After Dark

How much of that heat you still have at dawn comes down mostly to your glazing. The walls and roof of a greenhouse are almost entirely panels, so the glazing is doing the vast majority of the insulation work, typically 85 to 90 percent of the total heat retention equation. The frame matters at the margins.

Single-pane glass rates about R~0.9, which means heat leaves quickly once the sun is off it. Standard 6mm twin-wall polycarbonate, the material on most consumer greenhouses, improves on that at roughly R~1.6. The 10mm 4-wall polycarbonate we use on every greenhouse rates about R~2.1, with twice the walls and three times the insulating air chambers, holding roughly 30 percent more heat than that 6mm twin-wall.

In winter that difference translates into extra weeks of shoulder-season growing without supplemental heat, and if you are heating, into a smaller bill for it. Our polycarbonate vs. glass comparison works through all four glazing types in detail.

What You Can Grow Unheated, by Zone

USDA Zone	What an unheated greenhouse gives you
Zones 3 and 4	Season extended on both ends. Hardy greens under row cover carry a harvest into early winter, but production stalls through the deepest cold. Growing right through winter here means adding heat
Zones 5 and 6	Row cover laid directly over the plants typically carries the fall harvest into December or January. Spinach and mache are the reliable performers, with no supplemental heat at all
Zone 7	Cold-hardy greens including lettuce, kale, and spinach produce right through winter, though it's worth keeping row cover on hand for the coldest nights
Zones 8 through 10	Production continues through winter with minimal frost protection. Summer heat becomes the harder problem.

A single layer of floating row cover, laid directly over plants inside the greenhouse, adds roughly 5 to 8°F of frost protection. It costs very little and it's the difference between spinach continuing and spinach stopping. For what to plant and when, the zone planting calendar in our beginners guide breaks the year down month by month from Zone 3 through Zone 10.

Do You Actually Need to Heat It?

Not necessarily. It depends on your zone and on what you want out of the greenhouse. Work through it in order.

Start with row cover and thermal mass. Together they cost very little and cover most shoulder-season cold snaps, and a good deal of winter for hardy crops depending on your zone. Many owners find they don't need to go further than this.

Add heat if you want more than hardy greens in a cold zone. If you're in Zone 3, 4, or 5 and you want tender crops, tropicals, or citrus through winter, glazing and row cover won't get you there. That's a heating decision, and it comes with an electrical plan and a line on your winter utility bill worth pricing before you commit.

One safety note that applies regardless. The PowerPort™ Electric Hub in our greenhouses is designed for low-draw accessories like grow lights and fans. A space heater should run on a dedicated circuit, following your greenhouse manual, the heater manufacturer's guidance, and local electrical code.

The Winter Mistake: Sealing It Up

The instinct when the temperature drops is to close everything and trap what heat you have. Sealed and stagnant is where the trouble starts.

A sealed greenhouse in winter becomes a humid one. Moisture evaporates from the soil and transpires from the leaves, and with nowhere to go it condenses on every cold surface: the glazing, the frame, and the plants themselves. University extension research, including guidance from UF IFAS, consistently identifies 80 percent relative humidity as the point where fungal disease risk rises sharply, with rapid development above 90 percent. Cold, still, wet air is a better environment for botrytis and mildew than it is for your greens.

The fix costs you a few degrees. Crack a window for an hour or two on sunny winter days, or let the exhaust fan run briefly during the warmest part of the afternoon. You lose a little heat and remove a great deal of moisture, and that trade is almost always worth making. Watch the panels in the morning: water droplets on the inside of the glazing mean overnight humidity is too high and you need more air exchange during the day. Our ventilation guide covers humidity control and seasonal airflow in full.

Can a Greenhouse Handle Snow and Wind?

The other winter question isn't about plants at all. It's whether the building survives February.

Snow load is the figure that matters, and it's worth understanding how it's quoted. A rating in pounds per square foot tells you the engineering standard, and it's the number you can hold against any other greenhouse regardless of size. A rating in total pounds tells you what that particular roof is built to carry, which naturally rises with roof area. A small greenhouse and a large one built to the same standard will show very different total figures, so compare per square foot and use the total to understand the specific model.

Both numbers are worth having, because wet, heavy snow runs far denser than the light powder people picture, and a foot of it across a large roof adds up quickly.

Ask any manufacturer for a published, tested snow load and wind rating. If those numbers exist, you can compare structures on something real. If they aren't available, that absence is information too.

Wind matters for a different reason. A rating of 100 mph is well above what most locations will ever experience, but severe thunderstorms can produce gusts in the 60 to 80 mph range, and a greenhouse that folds in a storm is not just a financial loss but a safety hazard.

Every Backyard Discovery greenhouse carries a Pro-Tect™ rating verified through structural testing. Every model meets or exceeds 50 pounds per square foot of roof load and is rated to 100 mph winds. In total pounds that runs from 2,600 on the compact Poppy 9x6 up to 10,600 on the Larkspur, which is a difference in roof area rather than a difference in standard. You can see the ratings model by model in our buying guide.

Whatever you own, clear heavy accumulation from the roof rather than waiting to find out.

Cold Frame vs. Greenhouse

If winter growing is the main thing you want and a greenhouse feels like more than you need, a cold frame is a legitimate answer rather than a consolation prize.

A cold frame is a low, unheated box with a clear lid that captures solar heat close to the ground. It'll let you harvest later into autumn and early winter, then plant and harvest earlier in spring, at a fraction of the cost and the assembly. Our Aggie Cold Frame Box uses a double-wall polycarbonate lid that lets natural light through while retaining heat, and it solves the venting problem on its own: a heat-sensitive wax hinge opens the lid automatically once temperatures exceed 70°F and closes it again as things cool, with no electricity involved.

What it won't do is let you stand up inside, grow tall crops, hold shelving, or run power and water. If you want a place to work as much as a place to grow, that's a greenhouse. If you want season extension for greens and starts without a structure in your yard, a cold frame like our Aggie Cold Frame Box does the job honestly.

What Makes a Greenhouse Work Better in Winter

Winter is where the difference between greenhouses stops being cosmetic.

Glazing that holds overnight heat. 10mm 4-wall polycarbonate at R~2.1 rather than the standard 6mm twin-wall at R~1.6 found on most consumer kits. That margin is what buys you the extra weeks at each end of the season, and lowers what you spend if you heat.

Published structural ratings. Pro-Tect™ snow load and wind figures verified through structural testing, which matters most in exactly the climates where winter growing is hardest.

Ventilation that still works in January. A thermostat-activated exhaust fan comes on at your set temperature and cycles off when the air cools. Between the sunny afternoons that overheat a sealed greenhouse and the humidity that builds up when you close it against the cold, this is the feature that handles both while you're at work.

Frame material matters differently in winter than it does in summer, mostly in terms of what you want to maintain. Our cedar models use FSC-certified, kiln-dried cedar, naturally rot and insect resistant. Left alone it weathers to a silver-gray, which is cosmetic rather than structural and a look plenty of gardeners prefer. If you want the wood to keep its warm color, plan on a coat of penetrating oil stain or sealant every two to three years. The Larkspur pairs a galvanized steel and aluminum frame with wood-tone composite paneling that never needs sealing or staining, which is worth weighing if the idea of refinishing a structure in a cold, wet climate doesn't appeal. Both are covered by a 5-year warranty, and our guide to greenhouse frame materials compares cedar, aluminum, and steel in detail.

If winter is the season you're buying for, size matters differently than it does in summer. A compact model like the Willow is plenty for hardy greens and seed starting on shelves, and the PowerPort™ Electric Hub and interior spigot mean you're not running cords or hauling water across a frozen yard whatever size you choose. If you want to work in there through the cold months, or grow beyond greens, the larger models like the Bellerose XL, Savannah, or Larkspur give you floor space that stays usable when everything is under cover at once.

Browse all Backyard Discovery greenhouses

Greenhouses in Winter: Frequently Asked Questions

Do greenhouses work in the winter?

Yes. A greenhouse extends the growing season by roughly six to eight weeks on each end, more in warmer zones and less in colder ones, and will produce cold-hardy greens through winter in Zone 7 and warmer. In Zones 5 and 6, row cover carries the harvest into December or January. What it won't do is stay warm overnight without help, so tender and tropical plants need supplemental heat in cold zones.

How cold does an unheated greenhouse get?

An unheated greenhouse settles close to the outside temperature overnight. At 20°F outside, expect roughly 22 to 28°F inside, depending on glazing and thermal mass. During the day the difference is far larger, with a sunny day typically putting a ventilated greenhouse 20 to 30°F above outside air.

Can you use a greenhouse year-round?

Yes, though what year-round means depends on your zone. In Zone 7 and warmer, cold-hardy greens produce through winter in an unheated greenhouse. In Zones 5 and 6, row cover extends the harvest into December or January. In Zones 3 and 4, growing through the coldest months needs heat. A greenhouse is also useful year-round for overwintering dormant plants, which mainly need shelter from freezing wind and hard frost.

What can you grow in a greenhouse in winter without heat?

Cold-hardy greens are the mainstay, including kale, spinach, mache, arugula, and hardy lettuce varieties. Radishes also do well. Adding a layer of floating row cover directly over the plants inside the greenhouse buys another 5 to 8°F, which is often the difference between production continuing and stopping.

Do you need to ventilate a greenhouse in winter?

Yes, but for a different reason than in summer. In winter you're venting to remove moisture rather than heat. A sealed greenhouse builds humidity that condenses on the glazing and the plants, and fungal disease risk rises sharply above 80 percent relative humidity. Cracking a window for an hour or two on a sunny afternoon, or letting the exhaust fan run briefly, costs you a few degrees and prevents the problem. Condensation on the inside of the panels in the morning is the signal that you need more air exchange during the day.

Is a cold frame better than a greenhouse for winter?

For growing hardy greens and starting seeds, a cold frame does much of the same work for far less cost and takes up very little space. A greenhouse is the better choice if you want room to work standing up, grow tall crops, use shelving, or run power and water. Many gardeners use both.

Can a greenhouse handle heavy snow?

It depends entirely on how it was engineered, which is why published snow load ratings matter. Ask for a tested figure in pounds per square foot rather than a description, since that's the number that compares across sizes. Backyard Discovery greenhouses meet or exceed 50 pounds per square foot and are rated to 100 mph winds, which works out to between 2,600 and 10,600 total pounds depending on roof area. Whatever you own, clear heavy accumulation rather than waiting.