How to winterise a greenhouse

Practical walkthrough for getting a greenhouse ready for the cold months. This guide draws on "How To Prepare Your Greenhouse For Winter", which covers the key steps from cleaning down through to insulation and heating, with useful detail on bubble wrap lining technique.

1. Do it in October, before the first hard frost

The ideal time is mid to late October. The summer growing season is over, the worst of the warmth has gone, but you still have mild enough days to work comfortably and the first serious frosts have not arrived. Leave it much later and you'll be trying to line the walls in the cold with numb fingers.

East Kent can see hard frosts in November, so don't sit on it if you have tender plants to protect.

2. Clear everything out and give it a proper clean

Take out all the pots, trays, canes, and staging. Remove any summer crops that have finished. Sweep the floor, wipe down the staging, and scrub the glass or polycarbonate panels inside with warm water and a soft brush. Algae and grime on the glass cuts down the light coming in over winter, when the plants need all they can get.

This is also a good time to check for pests hiding in crevices or under staging. Red spider mite and vine weevil larvae in particular appreciate a winter refuge.

3. Check and repair any broken glass or cracked panels

Walk around the outside and look carefully at every pane. A small crack will let in a surprising amount of cold air and rain. Broken panes need replacing before you insulate - there is no point lining the walls if the structure itself has gaps. Horticultural glass is widely available and can be cut to size at a glazier; polycarbonate panels for aluminium-framed greenhouses are readily available online by the metre.

Check the glazing clips and putty lines too. Loose panes rattle in the wind and can break under pressure from frost or snow.

4. Seal draughts around doors, vents, and the base

Feel for draughts around door frames, vent edges, and where the greenhouse meets its base. Self-adhesive foam draught excluder strip from any hardware shop will seal gaps in door frames quickly and cheaply. For gaps at the base, a bead of exterior silicone sealant or some gravel boards piled against the outside will help on exposed sites.

5. Line the walls with horticultural bubble wrap

Horticultural bubble wrap - the large-bubble kind, sold specifically for greenhouses - can cut heat loss considerably and costs very little. Fix it to the inside of the frame using special greenhouse clips, or hang it with wire pushed through the bubbles and hooked over the frame bars. Fit it to the walls and ideally the roof too if you can manage it.

You don't have to do the whole greenhouse. If you're only overwintering tender plants on one bench, you can section off that area with a curtain of bubble wrap to create a smaller, easier-to-heat zone inside the larger space.

6. Think about heating

Whether you need heating depends on what you're trying to keep alive. Tender perennials and overwintering cuttings need to stay frost-free - typically above 2-3°C. Most East Kent winters will not drop below this in a well-insulated greenhouse, but a hard cold snap can. A small electric fan heater with a frost-stat setting is the simplest, most controllable option. Paraffin heaters work without electricity but produce moisture, which can encourage mildew.

A min/max thermometer inside the greenhouse is worth having so you know what temperatures you're actually hitting overnight.

7. Sort out ventilation

It sounds odd, but ventilation matters in winter too. A closed, damp greenhouse with no airflow is ideal for botrytis (grey mould) to take hold, which will spread through your stored plants quickly. On mild winter days, open a vent slightly for an hour or two to let damp air out. Automatic vent openers can be set to open at a specific temperature - useful if you are not out there every day.

8. Organise the plants and check them regularly

Put the most tender plants furthest from the door and the draughtiest spots, and group them together to share warmth. Keep them off the cold ground if possible - staging or upturned pots work as risers. Then check in every week or so through winter: remove any dead leaves promptly (they become a source of mould), water sparingly (much less than in summer), and keep an eye on the thermometer.

When to call a handyman

If your greenhouse needs physical repairs - broken glazing bars, a rotting timber base, or a door that has dropped and won't shut - those are worth sorting properly before winter. Richard can help with structural repairs to garden buildings and greenhouses around East Kent.