How to Grow Dahlias

Dahlias are popular for their stunning, unique flowers, but gardeners often find them intimidating to grow. Our dahlia care guide gives you all the information you need to grow dahlias successfully year after year.

Dahlias are some of the most popular and rewarding flowers to grow in your garden, giving you masses of showy blooms throughout summer and autumn. They sometimes have a reputation for being finicky, but as with many plants, they'll thrive if given the right conditions.

In this guide, I'll walk you through planting dahlia tubers and plants, caring for and supporting growing plants, and give you options for what to do with your tubers over winter. You'll also find out how you can propagate your dahlias to create new plants.

If you're struggling to choose from the huge variety of dahlia tubers on sale, read our companion guide which explains the different types of dahlias and how to choose dahlia flowers for your garden.

 

Where to plant dahlias

Dahlias are sun-loving flowers that like moist, fertile, but well-drained soil. They are prone to wind damage, so choose a sheltered spot and stake or support tall cultivars and those with large flowerheads. You can grow smaller cultivars in containers, but larger varieties are best planted in the ground.

Depending on the space you have available, you can plant a single dahlia as a statement plant or group multiple plants together for a cut flower bed. Dahlias also grow well with annual flowers such as cosmos, marigolds and sunflowers in a mixed border. Plant taller dahlias at the back of borders or in front of a fence or wall for extra wind protection. 

 

When to plant dahlias

You have two options when planting dahlias. The first method is to pot up tubers in March or April and keep them inside or in a frost-free greenhouse. This encourages shoots and flowers to form earlier. When you move them outside in May or June, the established plants should also be more resilient to slugs and other pests than new shoots.

If you don't have anywhere suitable to keep potted dahlia tubers, you can plant them outside, where they are to grow, at the end of April. This is likely late enough that the soil (and tubers) won't freeze, and by the time shoots emerge, all risk of frost should have passed.

 

How to plant dahlias

Dahlias are tender plants, which means you can't plant them outside until the risk of frost has passed. However, you can wake the tubers up early and give the plants a head start by potting up dahlia tubers in early spring and keeping them indoors or in a greenhouse. You can also take cuttings from the young shoots and grow these on to plant out when the weather is warmer.

How to plant dahlia tubers

Choose a pot that's deep enough for the compost to completely cover the dahlia tubers. It doesn't need to be too big; aim for a snug fit. Dahlia tubers come in all shapes and sizes, so it's handy to have an assortment of pots to hand.

Put some peat-free multi-purpose compost in the bottom of your pot and nestle in the tubers, with the short stem section pointing up. Add more compost until the tubers are completely submerged. Water the pot and place it in a bright, warm spot. 

You should see shoots appearing in 2–5 weeks, at which point you can begin to water the plant lightly. Take care not to overwater your dahlia tubers as this could cause them to rot. To encourage your dahlias to form bushy plants, pinch out the tips of the main shoot when the plant has formed at least three sets of leaves and is around 30 cm tall.

Planting dahlia plants

In late spring or early summer, once temperatures are consistently above 5°C, you can begin to harden off your dahlia plants and gradually acclimatise them to outdoor conditions. Dig plenty of organic matter, such as compost, into the planting area. If you have very heavy soil, you may also want to incorporate some grit to help with drainage.

As a general guide, you want to plant dahlias around 60–75 cm apart, depending on the eventual size of the plants. Dig a hole for each plant and position them so the tops of the tubers are just below the surface, then backfill with soil and water. Adding a layer of mulch helps keep the soil moist, but make sure the mulch doesn't touch the stems.

Planting dahlia tubers outside

Prepare the ground as above and dig a hole deep enough to accommodate your dahlia tubers. Place the dormant dahlia tubers in the hole and backfill to cover the tubers, leaving the stump of last year's stem just protruding. Water and add a layer of mulch. Be prepared to cover your dahlias with fleece or a cloche if temperatures drop suddenly, particularly if there are new shoots.

 

How to support dahlias

Dahlias need different types of support depending on the size of the plant and its flowerheads. It's best to put support structures in place when you plant out tubers or young plants to avoid disturbing the roots. Small varieties of dahlia don't need staking.

Tall dahlias with large flowers benefit from individual, heavy duty stakes. Place these around 30 cm away from the plant and tie in the stem at intervals as the plant grows. When the dahlia leaves grow, the stake will be less visible.

For smaller, less top-heavy varieties, you can use bamboo canes or straight branches. Place 3–4 canes around the dahlia and tie twine around them as a circular support. You can also buy metal dahlia supports. Alternatively, if you're planting a cut flower bed, you can put in stakes around the outside of the bed and stretch pea and bean netting across the bed. The dahlias will then be supported as they grow up and through the netting.

 

Growing dahlias in pots

You can grow many dahlias in pots, but dwarf varieties and those with smaller, lighter flowerheads will be easier to support. Choose a large container – around 30 cm wide for dwarf varieties and 40–50 cm wide and deep for standard cultivars – and make sure it has good drainage. Mix some slow-release fertiliser into multi-purpose compost and fill the pot.

Plant your dahlia plant or tuber with the tubers just below the surface and water well. Unless you're growing a dwarf variety, you're likely to need to add a support. Sturdy sticks or canes inserted around the outside of the pot with twine criss-crossed between them works well for most plants.

 

Dahlia care

Dahlias are hungry, thirsty plants once they get going. To get the best flowers, you need to water plants regularly during dry spells. For dahlias in the ground, water deeply a couple of times a week. Potted dahlias may need watering daily during dry spells. Dahlias grown in containers will also benefit from fortnightly feeding once they begin flowering in July.

Deadheading encourages dahlias to produce more flowers. Cut flowers back to a leaf joint lower down the stem. Ideally, snip off flowers before they die, as once the petals fall off, it can sometimes be tricky to separate new buds and spent blooms.

Overwintering dahlias

You have two options for overwintering dahlia tubers: leaving them in the ground or lifting and storing them.

The best option for most people is to lift the dahlia tubers out of the ground after the first frost, let them dry off and store them in a cool, frost-free location over winter (a dark garage or shed is ideal). You can store the tubers in dry soil, sand, or vermiculite, or wrap them in newspaper. Check the tubers occasionally and remove any soft or rotting tubers.

If you live in a mild part of the UK and you have light, sandy soils you may choose to leave the tubers in the ground over winter. It's a risk, as if you have a particularly cold winter it could kill the plants, but it's much less work than lifting and storing tubers. If you decide to leave your dahlias in the ground, cut the foliage down to near the ground after it's been blackened by the first frosts. Then dump a mound of mulch, such as compost or chipped bark, over the top of each dahlia to help protect it from frost. It's worth putting a label in the ground too, so you don't forget where you've planted it.

 

How to propagate dahlias

You can propagate dahlias by dividing groups of tubers in spring or taking cuttings from young plants. With both of these methods, the flowers will be identical to those of the parent plant. Dahlias grown from seed will be a hybrid of the parent plant and whatever other dahlia the pollinator has visited, so you can't be sure what the flowers will look like until they bloom.

Dividing dahlia tubers

When you divide dahlia tubers, each division needs to include a section of the crown that contains "eyes". The eyes are where new shoots come from, so if you don't have any eyes, you won't get a dahlia plant. The eyes can be hard to spot on dormant plants, so the easiest time to divide dahlia tubers is in spring when the first shoots emerge. You can also divide clumps of tubers in autumn after lifting them.

Use a sharp, disinfected knife or gardening scissors to divide your tubers. To keep the eyes, you need to split the crown of the plant rather than snipping off individual tubers at the neck. As a guide, you need at least a centimeter of the crown attached to a tuber for it to be viable. If your dahlia has shoots, make sure each division has at least one shoot and roots.

Dividing dahlia tubers every couple of years helps keep plants healthy and blooming, as well as creating new plants for your garden.

Taking dahlia cuttings

If you pot your dahlia tubers up in March and keep them undercover, you can take basal cuttings from the new growth. Once your tuber has a number of shoots around 7–8 cm long, you're ready to start.

Select a strong shoot and cut it at the base. If you're able to incorporate a small sliver of the tuber, that's ideal, as it will help the cutting to root, but it's not essential. Remove lower leaves and, if necessary, cut the top leaves in half to reduce moisture loss. Dip the cutting in rooting powder and place it in a pot filled with a 3:1 mix of compost and grit. Water lightly and place in a propagator or cover with a plastic bag.

After 2–4 weeks, the cutting should root and new leaves will appear on the stem. You can then pot up the cuttings and grow them on, planting them out in early summer once all risk of frost has passed.

Growing dahlias from seed

Growing dahlias from seed takes more effort than growing from tubers, but it's an economical way to get a lot of flowers! You can buy dahlia seeds or collect them from your own flowers in autumn and store the seeds over winter. Plants grown from seed will flower in their first year, though they may take a bit longer to get going than plants grown from tubers.

Follow the steps in our Beginner's Guide to Planting Seeds to plant your seeds. Start seeds indoors and work back from when you want to plant them out. Harden off your dahlia seedlings before planting them outdoors.

 

Common dahlia problems

Gardeners and flower lovers are not the only fans of dahlias. Slugs and snails love dahlia shoots, and you'll need to protect young plants when moving them outside to prevent them from being decimated.

Earwigs also enjoy munching on dahlia flowers and foliage. Placing upside down pots filled with straw on canes is one way of getting rid of earwigs – they crawl into the straw to escape the sun, meaning you can move them elsewhere. However, earwigs do feed on aphids, which can also affect dahlias.

Dahlias may be affected by leafy gall or crown gall (either of which may also be referred to as dahlia gall). Galls are caused by bacteria entering the plant through wounds in stems or roots. Crown gall leads to swellings on the tubers and roots, and leafy gall is identifiable by clusters of small, leafy shoots at ground level. Although dahlias will still flower when affected by dahlia gall, it's best to dispose of affected plants to prevent the disease from spreading.

Dahlias can also be affected by mosaic virus and fungal diseases such as Botrytis cinerea. If this is the case, you'll need to destroy the affected plants.

 

Our favourite dahlia flowers

There are so many types of dahlias, it's hard to pick a top five, but these are some of my personal favorites. Find out more about the different types of dahlia flowers in my guide to dahlia types.

Dahlia 'Thomas A. Edison' has large, decorative, purple flowerheads, up to 20 cm in diameter. This "dinner plate dahlia" looks dramatic on its own or combined with deep red and black dahlias for an elegant, moody display. 

Dahlia 'My Love' is a pure white dahlia with rolled petals that give the large flowers a spiky appearance. Pair it with pink and purple varieties for a classic cottage garden look or with a black dahlia for maximum contrast.

Dahlia 'Lake Ontario' is a striking decorative dahlia with full double blooms. The golden yellow blooms are flushed with red and have a defined red edge, making them a wonderful addition to a warm, late-summer border.

Dahlia 'Duet' has large flowerheads with unusual white-tipped red petals and strong stems, which make it a favourite dahlia for cut flowers and shows.

Dahlia 'Franz Kafka' is a delightful pompon dahlia with small, spherical pink flowerheads. These playful blooms are the perfect flowers for buttonholes or posies, as they last a long time once cut.

 

FAQ

Are dahlias perennials?

Yes, dahlias are classed as tender perennials in the UK. This means they will grow back year after year, but they can't always survive winter conditions outside. The safest way to look after your dahlias is to life and store the tubers over winter.

How do you plant dahlia tubers?

Plant dormant dahlia tubers in a pot of peat-free multipurpose compost in March. Choose a pot that fits the tubers snugly, but is big enough to fully submerge the tubers. Place the tubers with the stem up, cover with compost and lightly water. Keep in a bright, frost-free place until outdoor temperatures are above 5°C, when you can harden off the plants and plant them outside.

Are dahlias poisonous to dogs?

Dahlias are mildly toxic to dogs and cats and could cause skin irritation, vomiting and diarrhoea. If you're concerned about your pet and think they have consumed part of a dahlia, contact your vet for advice.

Do dahlias need full sun?

Dahlias are sun-lovers and need 6–8 hours of direct sunlight a day to grow well. You could try growing them in a slightly shaded area, but the dahlias may produce fewer flowers.

How do I store dahlia tubers?

Store dahlia tubers in a cool, dark, frost-free place over winter; a garage or shed is ideal. Make sure the tubers are completely dry before storing them to prevent rot. You can bury the tubers in dry soil, compost, vermiculite or sand, or wrap them in newspaper. Check them periodically and discard any tubers that feel soft or look mouldy.