Heating a Greenhouse in Winter: 9 Practical Ways to Keep Your Plants Warm

Heating a greenhouse in winter is the difference between a thriving little jungle and a sad glass box of dead seedlings. I learned this the hard way a few winters ago when a single hard frost wiped out every tender plant I had smugly “protected” under glass. An unheated greenhouse is better than nothing, but once temperatures plunge, glass alone will not save your plants.

The good news is that keeping a greenhouse warm through winter does not have to mean a shocking energy bill. With a mix of insulation, free solar heat, thermal mass, and one well-chosen heater, you can hold your greenhouse frost-free all winter for less than you probably expect. Here are the nine methods that actually work, starting with the free ones.

How Warm Does Your Greenhouse Really Need to Be?

Before spending a penny, decide what temperature you are aiming for, because “warm” means different things to different plants. For most home gardeners, the goal is simply frost-free — around 4°C (39°F) — which is enough to overwinter tender perennials like pelargoniums, fuchsias, and young citrus.

If you want to actively grow through winter, say salads or early seedlings, you will need closer to 10–15°C, and that costs considerably more to maintain. Every extra degree you demand from your heater multiplies the running cost, so be honest about what your plants actually need rather than heating the whole space to T-shirt weather. Once you know your coldest overnight low, you can pick the cheapest combination of methods below that closes the gap.

1. Start With Free Heat: Make the Most of Weak Winter Sun

The cheapest heat source you own is the winter sun, feeble as it feels. A greenhouse is essentially a solar collector, and small tweaks can noticeably raise daytime temperatures. First, clean the glass — inside and out — because a winter’s worth of algae and grime can block a surprising amount of already-weak light.

Second, check the orientation: a ridge running east to west catches the most low winter sun on its long south-facing side. None of this costs more than an afternoon’s work, and on a bright January day you will be amazed how warm a clean, well-oriented greenhouse feels by midday. The trick, of course, is holding onto that warmth after sunset, which brings us to the next method.

2. Bank Daytime Warmth With Thermal Mass

Thermal mass is just a fancy term for heavy stuff that soaks up heat during the day and releases it at night. Water is the star performer: a few 200-litre barrels painted black and placed along the north wall of your greenhouse will absorb solar energy all day and radiate it back for hours after dark.

Stone, brick paths, and concrete slabs do the same job on a smaller scale, which is one reason a greenhouse with a solid floor stays warmer than one sitting on bare soil. The effect is not dramatic — expect a degree or two of frost protection on a cold night — but it is completely free, silent, and maintenance-free once installed.

For the best results, combine thermal mass with good insulation (method 3), because stored heat is useless if it leaks straight through single glazing.

3. Insulate With Bubble Wrap: The Cheapest Big Win

If you do only one thing on this list, line your greenhouse with bubble wrap. Horticultural-grade bubble wrap — the large-bubble kind sold for greenhouses — can cut heat loss dramatically, and gardeners routinely report their heating bills dropping by a third or more after fitting it.

It works by trapping a layer of still air against the glass, exactly like double glazing, while still letting most of the light through. Fit it on the inside of the glass, bubble-side facing the glass, fixing it with clips or battens rather than tape where possible so you can remove it in spring. Cover the roof and walls but leave vents accessible, and accept a small loss of light — roughly 10 to 20 percent — as the price of warmth.

For the cost of a takeaway dinner, this single step does more for winter warmth than almost anything else.

4. Seal Every Gap Before You Pay for Heat

Heating a draughty greenhouse is like filling a bathtub with the plug out, so go on a draught hunt before the cold sets in. Check where the glazing meets the frame, around the door, and especially around roof vents and louvres, which are notorious for leaking warm air all winter.

Self-adhesive foam draught excluder strips, silicone sealant for fixed gaps, and a simple brush strip along the bottom of the door will close most of the leaks in an afternoon. Pay particular attention to automatic vent openers, which can stick slightly open in cold weather and silently vent your expensive warm air all night. Every gap you seal is heat your heater no longer has to replace, which means this boring job pays for itself within weeks.

5. Pick the Right Heater: Electric, Propane, or Paraffin

Sooner or later most winter gardeners buy a heater, and the choice matters. Electric fan heaters designed for greenhouses are the easiest option: they are thermostatically controlled, distribute warmth evenly so there are no cold corners, and produce no fumes or moisture. Look for one with a built-in thermostat and a frost-protection setting, and size it to your greenhouse — a small 6x8ft structure usually needs far less power than people assume. Propane heaters are powerful and work during power cuts, but they release water vapour and carbon dioxide as they burn, which can encourage fungal disease in a closed winter greenhouse.

Paraffin heaters are the old-school budget choice, but they need regular refuelling, smell, and also add moisture to the air. Whichever fuel you choose, never run a heater without a thermostat — heating blindly wastes money and can cook plants on mild nights. The Royal Horticultural Society’s guide to heating greenhouses efficiently is well worth a read for matching heater size to your setup.

6. Try Compost Heating for a Free Temperature Boost

Here is a wonderfully old-fashioned trick: a pile of fresh manure and straw generates real heat as it decomposes. Victorian gardeners built “hotbeds” — frames filled with fresh horse manure topped with soil — and grew melons in them while snow lay outside. You can use a smaller version of the same idea: a compost bin or a heap of fresh manure inside or against the greenhouse releases a steady low-level warmth for weeks.

It will not replace a heater on the coldest nights, but it can lift the temperature by a few valuable degrees exactly when you need it, and it costs nothing if you have access to manure. Just keep fresh manure away from direct contact with plant roots and make sure the heap does not block light or airflow.

7. Control It All With a Thermostat and a Max/Min Thermometer

A heater without a thermostat is just an expensive way to overheat your greenhouse on mild nights and still freeze your plants when it really matters. Set the thermostat to the minimum temperature your most tender plant needs — usually around 4°C for frost-free overwintering — and let it do the thinking. But do not trust the thermostat dial blindly: the sensor sits right next to the heater, so it reads warmer than the far corners where your plants actually live.

That is why every winter greenhouse needs a max/min thermometer placed at plant height in the middle of the structure, well away from the heater. Check it each morning through the coldest months and adjust the thermostat until the overnight minimum holds steady where you want it. This simple feedback loop is the difference between guessing and knowing, and it stops you paying to heat air your plants never feel.

8. What NOT to Do When Heating a Greenhouse in Winter

A few common mistakes undo all the good work above. First, do not seal the greenhouse completely airtight — plants still need fresh air, and a totally sealed, damp greenhouse is an invitation to grey mould and other fungal diseases. On bright winter days, crack a vent open for an hour around midday to let humid air escape; if you want a full low-energy approach to airflow, see my guide on how to ventilate a greenhouse without electricity. Second, do not water in the evening — wet foliage going into a cold night is asking for disease, so always water in the morning.

Third, do not buy a massively oversized heater “just in case”; it will cycle on and off, create wild temperature swings, and cost more to run than a correctly sized one working steadily. Finally, do not heat the whole greenhouse if you only need to protect a few plants — a small heated propagator or a fleece tent inside the greenhouse can protect tender specimens for a fraction of the cost.

Conclusion

Heating a greenhouse in winter is not about brute force; it is about layering smart, mostly cheap methods so each one makes the next more effective. Clean glass and dark surfaces catch free solar heat, water barrels and stone store it, bubble wrap and draught-proofing keep it from escaping, and a modest thermostat-controlled heater covers whatever gap remains.

Add the free bonus of a compost heap and the discipline of a max/min thermometer, and you can hold a home greenhouse frost-free all winter without dreading the energy bill. Start with insulation — it is the single biggest win for the money — and build from there. Come spring, when your overwintered plants burst into growth weeks ahead of everyone else’s, you will wonder why you ever let a frost decide your gardening calendar.