Time It Right: Moving Plants Under Cover

Most tender plants in pots are best moved under cover between late September and mid-October, before the first proper frost rather than after it. A light touch of cold does them good: pelargoniums, fuchsias, cannas and citrus all cope with a few nights at 5°C, and a gradual move into cooler, duller conditions is far kinder than a sudden shock. Watch the forecast rather than the calendar, and bring everything in ahead of a clear, still night when frost is forecast.

Before you move anything, give each plant a tidy-up. Pick off dead and yellowing leaves, trim back leggy growth by a third or so, and check the rim of the pot for slugs and snails that have taken up residence. If a plant is riddled with aphids, deal with that outside while you still can — you really do not want to carry an infestation into a confined space.

Choosing and Preparing Your Frost-Free Space

A frost-free greenhouse is ideal, but a sheltered porch, conservatory, cool spare room or even a garage with a decent window will do perfectly well provided it stays above freezing. Aim for a minimum of 5°C for citrus, olives and most succulents; pelargoniums, fuchsias, agapanthus and cannas will manage at 2–5°C as long as they stay dry.

Cooler is generally better than warmer. A hot, dark room encourages pale, weak growth and invites mould, whereas a bright, chilly spot keeps plants ticking over quietly until spring. Before the plants move in:

• Clean the glass or windows on both sides to let in every scrap of winter light.
• Line the greenhouse with bubble wrap for insulation, leaving vents and doors clear.
• Set pots on feet, bricks or slatted staging so they are not sitting on cold, damp slabs.
• Group pots together for mutual shelter, but leave a little air between them.
• Insulate terracotta and glazed pots with bubble wrap or hessian — they crack when wet compost freezes.

Water Less, Feed Hardly at All

Overwatering is the single biggest killer of overwintering plants, and it happens with the best intentions. Growth slows dramatically in low light, so roots use very little moisture. From October onwards, water only when the compost is nearly dry — check by weight, or push a finger in up to the second knuckle.

When you do water, do it in the morning so any surplus can dry off before the cooler night, and use tepid water rather than straight from a cold tap. Never leave pots standing in saucers of water; empty them within the hour. Plants that drop their leaves and go properly dormant, such as dahlias and some begonias, need barely any water at all — just enough to stop the tubers or corms shrivelling, perhaps once a month.

Stop feeding altogether from mid-autumn. You can offer a weak, half-strength feed from late February, but only if there is enough light for new growth to make use of it. Otherwise you are simply feeding mould.

Watch for Pests and Rot

Twenty minutes a week with a mug of tea and a torch will save you a great deal of heartache. Check the following regularly:

• Under leaves and in growing tips: aphids, whitefly, mealybug and red spider mite all thrive in the warm, still air of a greenhouse.
• The compost surface: fungus gnats are a nuisance, but vine weevil grubs are worse — white, C-shaped and capable of killing a plant outright.
• Stems and buds: grey, furry mould is botrytis, and it spreads fast. Cut affected growth back to clean tissue and bin it, don't compost it.
• Dropped leaves and debris: clear them away every week, as they harbour both slugs and spores.

Ventilate on mild, dry days whenever the temperature allows, even in January. Fresh air is the cheapest disease control you have. If you spot mealybug or whitefly, dabbing them off with a cotton bud dipped in methylated spirits works surprisingly well on small numbers.

Getting Them Going Again in Spring

As the light strengthens from March, increase watering gradually and start a balanced feed once you see fresh growth. Repot root-bound plants into slightly larger containers, using fresh, free-draining compost, and trim out any winter-killed stems.

Harden everything off slowly before planting out or moving pots back outside. Put plants out for a few hours on mild days, bringing them back in at night, and build up over two or three weeks. In most of the UK, do not assume frost risk has passed until late May, and give it until early June in northern areas and frost pockets. A sheet of horticultural fleece kept nearby is cheap insurance for those last cold nights.