Skip to content
Grow In Backyard
Design Ideas

Do Greenhouses Need to Be Heated? Only for Some Jobs

An unheated greenhouse extends the season by weeks at each end. Heating buys frost protection and winter growing, and the cost scales with ambition.

By Zakaria 3 min read Save to Pinterest — opens Pinterest in a new tab
An aluminium-framed greenhouse with the door open, surrounded by flowering shrubs
Photo: EME, Pixabay Content License, via Pixabay
On this page

No — not for most of what people use them for. An unheated greenhouse already gives you warmer days, no rain, no wind and several extra weeks at each end of the season. Heating buys three specific things, and each costs more than the last.

What heating actually buys

This table answers the question on its own
TargetTemperatureWhat it allowsRunning cost
UnheatedAmbient +1–2°CHardy seedlings, winter salads, dormant plantsNone
Frost-freeJust above 0°C / 32°FOverwintering tender plants in dormancyLow
Cool7–10°C / 45–50°FTender perennials in leaf, slow growthModerate
Warm~18°C / 64°FGerminating warm-season cropsHigh

Buy the warm band with a propagator, not a heater

Holding 18°C across a whole greenhouse to germinate tomatoes is expensive. A heated propagator holds it in a box, which is where the seeds actually are, for a tiny fraction of the energy.

An unheated greenhouse is not frost protection

This is the misconception worth correcting. A greenhouse typically sits a degree or two above outside air on a cold night, and on a clear still night it can fall close to the outside figure — glass radiates heat away efficiently once the sun is gone.

What it protects well against is wind chill and freezing rain, which matters more than people assume. But a tender plant in an unheated greenhouse during a hard frost is not safe.

What grows in a cold greenhouse over winter

More than most people try:

  • Winter salads — mizuna, mustards, corn salad, winter lettuce
  • Spinach and chard
  • Hardy herbs — parsley, chervil
  • Overwintering onions and garlic

Sow early enough, because light is the real limit

Growth nearly stops from late autumn to midwinter — not because of cold but because of day length. Sow early enough that plants are established before the light collapses, then harvest through winter rather than expecting them to grow through it.

Cheapest ways to add warmth

  1. Heated propagator or soil warming cable — heats the seeds, not the air
  2. Bubble wrap on the inside of the glazing — cuts heat loss substantially, costs little
  3. Horticultural fleece over plants on frosty nights — a few degrees, instantly
  4. Partition off a corner with polythene and heat a quarter of the volume
  5. Thermostatic electric fan heater when real heating is genuinely needed

What drives the running cost

Not floor area on its own. It is the temperature difference you hold and how leaky the structure is.

  • Frost-free, small, well-insulated, mild winter: modest
  • 15°C, single-glazed, cold winter: a serious bill
  • Insulate first, heat a partition rather than the whole, and always use a thermostat

Common questions

Do greenhouses need to be heated?

No, for most of what gardeners use them for. An unheated greenhouse still raises daytime temperatures substantially, keeps rain off, blocks wind and extends the growing season by several weeks at each end, which is enough for raising hardy seedlings, overwintering dormant plants and growing winter salads. Heating becomes necessary only when you want reliable frost protection for tender plants, or want to raise warm-season seedlings early, or want to grow actively through winter.

What can you grow in an unheated greenhouse over winter?

A good deal more than people expect. Winter salads such as mizuna, mustards, corn salad and winter lettuce, plus spinach, chard, hardy herbs like parsley and chervil, and overwintering onions and garlic all do well in a cold greenhouse. Growth is very slow from late autumn to midwinter because light rather than temperature is the limit, so sow early enough that plants are established before day length collapses, then harvest through the winter.

What temperature should a greenhouse be kept at in winter?

It depends entirely on the job. Frost-free, meaning kept just above 0°C / 32°F, is enough to overwinter most tender plants in dormancy and is by far the cheapest target. Around 7 to 10°C / 45 to 50°F supports slow active growth and overwintering of tender perennials in leaf. Anything approaching 18°C / 64°F for germinating warm-season crops is expensive in a whole greenhouse, which is why a heated propagator is the sensible way to buy that temperature.

What is the cheapest way to heat a greenhouse?

Heat the plants rather than the building. A heated propagator or a soil warming cable raises temperature where the seeds are for a fraction of the cost of heating the whole volume. After that, bubble wrap insulation on the inside of the glazing cuts heat loss substantially, horticultural fleece thrown over plants on frosty nights adds a few degrees, and partitioning off a small section with polythene lets you heat a quarter of the space. An electric fan heater with a thermostat is the usual choice when real heating is needed.

Does an unheated greenhouse protect from frost?

Partially, and not reliably. A greenhouse typically keeps the air inside a degree or two above outside on a cold night, which is not enough to protect tender plants during a hard frost, and on a clear still night the temperature inside can fall close to the outside figure. It protects far better against wind chill and against freezing rain. For actual frost protection you need either heat or added insulation such as fleece over the plants.

Is greenhouse heating expensive to run?

It can be, and the cost scales with the temperature difference you are maintaining and how leaky the structure is, not with the greenhouse's size alone. Holding frost-free in a small well-insulated greenhouse through a mild winter is modest; holding 15°C in a single-glazed greenhouse through a cold one is a serious running cost. Insulating first, heating a partitioned section rather than the whole, and using a thermostat rather than running continuously are what keep it reasonable.

Backyard Weekly

Know what to plant, week by week

Join the weekly email for seasonal jobs, fixes for what is going wrong, and gear worth buying.

Not ready to subscribe? The planting calendar is free and needs no email.

By subscribing you agree to our privacy policy. Grow In Backyard will never sell your address.

Keep reading