The Best Greenhouse Heaters

For a hobby greenhouse, buy a 1500 W electric fan heater with a splash-proof rating and a thermostat you can set to an actual number, and run it on a frost setting rather than a comfort setting. If your heater's control is a knob, add a plug-in thermostat controller rated for 1500 W -- that single part cuts the running cost more than any heater upgrade. And before you buy either, try a layer of row-cover fabric over the crop, because a few degrees is often the entire gap between a loss and a harvest.

Almost every bad greenhouse-heating decision comes from skipping this question. The equipment for keeping frost off a bench of overwintering pelargoniums is not the equipment for raising peppers from seed in February, and neither of them will grow a tomato in a December that has eight hours of weak daylight.

Quick picks

#	Product	Best for	Score	Price
1	Top pickBio Green Palma Greenhouse Heater1500 W, IPX4 splash-proof, digital thermostat	Frost protection you can set and leave	9.0★★★★★	Check price on Amazon
2	VEVOR Greenhouse Heater, 1500 W IPX41500 W, IPX4-rated steel body	The cheapest heater that belongs in a greenhouse	8.0★★★★★	Check price on Amazon
3	BN-LINK Digital Thermostat Controller Plug15 A / 1875 W, two-stage heat and cool outlets	Making a cheap heater behave	8.8★★★★★	Check price on Amazon
4	Agribon AG-19 Floating Row Cover0.55 oz/sq yd fabric, about 85% light transmission	A few degrees for almost no money	8.4★★★★★	Check price on Amazon
5	VIVOSUN Seedling Heat MatWaterproof mat sized for 1020 trays	Peppers, tomatoes and slow germinators	8.8★★★★★	Check price on Amazon
6	Min/Max Greenhouse ThermometerRecords the high and low since last reset	Knowing what actually happened overnight	8.6★★★★★	Check price on Amazon

#ad We may earn a commission - prices are shown by Amazon, not by us

Scores are our own editorial judgment, formed from published specifications and independent sources. They are not aggregated customer ratings, and they are not the result of hands-on testing, which we do not currently do. See how we choose.

This is the thing most first-time owners have backwards. A greenhouse in January is not failing because it is cold. It is failing because there is not enough daylight to drive growth, and warming the air does not add any. Colorado State University Extension puts the constraint plainly: winter vegetables have a slower growth rate due to low light intensity, and artificial lighting for vegetable production -- as opposed to raising transplants -- is generally not cost effective.

The same guidance is blunt about the economics of the whole exercise: winter vegetable production in a greenhouse is only cost effective with an energy-efficient structure, a well-designed solar collector and optimum management. A 1500 W fan heater in a 6 x 8 polycarbonate kit is none of those things.

Which is why the realistic winter model is harvesting rather than growing. Crops established in autumn sit through the dark months and get picked from; a heater's job is to stop the night from killing them, not to make them grow. Once you accept that, the heater you need gets smaller, cheaper and much cheaper to run. What to grow in a greenhouse sorts the crops that accept those conditions from the ones that will not.

We do not lab-test heaters. These picks come from published manufacturer specifications, extension guidance on greenhouse heating, and one hard filter: would this thing be safe and controllable in a wet, unattended building? That knocks out most of the category, which is dry-room appliances with greenhouse keywords in the listing.

• A real splash rating. Greenhouses are wet by design. An IPX4-rated housing is the entry specification; an IP20 grow-tent heater is not the same product, whatever the listing title says.
• A thermostat with a setpoint, not a dial. The difference between asking for 36 degrees F and asking for 'low' is most of the annual running cost.
• 1500 W or less. A 120 V outlet caps you at about 1500 W, roughly 5,100 BTU/hr. Anything claiming to heat a large structure from a standard plug is claiming the impossible.
• Tip-over and overheat protection. The heater will run unattended, at night, next to trays and plastic.
• A fan. Moving air mixes the warm layer down off the roof, which is why extension guidance recommends circulating fans that can run continuously in cold weather even when the heater is off.
• No unvented combustion. Every fuel-burning option on the market is a ventilation problem first and a heater second -- see below.

Heater sizing is a heat-loss problem, not a square-footage problem. A greenhouse loses heat through every pane, so the number that matters is glazed surface area, the insulating value of that glazing, and the temperature gap you are asking the heater to hold. The University of Georgia's greenhouse heating and ventilation bulletin sets the design temperature this way: take the average daily minimum January temperature for your area and subtract 15 degrees F. That is the night your heater has to cope with, and it is colder than the one people plan for.

Then decide the setpoint, which is where most of the money is won or lost. UGA's hobby greenhouse bulletin notes that most greenhouse plants are adequately served by a night temperature of 55 to 65 degrees F, with 40 degrees F as the practical minimum in most situations. Those are two very different heating bills, and for overwintering dormant or semi-dormant plants the lower one is usually the correct answer. The same bulletin's rule of thumb for electric heat in a hotbed -- 10 to 15 watts per square foot of growing area -- is a useful sanity check, with the caveat that a walk-in greenhouse loses far more per square foot of floor than an insulated frame does, because its glazed area is several times its footprint.

The practical version of all this: a 1500 W heater on a frost setpoint is a sensible match for a sealed 6 x 8 or 8 x 10 twin-wall kit. It is not a match for a single-skin tunnel, a bigger structure, or a target of 60 degrees F in January. If you are choosing glazing with winter in mind, polycarbonate vs glass is the decision that determines how much heater you will ever need, and the best greenhouse kits for winter covers the structures built for it.

Fuel-burning heaters are popular in this category because they are cheap to buy and do not need a circuit run to the end of the garden. They also burn fuel inside a sealed building full of plants and, periodically, you. Extension guidance is unambiguous: all greenhouse heaters should be ventilated for both plant and human health, and if you use a gas, oil or coal heater you need a fresh air supply and an unobstructed chimney so carbon monoxide will not build up.

Plants are affected too, and earlier than people are. Incomplete combustion produces carbon monoxide and ethylene, and sulfur-bearing fuels can cause sulfur dioxide damage -- which shows up as leaf injury on the crop you were trying to protect. UGA's heating bulletin frames the requirement simply: any heating system that provides uniform temperature control without releasing material harmful to the plants is acceptable. An unvented propane heater in a closed hobby greenhouse does not clear that bar.

The arithmetic is simple and worth doing before you buy. A 1500 W heater running flat out uses 1.5 kWh for every hour it runs; multiply that by your own utility rate to get a number that means something where you live. A heater that runs all night, every night, through a cold month is a genuine bill -- which is why UGA's hobby greenhouse bulletin says plainly that electric heat can be very costly and is usually used only in very small greenhouses.

Everything that reduces that bill is about run time rather than wattage. A thermostat set to just above freezing instead of a comfortable room temperature cuts hours of running. A probe at plant height instead of next to the heater stops short-cycling. Fleece over the crop on the coldest nights means the heater cuts in later and less often. Water barrels in the sun flatten the day-night swing for nothing. Sealing the gaps around the door and base does more than another 500 W would.

1. Cover the crop. A light row-cover fabric laid over plants slows the radiant loss that causes frost damage on clear nights, for a tiny fraction of a heater's price and nothing to run.
2. Add thermal mass. Black-painted water barrels or jugs along the sunny wall absorb heat all day and release it overnight. Free, silent, and it works whether or not anyone is home.
3. Heat the root zone, not the air. For germination and propagation, a thermostatically controlled heat mat under the trays does what a 1500 W heater cannot do economically -- see seedling heat mats.
4. Build a frame inside the frame. A cold frame or a low tunnel inside the greenhouse doubles the protection on your most tender plants without heating the whole volume. Extension guidance names cold frames and tunnel gardening as the less expensive way to extend the season.

• A proper outdoor circuit. A 1500 W heater draws 12.5 A at 120 V, which is most of a 15 A circuit. It should not share that circuit with anything else, and the supply should be GFCI-protected outdoor-rated cable, not a domestic extension lead run through a gap in the door.
• Off the floor, out of the wet. Stand the heater on a dry block or shelf, not on a floor you hose down.
• Clear space around it. Keep trays, bubble wrap, fleece and bags of compost well clear of the outlet and the intake.
• Probe at plant height, shaded. Not in the heater's air stream, not in direct sun, and not on the floor.
• Aim along the structure, not at plants. Direct hot air scorches leaves; the heater should stir the whole volume. Circulating fans can run continuously in cold weather even when the heater is off.
• Keep venting. A heated greenhouse still overheats on a sunny day. An automatic vent opener keeps working while the heater sleeps -- greenhouse accessories covers the rest of that kit.

If you are still choosing a structure, start with the best greenhouse kits and read the winter section carefully -- glazing thickness decides your heating bill for the life of the greenhouse. If you already have one and want it working harder, greenhouse gardening tips covers the running of it through the year.