How do I overwinter plants in an unheated or barely heated greenhouse?

A greenhouse in winter is a still, damp, dark box, and that is a far better environment for grey mould than for plants. The instinct is to close everything up and add heat. The regime that actually works is closer to the opposite: cool, dry at the roots, and ventilated whenever the weather allows.

Step by step

1. Clean and line the greenhouse before anything goes in.Autumn clean, then bubble lining with the vents cut around and still working. Doing this after the plants are in is far harder and disturbs them.
2. Prepare the plants before they come in.Cut back soft growth, remove all dead and yellowing leaves and spent flowers, check for pests, and top-dress or repot if needed. Anything with whitefly or vine weevil brought inside will spread through the house all winter.
3. Cut watering right back.Most losses are root rot. Dormant or semi-dormant plants need the compost just moist and no more. Check by weight, water in the morning so foliage dries, and water the compost rather than over the plant.
4. Space the plants out.Air must move between them. A greenhouse packed shoulder to shoulder is where botrytis spreads from one plant to the next. Put the most susceptible — pelargoniums, fuchsias, anything soft-stemmed — where air moves best.
5. Ventilate on every mild day.Crack a vent and a louvre open for a few hours whenever it is above freezing and not blowing a gale. Changing the air is the single most effective disease control available in winter.
6. Inspect weekly and remove dead material immediately.One rotting leaf becomes a botrytis colony that spreads to the stem. A five-minute walk round once a week, picking off anything dead, prevents most of the trouble.
7. Use fleece for the coldest nights rather than heating throughout.Horticultural fleece thrown over the plants on frosty nights buys several degrees for nothing, and comes off in the morning. It is far cheaper than heating the whole house and often sufficient.
8. Partition and heat only if you must.If you have genuinely tender plants, curtain off a section and keep just that zone frost-free with a thermostatically controlled heater. Heating the whole volume for three plants is what makes greenhouse heating expensive.
9. Keep the plants in as much light as possible.Winter light is scarce. Clean glass, lining only where it does not cost light, and no shading. Plants in low light grow drawn and weak and are more susceptible to disease.

Tips

• Pelargoniums overwinter better almost dry than damp. Many growers cut them back hard, knock most of the compost off, and keep them barely moist in a frost-free spot.
• Raise pots off a cold concrete floor onto staging or a slat. Cold, wet concrete conducts heat out of the pot and holds water underneath.
• Deciduous tender plants such as fuchsias can be kept fully dormant and almost dry under the staging, which frees the bench for anything that needs the light.

Common mistakes

• Sealing the greenhouse up for the winter — Still, humid air is exactly what grey mould needs. Ventilating on mild days costs almost no heat and prevents most winter losses.
• Watering on the same schedule as summer — A dormant plant in cold, low light uses a fraction of the water. The compost stays wet, the roots rot, and the plant collapses in February.
• Bringing plants in with pests already on them — A warm, enclosed, predator-free greenhouse is ideal for whitefly and red spider mite. Inspect and treat before the plants come inside.
• Watering in the evening — The foliage stays wet all night in cold still air. Morning watering gives the day to dry off.

Checklist

• Greenhouse cleaned and lined, vents still working
• Plants cut back, dead material removed, pests checked
• Pots raised off the cold floor
• Plants spaced with air between them
• Watering reduced to occasional and morning only
• Fleece to hand for frosty nights
• Weekly inspection to remove dead leaves

Questions people ask