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9 July 2021

How to grow dahlias

Dahlias are stars of the late-summer border. Discover our best dahlia varieties and tips for how to grow them.
CT
Ceri Thomas
Dahlia

The flower types and colours of dahlias are wide ranging, from simple singles through symmetrical balls to crazy cactus blooms, and from white all the way through the spectrum to deep red and purple. 

They 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.

How to grow dahlias: month by month

January
February
March
April
May
June


POT UP TUBERS
TAKE CUTTINGS
PLANT
PLANT
July
August
September
October
November
December

FLOWERING
FLOWERING
FLOWERING
LIFT TUBERS

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.

Best Buy pale-coloured dahlias
What it looks like
Variety name
Height x spread
140 x 80cm
This decorative type produced masses of stems topped with 12cm-wide blooms in jolly hues, held high above the foliage. The twisted petals in shades of apricot orange, tipped with white around a yellow centre, put on a vibrant, statuesque show. The stems were a little wayward, but minimal staking kept them in check and the plants didn’t suffer in high winds or rain. This variety gave us a good two months of colour, the blooms lasted well in a vase and the bees loved it, too. Peak flowering: August to October.
What it looks like
Variety name
Height x spread
50 x 80cm
As popular with bees and hoverflies as it was with human visitors to our trial site, this variety was one of the earliest to flower in our trial, producing its first yellow-centred, pinkish-orange blooms in July. Our plants were in full flower by the end of the month and from then on they just kept on giving, with wave after wave of simple, star-shaped flowers atop fine, purple-tinged foliage. By October, the display was less abundant and more yellow in hue. Peak flowering: July to September.
What it looks like
Variety name
Height x spread
120 x 70cm
This previous Best Buy and RHS Award of Garden Merit (AGM) holder is a perfect blend of flowers and foliage. The deep-purple, almost black leaves complement the dark pink and coral-peach flowers that fade through to ice-cream shades of apricot as they age. The flowers, which were a big hit with bees and hoverflies, kept coming all summer long and our plants needed minimal deadheading thanks to the fact the blooms die so neatly. Our plants didn’t need any staking and stood up well to wind and rain. Peak flowering: August to October.
Best Buy single-flowered dahlias
What it looks like
Variety name
Height x spread
120 x 70cm
A classic single-flowered dahlia, this variety has real presence, with the mass of golden-yellow petals around a darker centre set off perfectly by the dark greenish-purple foliage. The elegant blooms were self-supporting so would suit herbaceous borders. They hummed with insects, attracting butterflies as well as bees. Their strong shapes and striking contrasts made them stand out from the more frilly types.  Peak flowering: August to October.
What it looks like
Variety name
Height x spread
115 x 65cm
Judging by the attention this got from insects, the paler colours of this variety are just as attractive to pollinators as more vibrant rivals. The combination of a pale pink corona of petals around a centre ruff of white with a gold centre was very pretty. Despite the delicate flowers, it was a big, bushy variety with green leaves that held the flowers clear to give a great display of blooms that the bees fought over. Peak flowering: August to October
What it looks like
Variety name
Height x spread
135 x 65cm
The flower is the star with this variety. The almost playful mix of bright orangey-red rounded outer petals with yellow edging and bright yellow spiky inner petals around a gold centre is eye-catching. It was popular with pollinators, too, attracting bees and other insects. The plants are quite tall and leggy, and would need support if used at the edge of a border. However, they gave us continuous colour until the first frost in November.  Peak flowering: August to October.
What it looks like
Variety name
Height x spread
120 x 70cm
This previous Best Buy and RHS Award of Garden Merit (AGM) holder is a perfect blend of flowers and foliage. The deep-purple, almost black leaves complement the dark pink and coral-peach flowers that fade through to ice-cream shades of apricot as they age. The flowers, which were a big hit with bees and hoverflies, kept coming all summer long and our plants needed minimal deadheading thanks to the fact the blooms die so neatly. Our plants didn’t need any staking and stood up well to wind and rain. Peak flowering: August to October.

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.

Caring for your plants

Starting into growth

Dahlia tubers are usually available to buy from late February and need to be started off in the greenhouse in a Best Buy compost for raising young plants. 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.

Taking 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. 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. 

Planting

Dahlias need rich soil in a sunny spot, so dig in spent mushroom compost or garden compost before planting. Plant out when all risk of frost has passed and setup supports as early as possible, tying in shoots as they grow.

Planting dahlia

Deadheading

Keep on top of deadheading to encourage more flowers. 

Feeding

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

Cut flowers

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 are tender 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.

Preparing dahlia tubers for winter

Common 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 slugs and snails.

Earwigs

Earwigs can nibble the flowers and leave them looking tatty. Shake the flowers to remove them.

Read more about earwigs.

Powdery mildew

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

Read more about powdery mildew.