How to Overwinter Dahlias: Lift, Store, Replant

Dahlias flower harder than almost anything else in the late-summer garden — and then the first frost turns them to black mush overnight. The tubers underground are still very much alive, though. Whether you lift them or mulch them, an hour of work in autumn saves you the cost of replacing every plant next year.

Should you lift, or leave them in the ground?

It depends on your winter. Dahlia tubers are killed by freezing and by sitting in cold, waterlogged soil. In mild-winter regions — roughly USDA zone 8 and warmer, or climates where the ground never freezes more than a crust — you can cut plants down after frost and leave the tubers in place under a 10–15 cm dry mulch of bark, straw or leaf mould, ideally with something to shed rain. In zone 7 it's a gamble that works best in sandy, free-draining soil. In zone 6 and colder, lift them: a frozen tuber is a dead tuber.

Tip: even in mild areas, lift your favourite or most expensive varieties. Storage indoors is the only method that protects against a freak cold snap and winter slugs at the same time.

When to lift dahlia tubers

Don't rush. Leave plants growing as long as possible — the tubers do much of their bulking in autumn — and lift only after the first real frost has blackened the foliage. Then, if you can, wait one to two more weeks before digging: the skins toughen and store better. If hard frosts arrive early or your ground is very wet, dig straight away; a slightly thin-skinned tuber beats a frozen or rotten one.

How to lift and dry them

• Cut all stems back to about 10 cm and label each plant now — every dahlia looks identical as a bare tuber.
• Push a garden fork in a good 30 cm out from the stem and lever the clump up gently; tubers snap off easily at the neck, and a snapped neck means a dead tuber.
• Shake or brush off the loose soil and turn the clump upside down for a few days somewhere frost-free and airy, so moisture drains out of the hollow stems.
• Trim off fine roots and any damaged or mushy pieces; dust cuts with sulphur if you have it.

Storing tubers over winter

Pack the dried clumps in a crate or cardboard box, buried in a barely damp medium — vermiculite, wood shavings, sand, or old potting compost all work — so the tubers don't touch each other. Store at 4–10°C, dark, frost-free and ventilated: an unheated garage, cellar or shed that never freezes is ideal. Do not seal them in airtight plastic; trapped moisture means mould.

Check the box once a month through winter. Wrinkled, shrivelling tubers need a light mist of water on the packing medium; any tuber going soft or mouldy comes out immediately before it infects the rest.

Warning: the most common storage failure isn't cold — it's rot from packing tubers away wet, or in a heated room where they sweat. Dry first, store cool.

Waking them up in spring

From mid-spring, pot the clumps into moist compost in a bright, frost-free place and they'll sprout within a few weeks; plant out only after the last frost. Large healthy clumps can be divided just before potting — every division needs a piece of the old stem collar with at least one visible eye, because the eyes grow only from the collar, never from the tuber body itself. Division keeps plants vigorous and doubles your stock for free.

Not sure whether that late-season wilt is frost damage or something infectious? Snap a photo in PlantMe and the app will tell you what's wrong — and log a reminder to lift your tubers before winter bites.