How to grow dahlias and the best varieties

Dahlias thrive in a sunny spot in fertile, well-drained soil. 

They're tender plants so can't be planted out until the danger of frost has passed in late spring, but on light soil their tubers can be left in the ground to flower year after year in late summer and early autumn.

Which? Gardening magazine grew a range of popular varieties to see which would give us the best display.

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Key facts

• Plant type - Tender perennial
• Position - Sun
• Soil - Well-drained and fertile

How to grow dahlias: month by month

How to plant dahlia tubers

Dahlia tubers can be bought from late February and need to be started off in a greenhouse as they're tender and don't like frost. Choose pots twice as wide and deep as the tubers, and pot up so the top of the tuber is just below the compost surface.

Keep moist until shoots emerge and then water as necessary, feeding weekly with a balanced liquid fertiliser. Put them in a coldframe for hardening off before planting outdoors in mid-May to June.

Dahlias need rich soil in a sunny spot, so dig in soil improver before planting. Plant the tubers out when all risk of frost has passed and set up supports as early as possible, tying in shoots as they grow.

If you're growing dahlias in patio pots, use a Best Buy compost for containers and mix in a Best Buy controlled-release feed.

Best dahlia varieties

Which members can log in now to see the full results and which are our Best Buy varieties. If you're not a member, join Which? to get instant access.

Full testing results for dahlias

Pale-coloured dahlias

Single-flowered dahlias

Dark-leaved dahlias

Best dahlias for pots

How we test dahlias

We selected different dahlia varieties, ordering three tubers of each. Dahlia tubers started to arrive from February onwards. We inspected all the tubers on arrival, checking for any soft or rotten areas and making a note of their condition. We potted up all our tubers at the end of March using a Best Buy compost for raising young plants.

We kept them in the polytunnel at the Which? Gardening magazine trial grounds at Capel Manor, north London. During May, we moved our plants to the coldframe to harden off, before planting them into their final positions in a sunny, fertile bed at the start of June.

During the summer, we assessed each variety for flowering impact and duration, attractiveness of foliage, and whether they suffered from any problems. We also tested how long they lasted as cut flowers.

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Caring for your plants

Taking dahlia cuttings

Once your tubers have sprouted, you want about four or five shoots. Any extras can be used as cuttings. Choose ones that are about 10cm tall, cutting them off at the base with a sharp knife. Use shoots that are solid, as hollow ones are less likely to root. 

Trim to a clean cut, reduce the foliage, and insert the cuttings in a Best Buy compost for raising young plants. Seal a clear plastic bag around the pot and keep in a greenhouse or on a bright windowsill. They should root in two to three weeks. 

Deadheading dahlias

Remove the old flowers as they fade to encourage the plant to produce more flowers. It can be confusing about which to remove: flower buds are round and old flowers are pointed.

Try the best secateurs from our tests

Feeding

Give plants a boost with a liquid tomato feed towards the end of summer.

Cutting dahlias for vases

Pick dahlias first thing in the morning or in the cool of the evening, choosing flowers that are fully open but with the petals at the back still looking fresh. Cut above a pair of leaves; axillary buds between these leaves and the main stem will develop to give you more flowers. They should last around four days in a vase. 

Overwintering dahlias

Dahlias grow from tubers and are perennial, meaning they'll flower again for years to come. 

They're tender though and, although on light soils they can be left in the ground covered with a thick mulch, it’s safest to lift the tubers after the first frosts.

 Cut down the blackened foliage, then lift with a fork. Remove soil and leave the tubers upside down in a cool place to dry. Store in a frost-free place in trays or boxes filled with dry sand, soil or compost.

Common dahlia growing problems

Slugs and snails

These two pests both enjoy eating the soft, young foliage. Remove any that you find or use organic slug pellets. Slugs can be tackled using biological control.

Read more about how to get rid of slugs and snails.

Earwigs

Earwigs can nibble the flowers and leave them looking tatty. Shake the flowers to remove them or you can take advantage of their habit of seeking daytime shelter to trap them. 

They like really tight spaces to hide in. Make traps of pots filled with straw or corrugated cardboard and place this on top of stakes or canes holding up affected plants, or around the base of the plants.

Other possibilities are: using lengths of garden hose cut on a slant, hollow bamboo canes, rolls of corrugated cardboard, pieces of sacking placed on the ground, or boards with grooves in them placed groove-side down on the ground. Next morning shake the earwigs out of the traps away from your plants.

Powdery mildew

In late summer, the leaves can get a white powdery coating. It's most common in dry summers.

Read more about how to treat powdery mildew on plants.