Overwintering Tender Plants: Dahlias, Pelargoniums and More

Tender plants don't have to be thrown away in autumn. Dahlias, pelargoniums, fuchsias and chillies are all perennials in warmer climates: it's only our frosts that kill them. Give them a frost-free winter and the same plants come back next year, often bigger and earlier than anything you'd buy in May.

Overwintering is mostly about doing very little, carefully. This guide covers the four plants UK gardeners most often want to keep, and the two mistakes that kill more of them than cold does.

Where to keep tender plants over winter

The one thing every tender plant needs is to stay frost-free. Beyond that, they split into two groups:

• Plants that keep their leaves (pelargoniums, fuchsias, chillies, citrus) need light as well, because they're still living, slowly. A cool windowsill, a porch, a conservatory or a heated greenhouse.
• Plants that die back to a tuber or root (dahlias, cannas) need only a dry, frost-free, dark place: a frost-free garage, a cellar, a cupboard in an unheated room.

What doesn't work is an unheated greenhouse, cold frame or shed for the leafy group. They feel sheltered, but on a hard night they freeze too. If you want to keep tender plants under glass, you'll need a heater on a thermostat set just above freezing: heating a greenhouse in winter explains how, and the greenhouse heater cost calculator shows what it'll cost to run.

Warm isn't better, either. A plant kept in a warm, dark living room grows long, pale and weak. Cool, light and frost-free is the aim.

Dahlias

Dahlias are the one plant where you wait for the frost.

1. Let the frost blacken the leaves. The first frost turns dahlia foliage black overnight. That's your signal that the plant has stopped growing and the tubers have finished feeding up. Lifting earlier just gives you smaller tubers.

2. Cut the stems back. Cut the blackened stems down to a short stump above the ground. Label each plant now. A bare tuber in March gives nothing away.

3. Lift the tubers. Ease them out with a fork, working from well outside the clump so you don't spear them. Shake or gently wash off the soil.

4. Drain and dry them. Stand the clumps upside down for a few days somewhere frost-free and airy, so water drains out of the hollow stems. Water left sitting in the stem is a common cause of rot.

5. Store them dry and frost-free. Pack them in a box or tray of dry compost, sand or shredded newspaper, with the crowns just showing, and keep them in a cool, dark, frost-free place.

6. Check them every few weeks. Cut out any soft or rotting bits. If a tuber is shrivelling badly, mist the packing very lightly.

Can you leave them in? In mild areas with free-draining soil, many gardeners leave dahlias in the ground and cover the crown with a thick mulch of compost, bark or straw once the stems are cut down. It's a gamble that pays off in a mild winter. In cold areas, or heavy soil that stays wet, the tubers are likely to rot or freeze, and lifting is the safer bet.

Pelargoniums

The bright "geraniums" of summer pots and window boxes are tender, but easy to keep.

• Bring them in before the first frost. Lift bedding plants and pot them up, or bring the whole container in.
• Cut them back to a compact, bushy shape, and remove any yellowing leaves and spent flowers, which rot in damp winter air.
• Keep them light and frost-free: a cool windowsill, porch or heated greenhouse.
• Keep them on the dry side. Water only when the compost is nearly dry, then just a little. Pelargoniums cope with drought far better than with soggy compost.
• Don't feed until spring.

From late February, as the days lengthen, they'll start to push new shoots. Water a little more, start feeding, and pot them on if they need it. They go back outside after your last frost.

Fuchsias

Fuchsias come in two kinds, and it's worth knowing which you have.

• Hardy fuchsias are shrubs that stay in the ground all year. Leave them be; in a cold area, a mulch over the base helps.
• Tender fuchsias (most trailing basket kinds and many with big, showy flowers) need the same frost-free treatment as pelargoniums.

Bring tender fuchsias in before the first frost. They usually drop their leaves as they slow down, which is normal. Keep them cool, frost-free and nearly dry, with just enough water that the compost never dries out completely. They don't need much light while leafless, but a bright spot helps them wake up in spring. Trim them back to a tidy framework in late winter, just before growth starts.

Chillies

Chillies are perennials too, and an overwintered plant fruits earlier and harder the following year. The full method is in overwintering chillies, but the short version is:

• In October, before the first frost, pick all the fruit and cut the plant back hard, removing about two-thirds of the top growth.
• Keep it in a bright, frost-free spot at around 8–12°C: a cool windowsill or a heated greenhouse, not an unheated one.
• Let the compost go almost dry between small drinks, and don't feed until late February or March.

Take cuttings as insurance

Whatever you overwinter, some plants won't make it. Cuttings are cheap insurance, and small rooted cuttings take up far less windowsill space than whole plants.

Pelargoniums and fuchsias root easily from cuttings taken in late summer or early autumn, before the plants come in. Take a healthy, non-flowering shoot tip, trim it below a leaf joint, remove the lower leaves, and push it into the edge of a pot of gritty compost. Keep it light and barely moist. Rooted cuttings are overwintered on a windowsill exactly like the parent plants.

Mediterranean herbs

Rosemary, thyme, sage and oregano are a different case. Most established plants are hardy in a normal UK winter; it's winter wet, not cold, that kills them. They don't need bringing in so much as keeping dry: good drainage, pots raised off the ground, and fleece only for young or tender kinds. Overwintering Mediterranean herbs has the full method, and suggests moving tender pots under cover between late October and mid-November.

The two mistakes that kill overwintered plants

Too much water. In winter, plants use almost nothing. Compost that stays wet and cold rots roots and tubers. When in doubt, don't water.

Too much warmth. A warm, dark room makes plants grow when they can't feed themselves, so they turn long, pale and weak. Cool and light is better.

Get those two right, keep everything frost-free, and most tender plants come through. Not sure what's tender in the first place? Hardy or tender? sorts the common ones, and how to protect plants from frost covers everything that stays outside.