Monty Don 6-Step Dahlia Lifting Routine Before First Frost

Cut the stems, then leave the clump in place

Digging on the morning the foliage turns black is the usual mistake. A frost at minus 2C or minus 3C can kill the top growth overnight, leaving a collapse of stems and leaves that looks ready for clearing. Cut the stems back to roughly 10cm to 15cm above soil level with secateurs, keeping a short stub for handling and marking, then leave the clump in the ground for one to two weeks if the weather allows.

During that pause, the tuber finishes drawing energy down and the dormant eyes set near the old stem base. Those eyes sit at the crown, where stem and tuber meet, and they are the only part that produces next year’s shoots. A tuber without an eye is a lump of stored starch that will never grow.

The stub has a second use once the leaves have gone. It marks each clump in a crowded border where three or four varieties have run together and the labels have disappeared to slugs.

Lift the whole mass with a garden fork

Push a garden fork into the soil a good 30cm out from the central stub, work all the way round the plant, and lever gently. A spade cuts straight through the necks where the individual tubers join the crown. A cut neck will not grow, however plump the tuber behind it looks, so lift the whole clump as one mass with the soil still around it.

Dahlia clumps become wider and heavier than many gardeners expect after a season in good ground. A single ‘Cafe au Lait’ or ‘Bishop of Llandaff’ can make a clump the size of a dinner plate. Tip the fork back and ease the weight up from underneath. Pulling on the stem stub can tear the necks.

Once the clump is out, turn it stem-down for ten minutes. Hollow stems can hold water, and water sitting against the crown is one of the commonest routes into winter rot.

Clean enough to inspect the necks

Some gardeners hose every trace of soil away so each neck can be inspected. Others knock off the loose clods and store the tubers with a dry skin of soil still on them. Both methods work. In a damp storage spot, washed and fully dried tubers usually store more reliably, because soft tissue can be seen and removed before it spreads.

Use a low-pressure hose if you wash them, and turn the clump so water does not pool in the crown. Then check every neck with your fingers. A firm neck is pale and resists a light squeeze. A brown, hollow, or squashy neck has gone, and the tuber attached to it will not produce. Snap or cut those away.

Count the eyes while the crown is exposed. A clump that can be split into five fat tubers, each carrying an eye, is worth far more next spring than one huge clump with the eyes gathered in a centre that is beginning to rot.

Dry the tubers before boxing

Lay washed clumps upside down on newspaper or a slatted bench in a frost-free shed or porch. Around 7C to 10C suits them, with air moving over the skins.

A washed tuber packed away wet can rot from the surface inward within weeks, even when the storage medium is otherwise sound. The outside needs time to dry before any box is filled.

Leave the clumps for one to three days. Thick necks and humid air need the longer end of that range. The skin should feel dry to the touch, and cut surfaces should have callused slightly before they go near storage material.

Greenhouse staging works well if night temperatures stay above freezing. A cold garage with a door that lets frost creep in puts the clumps at risk.

If the clump has already been divided into single tubers, dust the cut faces with sulphur powder or leave them an extra day so the wounds seal. The finished tuber should be dry on the outside while staying firm and heavy in the hand. A shrivelled husk has gone too far.

Watch the necks as they dry. A borderline neck that looked doubtful when wet often firms up and proves sound. A neck that stays soft after two days in moving air is finished and should be cut off before it touches its neighbours in the box. Sort sound tubers to one tray and doubtful ones to another, then check the doubtful tray weekly through winter.

Box them in dry, loose material

Use a cardboard or wooden box, add a layer of dry material, lay the tubers so they do not touch, and cover them with more of the same. Vermiculite, dry sand, spent compost, wood shavings, and shredded paper can all work, provided they hold the tubers apart and stay barely moist at most. Soaking-wet peat or compost rots them. Bone-dry material can let them shrivel. The feel should be no wetter than a barely damp wrung cloth.

Label each variety as it goes into the box. A dormant tuber from one dark red dahlia is indistinguishable from another.

Store the boxes somewhere that stays between 4C and 10C and never freezes. An unheated spare room, a frost-free shed, or the cool end of a utility room can work. A garage is safe only if it genuinely remains above freezing on the coldest nights, which many garages do not.

Check the boxes every two to three weeks. Pull out anything soft or mouldy, because rot spreads quickly from tuber to touching tuber. One missed soft tuber can take half a tray with it by February.

When lifting is the wrong call

In mild, free-draining gardens, many gardeners leave dahlias in the ground under a thick dry mulch of bark or straw and accept the occasional loss in a hard winter. On heavy wet clay, lifting wins almost every time, because winter wet around the crown rots the tissue before spring.

One clump becoming five plants

Take a three-year-old ‘Bishop of Llandaff’ that has been left undisturbed and has bulked into a dense clump. Lift it whole with a fork from 30cm out, drain it stem-down, and wash the soil away. What first looked like one plant becomes a crown carrying perhaps eight to ten individual tubers radiating from the old stem base.

Some go straight to the compost heap. In this example, perhaps three have soft, hollow necks. Of the remaining seven, trace the eyes: small pinkish bumps clustered on the crown close to the stem.

Divide the crown with a clean knife so each section carries at least one eye and one or two firm tubers. Sections without an eye are discarded, however fat the tuber, because they will sit in the box all winter and produce nothing.

From one parent clump, five viable divisions may remain, each capable of becoming a full plant. A plant bought once, three seasons earlier, has become five plants for the cost of a knife and a box of dry sand.

The divisions go through the same drying and boxing as whole clumps. Cut crown faces need an extra day to callus, with a dusting of sulphur if the store runs humid. Come March, each division with a visible eye can sit under cover in a pot. The eye remains the small point on which the whole winter’s work depends.