How to Grow Dahlias

Few flowers deliver quite as much impact for the effort as dahlias. From neat, pom-pom perfection to huge, dinner-plate showstoppers, they flower relentlessly from midsummer right through to the first frost. They’re also wonderfully versatile - equally happy in borders, large pots, and cutting gardens.

That said, dahlias can feel a bit intimidating to new growers. Do you start with dahlia tubers or dahlia seeds? When is it actually safe to plant them out in the UK? And what on earth does “pinching out” mean?

The good news is this: dahlias aren’t difficult - they just need the right timing. Get that right, and they’ll reward you with months of colour. This guide covers everything you need to know, from planting dahlia tubers in pots to overwintering dahlias in the UK, so you can grow them with confidence year after year.

Dahlias thrive in sunny, sheltered spots with rich, free-draining soil.

Before planting:

• Choose a position with at least six hours of direct sun
• Improve heavy soil with garden compost or well-rotted manure
• Avoid fresh manure, which can cause excessive leafy growth
• Ensure good drainage - dahlias dislike sitting in cold, wet soil
  
  

If your soil is heavy clay, raised beds or large containers will give far better results.

Dahlias can be started from seed or grown from tubers. Both work well - they just suit different goals.

How to Sow Dahlias

To grow dahlia seeds:

1. Sow seeds on the surface of the seed compost
2. Lightly cover with compost or vermiculite
3. Water gently and keep moist
4. Place somewhere warm (18–21°C)
   
   

Seedlings usually appear within 7–14 days.

When to Sow Dahlias

• Indoors: February to April
• Outdoors: Not recommended until late May, once frost risk has passed

Seed-grown dahlias are ideal for bedding schemes and experimentation - many will even form tubers you can lift and store for next year.

When to Plant Dahlias

Timing is everything when planting dahlias in the UK.

• Outdoors: Late April to May, after the last frost
• In pots: March to April, kept frost-free

If in doubt, wait. Cold soil can stall growth and cause tubers to rot.

How to Plant Dahlias

For dahlia tubers:

• Plant with the old stem facing upwards
• Set tubers 10–15cm deep
• Space plants 45–60cm apart (larger varieties need more room)
• Water lightly until shoots appear

When planting dahlia tubers in pots, choose a container at least 30cm wide and use a free-draining compost.

Dahlias are thirsty plants once established.

• Water deeply during dry spells
• Containers will need more frequent watering
• Avoid waterlogging early in the season

Feeding

Feed weekly once buds form using a high-potash fertiliser. This encourages flowers rather than leafy growth.

Mulching

Mulch around plants in early summer to:

• Retain moisture
• Suppress weeds
• Improve soil structure

Keep mulch clear of the main stem.

Weeding

Weed regularly, especially when plants are young. Hand weeding is best to avoid damaging shallow roots.

When to Harvest Dahlias

For cutting:

• Harvest from July until the first frost
• Pick flowers early in the morning or evening
• Choose blooms that are fully open - buds won’t open once cut

How to Harvest Dahlias

• Cut stems cleanly just above a leaf joint
• Place immediately in water
• Regular cutting encourages more flowers

Deadheading spent blooms also helps plants flower longer.

• Lots of leaves, few flowers: Too much nitrogen or insufficient sun
• Floppy stems: Stake tall varieties early - before they need it
• Slugs/snails: Protect young shoots - they’re highly attractive
• Frost damage: Dahlias are tender; even a light frost can kill foliage

Overwintering Dahlias in the UK

Dahlias are not frost-hardy, so in most parts of the UK they won’t survive winter left unprotected in the ground. Once the first frost has blackened the foliage, it’s time to decide how you’ll overwinter them.

Lifting and Storing Dahlias (The Most Reliable Method)

For guaranteed success, lifting the tubers is best. After frost damage, cut the stems back to around 10–15cm. Carefully loosen the soil with a fork and lift the clump, taking care not to snap the tubers. Shake off excess soil and leave them in a dry, frost-free area for a few days so the skins can dry and toughen.

Store the tubers in a cool, dark place such as a shed, garage or cellar, packed in dry compost, sand, or wrapped in newspaper. They should be kept just above freezing and checked occasionally through winter for signs of rot or shrivelling.

Leaving Dahlias in the Ground (Mild Areas Only)

In very mild or coastal areas, some gardeners successfully leave dahlias in the ground. This only works in free-draining soil and with good protection. Apply a thick mulch of compost, straw or bark over the crown once foliage has died back. Even then, winter wet is often a bigger risk than cold, so this method is less reliable.

Dahlias may look dramatic, but they’re surprisingly straightforward once you understand their rhythm. Get the timing right, give them sun and food, and they’ll perform tirelessly until autumn.

Whether you’re growing dahlias from seed, planting tubers in pots, or planning ahead for overwintering dahlias in the UK, they’re one of the most rewarding flowers you can grow.

Browse our full range of dahlia tubers, seeds, pots and growing essentials and get planting this season. Order now for fast UK delivery.