When to Plant Dahlias, and Whether to Lift Them
Dahlia tubers rot in cold wet soil, so the planting date is set by soil temperature. Starting them early under cover, and the honest case for leaving them in.
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Dahlia tubers behave much like potatoes: they rot in cold wet soil and shoot happily in warm soil. So the planting date is not a calendar date — it is a soil condition.
Plant too early and the tuber turns to mush underground while you wait for growth that never comes.
Timing
Outdoors: two to four weeks after your last frost, once the soil has genuinely warmed. Dahlias are frost-tender at both ends of the season.
Under cover, to gain time: pot the tubers up about a month before that, in just-moist compost, somewhere frost-free and bright. Plant out the growing plants once frost risk has passed. This brings flowering forward by several weeks and removes the rot risk entirely, and it is what most dahlia growers actually do.
Do not water a newly planted tuber
A dormant tuber has no roots to take water up, so watering it simply sits wet compost around it until it rots. Plant into just-moist soil or compost and wait until you see a shoot before watering at all. This is the single most common way dahlia tubers are lost, and it looks like generous care.
Planting
- Depth: 4 to 6 inches.
- Orientation: laid horizontally, with the old stem stub or the visible growing eyes pointing upward. The eyes are on the crown where the tubers join the stem, not on the tubers themselves.
- Spacing: 18 to 24 inches for bedding and border types, up to 3 feet for the very large-flowered varieties.
- Stake at planting. Anything over about 3 feet needs it, and driving a stake in later will spear the tuber.
- Full sun — six hours minimum — and free-draining soil with compost worked in.
Feeding, and why yours may not flower
Dahlias want low nitrogen and decent potassium. Too much nitrogen gives a big handsome leafy plant carrying very few blooms, which is the commonest complaint about them.
The frequent culprit is lawn feed applied nearby, or a general-purpose fertiliser used out of habit. Switch to a high-potassium feed — a tomato feed works well — every couple of weeks once buds form. It is the same nitrogen-early, potassium-late logic as feeding tomatoes.
The other two causes of poor flowering are not enough sun and not deadheading.
Pinching and deadheading
Pinch out the central growing tip when the plant is 12 to 16 inches tall with three or four pairs of leaves. It branches from below, and you get far more flowers on sturdier stems. It delays the first bloom by a week or two and roughly doubles the total.
Deadhead continuously. Once a dahlia sets seed it slows down, so removing spent flowers is what keeps it going until frost.
Telling a bud from a spent flower
This catches everybody out. An unopened bud is round — a smooth ball. A spent flower goes pointed and conical once the petals have dropped, and looks convincingly like a bud about to open. Feel the shape rather than trusting the look, and cut back to a main stem or a pair of leaves rather than snipping the head off on a bare stalk.
Lifting, or not
The traditional instruction is to lift every autumn. Whether you need to depends on two things.
| Your conditions | What to do |
|---|---|
| Ground freezes hard, heavy soil | Lift and store |
| Mild winters, free-draining soil | Mulch thickly and leave |
| Mild winters, heavy wet soil | Lift — wet is the real risk |
| Cold winters, raised beds, sharp drainage | Worth trying with a deep mulch |
| Expensive or irreplaceable varieties | Lift, whatever the conditions |
Wet kills more tubers than cold does. A dahlia in waterlogged clay rots through a mild winter, while one in gritty soil under a thick dry mulch often comes through a cold one.
To lift: wait until frost has blackened the foliage, cut the stems back to about 4 inches, lift carefully with a fork well outside the crown, and shake off the soil. Let them dry upside down for a week or two somewhere frost-free so moisture drains out of the hollow stems — which is where rot starts.
Then pack in crates of dry compost, vermiculite or sand with the crowns exposed, and store at 40 to 50°F, dark and dry. Check monthly and remove anything soft.
To leave in: cut back after frost and cover the crown with 6 inches or more of dry mulch — straw, bark, or bracken — kept in place until spring.
Dividing
Clumps get congested after a few years, flower less, and are worth splitting in spring once the eyes are visible.
Every division must include a piece of the crown with at least one eye. A tuber on its own, however plump, will not grow — the growing points are on the crown, not on the tuber. This is the detail that makes first attempts at dividing fail.
Pests
Slugs and snails are the serious one, and they target emerging shoots specifically. Protect from the moment growth appears; a dahlia can be eaten to nothing in a night.
Earwigs chew petals. They shelter by day — an upturned pot stuffed with straw on a cane is the traditional trap.
Aphids on soft growth; blast off with water and encourage predators.
Common questions
When should you plant dahlia tubers?
How deep do you plant dahlia tubers?
Do you have to lift dahlias over winter?
Why is my dahlia not flowering?
Should you pinch out dahlias?
How do you tell a dahlia bud from a spent flower when deadheading?
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