Key takeaways
- Wake stored tubers in pots in March and keep the new shoots frost-free
- Dormant tubers can go straight in the ground from mid to late April
- Leafy plants go outside only once the frosts are over
- Pick a sunny spot with rich soil that drains well
- Space most dahlias at least 60cm apart and put stakes in early
Start your stored dahlia tubers in pots in spring, then plant the leafy plants outside once the frosts are over. Dormant tubers with no shoots can go straight into the ground from mid to late April.
Dahlias are tender, so what the plant looks like matters as much as the date. A sleeping tuber under the soil is fairly safe. Fresh green shoots, though, can be killed by one cold night, so they need to wait until it’s properly safe.
Dahlia planting dates at a glance
| How you’re planting | When | What it needs |
|---|---|---|
| Start stored tubers in pots | March or early spring | A bright, frost-free spot |
| Plant dormant tubers straight outside | Mid to late April | Soil that drains well and no shoots showing |
| Plant out leafy pot-grown dahlias | Once the frosts are over | 10-14 days of hardening off first |
| Buy ready-grown plants | Late spring to early summer | A good drink and slug protection |
The RHS dahlia growing guide suggests planting dormant tubers outside from mid to late April. For leafy plants, it says wait until late May, or early to mid-June in colder areas.
When should you start dahlia tubers in pots?
March is a good time to wake up stored tubers under cover. In a pot, you’re in charge of the drainage, and the first shoots are safe from frost and slugs.
Pick a pot wide enough that the tuber fits without squashing its “necks” against the side. Set the old stem upright with the crown just below the compost surface. Water lightly at first. A tuber with no roots yet needs very little, and soggy compost makes it rot.
Keep the pot in a bright, frost-free greenhouse, conservatory or porch. Water more as the leaves and roots grow. If it outgrows the pot before it’s safe to go out, move it into a bigger one so it doesn’t keep drying out.
Crown just under the compost, and keep the new shoots frost-free.
Can you plant dormant tubers straight outside?
Yes. Plant dormant tubers from mid to late April, as long as the soil drains well and is easy to dig. They shouldn’t have long, soft shoots that would end up above ground.
Pick a sunny spot and loosen up compacted soil first. Dahlias love rich soil, but they hate sitting in water. On heavy clay, dig compost into a wide area, not just one hole, or the hole will fill with water like a sump.
Lay the tuber with the old stem upright and cover it so the crown sits just below the surface. Pop a cane or label in to mark the spot, so you don’t hoe through it. Don’t water much until you see growth, unless the soil is very dry.
Our what to plant in April guide has other spring jobs to do at the same time.
When can leafy dahlias go outside?
Once the frosts are over in your garden. That’s often late May, or early to mid-June in colder areas. Your own garden matters more than a date: high ground, shelter and frost pockets can make a big difference over a short distance.
Harden off indoor-raised plants for 10-14 days. Start by putting them out somewhere sheltered for part of a mild day. Leave them out a bit longer each day, but bring them in on cold nights. Shield the soft leaves from strong sun and wind for the first few days.
Before planting, water the pot and put in a stake for tall varieties. Plant at the same level it was in the pot, firm gently and water well. Slugs and snails love young dahlias, so see our guide to stopping slugs eating dahlias for wildlife-friendly ways to protect them.
Dahlias love sun, soil that drains well and a bit of room around them.
Where should you plant dahlias?
Dahlias do best in sun and rich soil that drains well. A sheltered spot helps protect tall stems and big flowers from wind. Avoid deep shade, where they get leggy reaching for light.
Space most plants at least 60cm apart. Check the label, as little bedding dahlias and big decorative ones end up very different sizes. Too close together, and they get stuffy and are hard to stake, deadhead and check for pests.
Put a cane, stake or support ring in when you plant. It’s much easier to guide young stems through a support than to rescue a big plant after a storm. Place stakes carefully so they don’t spear the tuber.
Dahlias look great in a sunny mixed border, a cutting bed or a big pot. In a pot, choose a sturdy one with drainage holes. For ideas on what to grow alongside them, the cottage garden planting plan is a good start.
A simple UK dahlia calendar
February and March
Check stored tubers and cut off any soft or rotten bits with clean tools. Start healthy tubers in pots if you have a bright, frost-free spot. Go easy on the water until they start growing.
April
Get the soil and supports ready. Plant dormant tubers straight into the ground from mid to late April. Keep any leafy plants under cover.
Late May and June
Watch the forecast and plant hardened-off leafy dahlias outside once the frosts are over. Water them in and protect the new growth from slugs and snails.
Summer
Tie tall stems loosely to their supports, water in dry spells and deadhead fading flowers. Feed pot-grown plants as the label says.
Stake tall plants early so the stems are easy to tie in and cut.
Autumn and winter
The first proper frost turns the leaves black. In cold or wet gardens, lift the tubers and store them somewhere frost-free. In mild areas with well-drained soil, you can leave them in the ground under a thick, dry mulch, but there’s more risk in a hard or soggy winter.
How to plant a dahlia step by step
- Check the tuber feels firm and the crown looks healthy.
- Pick a sunny spot where water drains away.
- Dig over the whole area and pull out perennial weeds.
- Set the crown just below the soil with the old stem upright.
- Mark the spot and put in a stake for tall varieties.
- Water leafy plants well, but water dormant tubers only lightly until they grow.
- Keep the area clear so you can spot slugs and new shoots.
If you split a tuber, make sure each piece has a bit of the crown. The fat tubers store food, but the new shoots only grow from buds on the crown.
Caring for dahlias after planting
Water new leafy dahlias regularly while they settle in. After that, give them a good soak in dry spells rather than little splashes that only wet the top. Check pots more often, as they dry out faster.
Tie in tall plants as they grow, using soft ties with room for the stem to thicken. Check hidden ties over the summer so they don’t cut in.
Deadhead regularly by following the old flower stem back to a leaf joint. It keeps the plant tidy and flowering. Picking flowers for the house does the same job.
Single-flowered dahlias are great for bees, as they can easily reach the pollen and nectar. Our bee-friendly garden plants guide shows how to keep flowers going for them all season.
What should you do with dahlia tubers in winter?
Either lift them or mulch them, depending on your soil and how cold your winters get. Lift them if your winters are cold or your soil stays wet. Cut off the blackened stems, then lift gently with a fork, keeping well back so you don’t spear the tubers.
Let them dry off somewhere airy and frost-free, then store them, labelled, somewhere cool, dry and frost-free. Check them now and then over winter and cut off any rot before it spreads.
Leaving them in the ground works best in mild gardens with well-drained soil. The RHS suggests a mulch about 15cm deep. Make sure the crown doesn’t sit in wet soil, and remember mulch lowers the risk but doesn’t guarantee they’ll survive. Our guide to storing flower bulbs and tubers walks you through lifting and labelling.
Lift with a fork from a little way out so you don’t pierce the tubers.
Common dahlia-planting mistakes
Putting leafy plants outside too early
Green shoots are tender. Keep them under cover until the frosts are over, even if a few spring afternoons feel lovely and warm.
Overwatering a dormant tuber
A tuber with no roots yet needs very little water. Use compost that drains well and water more as it grows.
Planting into a wet pocket
Improve the whole bed rather than filling one deep hole with rich stuff. Water sitting around the tuber can rot it.
Adding support after the plant flops
Put the stake or ring in when you plant. Staking late risks snapping stems and spearing the tuber.
Treating every variety as the same size
Check the label and give each one room for its full size. Little and very tall dahlias need different spots and supports.
Browse more seasonal advice in our growing guides, or read the full guide to growing dahlias in the UK.
Frequently asked questions
When should I plant dahlia tubers outside in the UK?
Leafy dahlias go out after the last frost, usually late May. Dormant tubers with no shoots yet can go straight into the ground from mid to late April.
Can I plant dahlias in March?
Yes, in pots under cover. Keep the new shoots somewhere bright and frost-free until it’s safe to go outside.
How deep should I plant dahlia tubers?
Just below the surface. Set the old stem upright with the crown, where the shoots grow from, just under the compost or soil. Don’t bury new shoots.
Do dahlias come back every year?
Yes, if the tubers are kept safe from frost and wet. Lift them and store them somewhere frost-free, or cover them with a thick mulch in a mild garden.
How far apart should I plant dahlias?
At least 60cm apart for most. Check the label, as little bedding dahlias and huge decorative ones need different amounts of room.
Our team produces practical UK gardening guidance from source research, contributor records and established horticultural practice. Editorial standards are overseen by Matt W.
