Do dahlias come back every year? How to overwinter, lift and store dahlias

Dahlias do come back every year, as long as the tubers under the soil get through the winter. The flowers, leaves and stems die back at the first frost, but the tubers can live for many years and grow a new plant each summer. The catch is that they can't survive being frozen, and they rot in cold, wet ground. So the real question isn't whether dahlias grow back every year, but whether yours will survive where they are, or whether you should lift them and store them somewhere frost-free.

Dahlias are tender perennials, not annuals

Originally from Central America and Mexico, dahlias are tender perennials. The RHS describes them as plants that can live for many years but need protection from freezing temperatures. Each plant grows from a clump of swollen roots called tubers, which store food through the winter while the top growth is dormant.

The RHS rates dahlias such as 'Bishop of Llandaff' H3, which means half hardy: down to about -5°C, hardy in coastal and mild areas except in hard winters, and at risk from sudden early frosts. A plant rated H3 can survive with artificial winter protection, such as a thick layer of mulch, but it isn't reliably hardy in an ordinary or cold garden.

So will dahlias come back after a winter outside? Only if the tuber stays alive: a dahlia comes back year after year as long as it does. If the tuber freezes or rots, it won't grow back, however healthy the plant looked in summer.

Should you leave dahlias in the ground or lift them?

There are two ways to overwinter dahlias: leave them in the ground under mulch, or lift the tubers and store them indoors. The RHS calls leaving them in "by far the easiest option, but it is a risk", and suggests it only if you live in a milder region or have light, free-draining soil. Lifting and storing dahlias is the safer choice, especially in colder areas and on heavy soil.

When leaving dahlias in the ground can work

Dahlias need a warm, sunny, sheltered spot and fertile, free-draining soil, and the same conditions that suit them in summer help them through winter. On light, free-draining soil in a mild garden, dahlias left in the ground often survive. Once frost has blackened the foliage, cut the stems down and cover the crowns with a thick layer of mulch, about 15cm (6in), using straw, garden compost, chipped bark or well-rotted manure. Plants overwintered this way may come into growth and flower a little later than tubers that were lifted, stored and replanted.

Why heavy clay and cold gardens push you towards lifting

The RHS notes that dahlias grow well in heavy soils, but that these tend to be colder and soggier in winter, so the tender tubers should be dug up at the end of autumn and stored indoors. That matters here: Durham County Council's landscape guidance describes the soils over most of the county as heavy, poorly drained gleys formed from glacial boulder clay. A tuber sitting in cold, wet clay from November to April is at real risk of rotting, even if a winter is never hard enough to freeze it.

Cold matters as well. Across the North East, the typical first autumn frost on our frost dates data ranges from late November on the coast at Sunderland, Seaham and Hartlepool to late October in Durham city and Barnard Castle, with frosts coming earlier and lasting longer on higher ground. If you garden on clay anywhere inland in County Durham, I'd lift your dahlias rather than gamble on them.

When to lift dahlia tubers in the North East: after the first autumn frost

Dahlias are lifted once the foliage has been blackened by the first frost. The blackened leaves show at a glance which plants are ready. If the frosts are late, the RHS advises digging up the tubers by late November anyway, which matters on the North East coast, where the typical first frost often doesn't arrive until the last week of November.

Inland it usually comes much earlier. Check your first autumn frost date for your postcode, and note the early date as well as the typical one: in Durham the typical first frost is around 30 October, but in 1 year in 10 it comes by 6 October. Have your crates, labels and storage space ready by that early date, and bring dahlias in pots under cover by then, so that you can lift the border plants as soon as the frost has blackened their leaves.

How to lift and store dahlias, step by step

Choose a dry day for lifting dahlia tubers if you can, so the soil falls away from the clump more easily.

1. Label each plant while you can still tell them apart, so you know which variety is which in spring.
2. Cut the stems back to 5 to 15cm (2 to 6in) above the tubers.
3. Dig up the tubers with a garden fork, starting well out from the stems and levering the clump up gently. Bruised, cut or snapped tubers can rot in storage.
4. Shake and rub off the soil by hand, and trim off any damaged tubers and thin roots.
5. Dry the tubers upside down in a cool, dry place for a few weeks, so the moisture drains out of the hollow stems.
6. Store your tubers in shallow crates or open-topped boxes, covered with dry spent compost, sand, straw or newspaper, with only the old stems showing.
7. Keep your tubers dry and frost-free in a dark, cool, well-ventilated place such as a shed or garage. The RHS defines frost-free as a night-time minimum of 4°C.

Don't divide dahlia clumps before winter storage: any cut is an entry point for rot. Leave the clump whole and divide your dahlia in spring instead.

Checking stored tubers over winter

Storing dahlia tubers is mostly a matter of keeping them dry, cool and frost-free, and looking at them now and then.

Overwintering dahlias in pots

If you're growing dahlias in containers, they're more exposed in winter than those in the ground. The RHS says the main danger for plants in containers in winter is the compost freezing, which may kill them, and wet compost makes it worse. You have two options:

• Lift the tubers out of the pot after the first frost and store them in the same way as tubers from the border.
• Keep them in the pot: cut the stems back and move the whole pot somewhere dark, cool and frost-free, such as a garage or shed. The same rules apply as for stored tubers: keep the compost dry until spring.

A pot left out on the patio through a North East winter is the most at risk. If it has to stay outside, group it with other pots against the house wall, remove the saucer and move it under cover if the compost becomes sodden.

Replanting dahlia tubers in spring, after the last frost

Stored tubers are best planted in pots in spring and kept under cover, in a greenhouse, porch or bright windowsill, so they get growing early. Keep them frost-free, in good light and only slightly moist; too much water at this stage can rot them. If you want more plants, divide clumps in early spring (February) in a shallow tray: each piece needs a bit of the old stem with a green bud, plus some roots.

Plant your dahlias out once the danger of frost has passed, which the RHS puts at usually late May, or early to mid-June in colder regions. In the North East, the safe date (when only 1 year in 10 has a later frost) is around 10 May in Peterlee, 18 May in Durham and 20 May in Barnard Castle; look up your own last spring frost before you move them out. Harden them off for a week or two first, and keep fleece handy for a late cold snap.

If you don't have room indoors, you can plant dahlia tubers straight into the border while they're dormant, in mid to late April, with the top of the tuber just below the soil surface. They'll flower a little later than plants started under cover.

Why dahlias might not return

When dahlias aren't growing back in spring, it's usually down to one of a handful of causes.

Slugs deserve a special mention, because a tuber that seems not to have come back may have had its new shoots eaten as they came through. The RHS advises against slug pellets, even organic ones, because of their effect on garden wildlife, and recommends cultural and biological controls instead.