Dahlia
A dahlia is a tender tuber pretending to be a border perennial, and its year is organised around one date: the last frost, which in the south east is around the middle of April. Everything before it happens under cover, everything after it happens outside.
They are gross feeders and heavy drinkers, they need staking from the day they go in rather than the day they fall over, and they flower until the first frost blackens them — which means a good dahlia is still going in November.
Care
Full sun.
Thirsty, hungry plants through summer once in growth.
Fertile, well-drained soil enriched with organic matter; avoid waterlogged ground, which rots the tubers.
No routine pruning; pinch out the growing tip of young plants to encourage bushier growth and more flowering stems.
Feed weekly with a high-potash liquid fertiliser — tomato feed is ideal — from the time the shoots are well away until flowering finishes. Switch off nitrogen-rich feeds once buds show: they give leaf at the expense of flower.
Divide the overwintered tubers in spring, making sure every division carries at least one eye — a tuber without one will never shoot. Basal cuttings from tubers started early under cover are the way to get many plants from one clump.
Month by month
Months change with the climate: in southern Italy tomatoes go out two months earlier than in England.
- MayPlant tubers out once frost risk has passed.
- JuneStake tall varieties at planting, before they need support. how ›
- JulyFeed weekly through summer to keep flowering going. how ›
- AugustDeadhead continuously — spent flowers left on reduce further blooming. how ›
- OctoberLift tubers after the first frost blackens the foliage, or mulch deeply in mild, free-draining gardens. how ›
Months based on climate: UK. Your choice stays on this device.
- AprilPlant tubers out once frost risk has passed.
- JuneStake tall varieties at planting, before they need support. how ›
- JulyFeed weekly through summer to keep flowering going. how ›
- AugustDeadhead continuously — spent flowers left on reduce further blooming. how ›
- OctoberLift tubers after the first frost blackens the foliage, or mulch deeply in mild, free-draining gardens. how ›
Months based on climate: Northern Italy. Your choice stays on this device.
- AprilPlant tubers out once frost risk has passed.
- JuneStake tall varieties at planting, before they need support. how ›
- JulyFeed weekly through summer to keep flowering going. how ›
- AugustDeadhead continuously — spent flowers left on reduce further blooming. how ›
- NovemberLift tubers after the first frost blackens the foliage, or mulch deeply in mild, free-draining gardens. how ›
Months based on climate: Central Italy. Your choice stays on this device.
- MarchPlant tubers out once frost risk has passed.
- JuneStake tall varieties at planting, before they need support. how ›
- JulyFeed weekly through summer to keep flowering going. how ›
- AugustDeadhead continuously — spent flowers left on reduce further blooming. how ›
- NovemberLift tubers after the first frost blackens the foliage, or mulch deeply in mild, free-draining gardens. how ›
Months based on climate: Southern Italy and islands. Your choice stays on this device.
Common problems
How to spot it. Major threat to young shoots.
What to do. Protect emerging growth.
How to spot it. Ragged holes in petals.
What to do. Trap with upturned pots stuffed with straw.
How to spot it. Fuzzy mould in storage.
What to do. Ensure good ventilation, let tubers dry before storage.
Popular varieties
Questions
Dahlia: is it hardy?
H3: Half hardy. It survives down to −5 °C.
Can it grow in a pot?
Dwarf varieties do well in large pots; taller varieties need a substantial container and firm staking.
Dahlia: when do you prune it?
No routine pruning; pinch out the growing tip of young plants to encourage bushier growth and more flowering stems.
Dahlia: when does it flower?
In the climate of UK it flowers in July–October.
When do you feed it?
Feed weekly with a high-potash liquid fertiliser — tomato feed is ideal — from the time the shoots are well away until flowering finishes. Switch off nitrogen-rich feeds once buds show: they give leaf at the expense of flower. Feeding months, UK: June–August.