The Dahlia Year: What to Do From Planting to Lifting

The shape of the season

A dahlia year runs from warm soil in spring to the first frost in autumn, and it divides into six phases: waking the tubers, planting, protecting the emerging shoot, pinching, the long flowering run, and lifting or mulching. Every one of them has a trigger you can actually observe, which is why no dates appear here. Soil temperature, frost and the count of leaf pairs are what tell you when to act.

The two phases people skip are protecting the emerging shoot from slugs and pinching the growing tip. Skipping the first costs you whole plants, and skipping the second costs you most of your flowers. Neither takes long, and both happen weeks before there is anything to look at, which is exactly why they get missed.

The two phases people rush are planting into cold soil and lifting before the frost. Both feel like being organized and both cost tubers. Waiting for 60F (15C) soil in spring and for a single blackening frost in autumn is the whole discipline.

Spring: the two weeks that decide the plant

Start by looking at the tubers in late winter. Eyes swelling on the crown mean they are ready. This is when to divide big clumps into single plants, each with a piece of crown and at least one eye, and when to pot them up under cover if your spring is too long and cold to plant directly.

Plant when the soil hits 60F (15C) at planting depth, with the stake driven in before the tuber goes into the hole, and give no water at all until the shoot appears. Then treat the emergence fortnight as the critical period it is: slugs take out more dahlias at this stage than anything else does in the entire year, and the plant has no spare leaf to lose.

As soon as the plant reaches four pairs of leaves, pinch the tip out. It will look like vandalism and it will delay your first flower by two or three weeks. Every stem that appears afterwards is a stem that would not otherwise have existed, and by midsummer the pinched plants are unrecognisably better.

The gap between planting and the first shoot is two to three weeks in warm soil and can be five or six in cold soil. Bare ground during that wait is normal. Mark the spot with a label so nobody digs it over.

Summer: keep it fed, tied and picked

From the first bud until the frost, the pattern repeats every fortnight: a high potassium liquid feed, a check on the ties, and a hard pick or deadhead. Water deeply twice a week at the base of the plant rather than sprinkling daily over the leaves, and keep the mulch topped up so the surface roots stay cool.

Disbudding is the optional layer. If you want long stems and large flowers, take the two side buds off each flowering stem while they are the size of a pea. If you want the maximum number of flowers in a border, leave all three. Whichever you choose, be consistent across a plant.

Cut flowers hard and often. A dahlia that is picked every few days flowers longer and more heavily than one left standing, because seed set is what tells the plant to stop. Cut back to a leaf joint or the next pair of buds rather than snipping the head off and leaving a hollow stub to fill with rain.

1. Every two weeks: feed high potassium Water first if dry, then feed at label strength. Stop entirely a month before your first expected frost so the tubers mature properly.
2. Twice a week: soak the soil, not the leaves Enough water to wet the soil to 12 in (30 cm). Check with a trowel occasionally, because most people water far less deeply than they think.
3. Every few days: pick and deadhead Cut back to a leaf joint. Squeeze anything you are unsure of: buds are round and firm, spent heads are pointed and soft.
4. After every storm: check the ties Wet foliage doubles the weight on hollow stems. Retie anything leaning and cut any snapped stem cleanly back to a joint.
5. Each morning through flowering: empty the earwig traps Straw filled pots on the stakes, tipped out well away from the bed. Set them before the first flowers open rather than after the holes appear.

Autumn: stop, wait for frost, then decide

Stop feeding roughly a month before your first expected frost. The plant should spend its last weeks thickening the tubers rather than pushing soft growth that the frost will blacken anyway. Keep deadheading and cutting for as long as flowers keep coming, because dahlias often run right up to the frost and the last flush is frequently the best of the year.

When the first frost blackens the foliage, cut the stems back to 4 in (10 cm). Then either lift and cure the clumps for dry frost free storage, or, where winters are unfrozen and the soil drains, cover the crowns with 6 to 8 in (15 to 20 cm) of dry mulch and leave them where they are. Which of those applies to you is settled by winter cold and winter wet, not by preference.

Finish the year with the notes. Write down which plants came up first, which needed extra staking, where the slug damage and earwig damage were worst, the date of the first flower and the date of the first frost. Add how many clumps you stored and how many were still sound in spring. Four or five seasons of that record turns dahlia growing from guesswork into something you can plan.

Do not tidy the bed before recording where each plant stood. Once the stems are cut to stubs and the mulch is on, the positions are gone, and next spring you will be forking blind through ground that has tubers in it.