How Do I Keep My Greenhouse Frost-Free Without High Heating Bills?

An unheated or minimally heated greenhouse gives you far more frost protection than the garden outside — but it is not automatically frost-proof. On a very cold night, temperatures inside an unheated glass greenhouse can drop nearly as low as outside if it is poorly sealed and uninsulated. The good news is that several low-cost interventions make a dramatic difference.

How Cold an Unheated Greenhouse Gets

A standard glass greenhouse with no additional insulation stays roughly 2–5 °C warmer than outside on still nights. On a −5 °C night that means the inside might be 0 to −3 °C — cold enough to damage or kill tender overwintering plants. A polytunnel performs similarly. However, adding bubble wrap insulation and a small heater changes the equation completely: even a 1 kW electric propagator heater set to 5 °C minimum can keep the whole greenhouse frost-free at minimal running cost.

Insulating the Greenhouse for Winter

Bubble polythene greenhouse insulation (large-bubble, sold in rolls specifically for this purpose) fixed to the inside of the glazing creates a still-air layer that dramatically reduces heat loss. Fix it with clothes pegs or drawing pins to the glazing bars. Cover side panels and roof glass separately so condensation drains correctly. Insulating the north wall permanently and removing south insulation on bright days maximises both warmth and light. Pay particular attention to the door: a draught-proof door seal and an inner curtain of bubble wrap over the doorway makes a significant difference on cold nights.

Heating Options for Frost-Free Status

A small electric tubular heater on a thermostat set to 3–5 °C is the most reliable frost-free solution. Running only when temperature drops near the threshold, it uses relatively little electricity. Paraffin heaters are a no-electricity alternative but produce moisture and require fuel management. Gas heaters (with mains connection) are more practical for large greenhouses. For purely frost-exclusion purposes, the smallest and cheapest heater that can maintain 2–3 °C above zero is entirely adequate — you do not need tropical warmth, just frost-free.

Using Thermal Mass Inside the Greenhouse

Water stores far more heat per litre than air. Black-painted water butts, large containers of water, or even large pots of moist compost inside the greenhouse act as thermal batteries: they absorb solar heat during the day and release it slowly at night. A greenhouse with several hundred litres of water inside will stay warmer on cold nights purely through passive heat release, sometimes preventing frost without any electrical heating at all in mild-to-moderate climates.

What to Overwinter in the Greenhouse

Tender perennials — dahlias, fuchsias, pelargoniums — need just frost exclusion to survive winter safely. Citrus trees in containers need a minimum of 5 °C. Overwintering young brassica plants for spring transplanting, rooted cuttings, and early-sown seedlings all benefit from the greenhouse environment. Tender herbs like lemon verbena and basil can be kept alive (just) at 5 °C if well drained. The greenhouse also provides space for early propagation of tomatoes and peppers in late winter, weeks before outside temperatures allow it.