How to grow Dahlias

Blousy dahlias make wonderful cut flowers Image: Shutterstock

Dahlias are enjoying something of a renaissance at the moment, and it's easy to see why. They come in many shapes and colours: from showy, double-form dahlias that look wonderful in a vase, to simpler, single-flowered varieties, which attract lots of bees. Dahlias are also easy to grow, giving you late-summer colour when many other plants are past their best.

Whether you want beautiful cutting blooms, or fabulous flowers to bring your patio containers to life, there's a dahlia for every purpose and taste. Read our guide to learn how to grow dahlias in your garden.

Where to grow dahlias

Dahlias love full sun and rich, fertile soil Image: Syda Productions

Dahlias do best in rich, fertile, well-drained soil and full sun. They're heavy feeders, so dig in plenty of well-rotted manure or compost before planting, and top-dress with a general-purpose fertiliser.

Seeds, tubers or potted plants?

Plant dwarf varieties 12" apart Image: Peter Turner Photography

Dahlias can be grown directly from seed or dormant Dahlia tubers, or grown in pots to be planted out once they're established. If you're growing dahlias from seed, sow them between February and April. If you're growing from tubers, plant them from April to May. Plant out potted dahlias in May and June when there's no risk of frost.

When it comes to spacing your dahlias, plant taller varieties, like 'Badger Twinkle', about 60cm (24") apart; medium sized varieties, such as 'Candy Eyes', 45cm (18") apart; and dwarf bedding varieties 30cm (12") apart.

How to grow dahlias from tubers

Plant out potted dahlias in late-May or June Image: Luisa Fumi

Plant dahlia tubers directly into the ground from around mid-April. Don't worry about frosts, as the soil will insulate the tubers.

Prepare a planting hole that is 10-15cm (4-6") deep, and wide enough to accommodate the dahlia tuber comfortably. Place the dahlia tuber inside with the 'eye' (where the tubers meet ? often with remnants of last year's stems) facing upwards. Backfill with the remaining soil, gently firming as you go, and water the tubers in thoroughly.

You can also pot up your dahlia tubers in April and grow them on in a greenhouse to give them a head-start; planting them out after all risk of frost has passed.

Growing dahlias in containers

Dahlias make great container plants Image: Krystian Duzynski

Growing dahlias in pots is a great way to brighten up your patio. Choose a container which is at least 30cm (12") in diameter and depth for best results. Remember, dahlias enjoy a rich, fertile soil, so use multi-purpose compost and add a slow-release fertiliser for strong growth. Plant your tubers 10cm deep ? as you would when planting them into the ground.

Frost protection

Make sure your dahlias are protected from more severe weather Image: Berschauer Joachim

Dahlias are half-hardy so they will need protection from frosts. Pot-grown plants should only be planted out after all risk of frost has passed in late May or June. Alternatively, leave your dahlias in the greenhouse all year round. If you planted dahlias in the ground and their shoots emerge while there's still the risk of frost, cover them with horticultural fleece for protection.

Staking dahlias

Stake tall dahlias to prevent them from flopping over Image: Shutterstock

Tall dahlias are prone to flopping over and are easily damaged by the wind. You can prevent this from happening by staking them. Stake dahlias as you plant them because it's easy to pierce the tubers once they are hidden underground. Use bamboo canes or twiggy sticks for a more natural look. Alternatively, use garden obelisks to support your growing dahlias, placing the obelisk directly over the plant.

The supports may look untidy initially, but your dahlias will soon grow to cover them. When you're tying in your dahlias, make a figure of eight with the string to prevent the stems rubbing against the canes.

Summer care

Pinching out dahlias encourages more flowers to grow Image: Flagman 1

Pinching out

Pinching out dahlias once they reach 40cm (16") tall encourages branching and more flowers. Simply pinch off the growing shoot just above a set of leaves. Dahlias can be pinched again once side shoots develop. For giant blooms however, it's best to only allow three to five flower stems to develop.

Feeding

Once flowers appear on your dahlia plants, feed with a high-potash liquid fertiliser, such as tomato feed, every two weeks until early September. Feeding dahlias will encourage more flowers.

Deadheading

Deadhead dahlia flowers as they fade to encourage more blooms to grow. Simply pinch or cut the flower stem back to a pair of leaves to keep the plant looking tidy.

Lifting dahlia tubers for the winter

Look after your dahlias through the winter and they'll reward you next summer Image: Cris D

If you live in a mild climate, you can leave dahlia tubers in the ground all year round, just make sure your soil is well-drained. If your garden suffers from frost, you'll need to lift dahlia tubers with a garden fork once the first cold snap has blackened their foliage. If leaving dahlia tubers in the ground, cover them with a 10-15cm (4-6") layer of bark chips or compost.

Once lifted, clean the tubers and trim off any fine roots. Cut the stems down to 15-20cm (6-8"). Place the tubers upside down for few weeks to dry off completely. When they're dry, put your tubers into boxes or trays of dry sand, peat-free compost, vermiculite, or coir fibre and store in a cool but frost-free place.

It's a good idea to check your dahlia tubers regularly throughout the winter to look for signs of mildew or rotting. Cut out any affected areas with a clean sharp knife.

For more help growing these beautiful, versatile flowers, check out our short how-to video: