Heating a Greenhouse in Winter | Keep Plants Alive Without Breaking the Bank

Heating a greenhouse in winter is most reliably done by combining passive solar gain, insulation, thermal mass, and a thermostatically controlled backup heater sized to prevent hard freezes.

For the full breakdown, see our best Greenhouse Heating System guide.

Come November, keeping a greenhouse above freezing while outdoor temperatures drop into the teens is a challenge. The mistake most new growers make is buying the biggest heater without first capturing free solar heat and sealing air leaks. The right sequence—site, seal, insulate, add thermal mass, then heat—cuts your electricity bill in half and keeps plants alive through January’s worst cold snaps.

Start With Solar Gain Before Adding Any Heat

A greenhouse that gets full winter sun starts several degrees warmer than a shaded one. Orient the ridge line east–west in colder U.S. climates so the long south-facing side catches low-angle winter sunlight all day. Double-layer polycarbonate panels or inflated double poly film hold solar heat much better than single-pane glass. A white or reflective north wall shines light back onto plants rather than letting it escape, raising interior temperatures noticeably.

Seal Drafts and Insulate Before the First Freeze

Drafty greenhouses lose heat faster than a heater can replace it. In late fall, seal every gap around vents, doors, panes, and the base frame with weatherstripping or greenhouse tape. The highest-return insulation trick is covering the inside glazing with bubble-wrap polythene—the bubbles trap a layer of still air that slows heat loss. Section off the used part with a polythene curtain to reduce the volume a heater must keep warm.

How Thermal Mass Works and What to Use

Water stores more heat per pound than almost any greenhouse material. A row of 55-gallon black barrels filled with water soaks up daytime solar energy and releases it overnight, often keeping the interior 4–6°F warmer without electricity. Black containers absorb heat best, but any sealed water container works. Stone paths, brick floors, and compost piles also act as thermal mass, though water is most effective for small to mid-size greenhouses. Arrange barrels along the north wall to avoid blocking light from plants, and leave air gaps for warmth to circulate.

Sizing the Heater and Getting the Controls Right

Even the best passive setup needs backup heat when a polar vortex arrives. Electric fan heaters with thermostats are the safest and most controllable option for most U.S. greenhouses. A heater set to 50°F is usually fine for overwintering tender perennials, seedlings, and leafy greens; you don’t need a tropical 70°F. Place a reliable max/min thermometer among the plants—about three feet off the ground and away from the heater—to dial in the setting accurately. Even on sunny winter days, crack the vents briefly to prevent overheating and excess humidity. A small low-setting fan circulates warm air and prevents cold spots where frost can hit overnight.

The table below summarizes the winter setup timeline so you can plan ahead.

Common Mistakes That Waste Heat and Money

Three errors appear in almost every cold-damage greenhouse: leaving small cracks unsealed (a vent that doesn’t close flush leaks as much heat as a six-inch gap), packing plants so tightly that air cannot circulate and cold settles into pockets, and running a heater without a thermostat—which makes it run full blast until turned off. Also, clean glazing passes more solar energy, so wash the glass or polycarbonate in November before the sun gets low.

A properly set thermostat is the cheapest insurance against losing a winter’s worth of seedlings. Test everything in late fall while the weather is still forgiving.