How do I grow dahlias and store the tubers over winter?
Starting tubers into growth, the staking and pinching that produce a good plant, and the lifting and storing routine that gets them through a UK winter.
- Difficulty
- intermediate
- Time
- 1 hr 30 min
- Read
- 4 min
Short answer
Start tubers in pots under cover in April, plant out after the last frost, stake at planting and pinch the growing tip out at four leaf pairs. Lift after the first frost blackens the foliage, dry the tubers upside down, and store them in barely damp compost somewhere frost-free.
Dahlias are tender perennials from Mexico. Everything about growing them in the UK is about getting them started early enough under cover and then keeping the tubers alive through a wet cold winter.
What you'll need
- Dahlia tubers
- Pots and compost
- Stakes and ties
- Slug protection
- Dry compost or vermiculite for storage
- Frost-free shed or garage
Step by step
- Start tubers in pots in April.Pot into just-moist compost with the crown at the surface, in a frost-free greenhouse, cold frame or on a windowsill. Water sparingly until shoots appear or the tuber rots.
- Plant out after the last frost.Late May or early June in most of the UK. Dahlias are killed outright by frost and there is no benefit in going early.
- Stake at planting, not later.Drive a stout stake in before the plant is in the hole. Staking an established dahlia means driving a stake through the tuber.
- Protect from slugs immediately.Emerging dahlia shoots are one of the most attractive things in the garden to slugs, and a plant can be lost overnight. Protect from the day it goes out.
- Pinch out the growing tip.When the plant has three or four pairs of leaves, take the tip out above a leaf pair. It produces a bushier plant with far more flowers.
- Feed with high potash from July.A tomato feed or similar every fortnight once buds appear. Nitrogen-rich feeds give leaf rather than flower.
- Deadhead relentlessly.Cut spent flowers back to a leaf joint. Spent dahlia heads look like buds — the pointed conical ones are spent, the round ones are buds.
- Lift after the first frost blackens the foliage.Cut the stems to about 150mm, lift the tuber carefully with a fork, and shake the soil off. In mild well-drained gardens tubers can be left in with a deep mulch, but lifting is more reliable.
- Dry them upside down.Stand the tubers upside down for a couple of weeks somewhere frost-free so moisture drains out of the hollow stems. Stem rot in storage is the main cause of losses.
- Store in barely damp compost.Pack in boxes of dry-ish compost, vermiculite or wood shavings with the crowns exposed, somewhere frost-free and cool — 5 to 10C is ideal.
- Check them monthly.Remove any that are soft or mouldy before they affect the rest. A shrivelled tuber can be lightly misted; a soft one is finished.
- Divide in spring if you want more.Split clumps when starting them into growth, making sure each piece has a section of old stem with a visible bud. A tuber without a bud will not grow.
Tips
- Stake at planting. There is no way to add a stake later without spearing the tuber.
- Pointed conical heads are spent flowers, round ones are buds. Everyone deadheads the buds at first.
- Dry the tubers upside down — the hollow stems hold water and rot the crown.
Common mistakes
- Watering newly potted tubers heavily — A dormant tuber with no shoots cannot use water and rots. Keep the compost barely moist until growth appears, then water normally.
- Storing tubers in a sealed plastic bag — Condensation forms and the tubers rot. They need cool, dark, frost-free conditions with some air movement and a dry-ish packing medium.
If it doesn't work
Tuber rotted in the pot before shooting
Cause: Overwatered while dormant — Fix: Keep new tubers barely moist until shoots appear.
Tall, floppy plants with few flowers
Cause: Not pinched, not staked, or too much nitrogen — Fix: Pinch at four leaf pairs, stake at planting, and feed high potash.
Tubers mouldy in storage
Cause: Not dried properly, or stored too damp — Fix: Dry upside down for a fortnight, store in dry-ish compost, and check monthly.
Tuber shrivelled and hard by March
Cause: Stored too warm or too dry — Fix: Mist lightly and pot up early. Store cooler next year, 5 to 10C.
Questions people ask
Can I leave dahlias in the ground over winter?
In a mild, well-drained garden, often yes, with a deep dry mulch over the crown after cutting back. In heavy wet soil or a cold garden, lifting is far more reliable.
When do I lift the tubers?
After the first frost has blackened the foliage, usually late October or November. The frost signals the plant to move reserves down into the tuber.
Why did my stored tubers rot?
Almost always water left in the hollow stems. Cut the stems short, stand the tubers upside down to drain for a fortnight, and store them in a dry-ish medium.