Dahlia
Dahlia
Dahlia × pinnata
📋Quick Facts
Water
💧💧 Average watering
Hardiness
Zone 7-10
About
Plant dahlia tubers from April under cover or after the last frost outdoors (mid-May to early June depending on region). Dahlias are frost-tender (RHS H3) but the tubers themselves can overwinter in mild south-coast soil under a thick mulch; everywhere else, lift them in November after the first frost blackens the foliage and store dry. The UK summer suits dahlias perfectly — cool nights, long days, plenty of rain. Slugs are the dominant problem on young shoots; protect with nematodes, copper rings, or hand-picking at dusk. Many forms — dinner-plate, decorative, cactus, pompon, single — and dahlias make exceptional cut flowers. Deadhead through summer to keep the flowers coming. Bishop of Llandaff, Café au Lait, and Karma series are reliable UK starters.
How to grow dahlia
- 1
Buy tubers or pot-grown plants
Tubers from January onwards from garden centres and seed merchants. Or buy pot-grown plants from May. Tubers are cheaper; pot-grown plants give an earlier crop and skip the early-spring care.
- 2
Pot tubers up from April
Plant tubers in 2-litre pots of multi-purpose compost in a frost-free greenhouse or porch from April. Eye facing up; cover with 5 cm compost; water sparingly until shoots appear. This head-start gives flowers from July rather than late August.
- 3
Prepare a sunny rich bed
Full sun, fertile, well-drained soil. Fork in well-rotted manure or compost. Dahlias are hungry; a poor bed gives weak plants and few flowers.
- 4
Plant out after last frost
Mid-May in southern England; early June further north. Harden plants off for 7–10 days first. Space 60 cm apart for medium varieties, 1 m for dinner-plate types. Stake at planting (in afterwards damages tubers).
- 5
Protect young shoots from slugs
Slugs love emerging dahlia shoots. Apply nematodes (Phasmarhabditis hermaphrodita) to warm moist soil before planting. Beer traps, copper rings on the bare soil around each plant, hand-picking at dusk in damp weather. Once shoots are 30 cm tall, slug pressure drops.
- 6
Pinch out the growing tip
When plants are 30 cm tall, pinch out the central growing tip to encourage branching and more flowers. Don't pinch dinner-plate varieties — you want the show-stopping single bloom on each stem.
- 7
Feed and water
Water deeply twice a week through July and August. Weekly high-potash liquid feed from the first buds. Mulch heavily.
- 8
Deadhead continuously
Cut spent flowers back to the next bud. Deadheading is the single biggest factor in long flowering — dahlias stop blooming if seed heads form. Cut for vases too; the more you cut, the more they flower.
- 9
Lift tubers in November
After the first frost blackens the foliage, cut stems back to 10 cm, lift tubers with a fork, brush off soil, label, and store dry in a frost-free shed in trays of dry sand or vermiculite. In mild south-coast gardens, you can leave them in the ground under a thick mulch of bracken or straw.
Common questions
Pest Resilience
Earwigs, aphids, slugs, and capsid bugs all target dahlias; tubers attract vine weevil.
The dahlia year in your garden
How to Propagate
Hardiness Zones
USDA 8–9 equivalent