A cold greenhouse is a wonderful ally for the UK gardener. It keeps the worst of the wind, rain, and frost at bay, giving tender plants a fighting chance through the darkest months. But it is not a heated conservatory. Simply moving pots inside in October and forgetting about them until spring is a recipe for rot, mould, and disappointment. The secret to successful overwintering is to manage the environment carefully – and that means paying close attention to water, air, and insulation.

Know what your cold greenhouse can and cannot do

An unheated greenhouse will still drop below freezing on the coldest nights. So it suits plants that are half-hardy or tender but not those that demand tropical warmth. Pelargoniums, fuchsias, dahlias, and many succulents will survive happily if kept dry and airy. The goal is to slow their metabolism, not encourage new growth. A cold greenhouse is about survival, not thriving. If you have a really precious plant, be prepared to add a layer of fleece or a cloche over it during a severe cold snap.

Cut watering right back

Overwatering is the number one killer of overwintering plants. In winter, growth slows to a crawl, so roots take up very little moisture. Compost that stays wet for weeks will quickly turn sour and rot the roots. For most tender plants in a cold greenhouse, water only when the compost is almost dry – check with your finger or a wooden stick at a depth of 2cm. That might mean once every two to three weeks, or even less. Cacti and succulents often need no water at all from November to February.

Always use tepid water, not cold, and water in the morning so any excess can drain before nightfall. Never let pots stand in saucers of water; empty them an hour after watering. If in doubt, err on the side of dryness. A slightly thirsty plant is far more likely to survive than a soggy one.

Remove dead foliage and tidy up

Good hygiene is your best defence against mould and damping off. Before you settle plants in for winter, go over each one and remove any yellowing leaves, spent flowers, and dead stems. Use sharp scissors or secateurs, and clean them between plants if you suspect disease. Fallen leaves and debris on the greenhouse floor should be swept up and binned – they harbour slugs, vine weevil, and fungal spores.

Damping off is a particular risk for cuttings and young seedlings. It causes stems to collapse at compost level, often with a tell-tale fuzzy growth. To avoid it, use clean pots, fresh compost, and never overcrowd. If you see signs of grey mould (Botrytis) – a fuzzy grey coating on leaves or stems – remove the affected material immediately and improve ventilation.

Insulate the glazing with bubble wrap

Bubble wrap is the cold greenhouse's best friend. A single layer fixed to the inside of the glazing can raise the minimum temperature by 2–3°C, which is often enough to keep the frost at bay. Use large-bubble wrap (around 10mm bubbles) for the best insulation. Attach it with double-sided tape, clips, or plastic fixings, with the bubbles facing into the greenhouse.

Prioritise the north-facing side and the roof, where heat loss is greatest. Leave vents, doors, and any automatic openers clear so you can still ventilate. Be aware that bubble wrap reduces light by around 30%, so don't cover every pane if you are overwintering light-hungry plants. A strip along the coldest wall is often enough. You can also insulate the floor with a layer of straw or old carpet, but keep it dry.

Ventilate on mild days to prevent mould and damping off

Still, damp air is the enemy of overwintering plants. It encourages grey mould, damping off, and a host of other fungal problems. The answer is ventilation on mild days. When the temperature rises above about 7°C and the air is dry, open a vent or the door for an hour or two, ideally around midday. This exchanges humid air for fresh, drier air and helps prevent condensation.

Even in winter, a closed greenhouse can heat up to 20°C in bright sunshine, then cool rapidly at night. That swing creates condensation on the glass and leaves. A max-min thermometer is a cheap and useful tool – check it regularly. If you see condensation most mornings, you need more ventilation. And if a mild spell is forecast, open up a little wider. Just remember to close everything well before dusk to trap the day's warmth.

A simple winter routine

Overwintering successfully comes down to a few regular habits. Once a week, walk through your greenhouse and:

• Check every pot for signs of rot, mould, pests, or dryness.
• Remove any dead or diseased material straight away.
• Test the compost of a few plants with your finger – water only those that are truly dry.
• Open the vents on mild days, and close them well before dusk.
• Keep an eye on your max-min thermometer, especially during cold snaps.

With a little care, your tender plants will come through the winter in good shape, ready to burst back into growth when spring arrives. A cold greenhouse is not a miracle worker, but with the right approach it is a remarkably effective one.