How to achieve dazzling displays this dahlia season

Dahlia season is here and if you treat them right they can flower into October and even November.

The big colourful flowers come in a huge range of shapes and sizes, from dwarf collarettes to spiky cactus, dinner plate, pom pom, waterlily and anemone-flowered varieties, all of which have their own charm.

Neil Miller, head gardener at Hever Castle and Gardens in Kent, which houses 300 specimens and 150 varieties of dahlia, will by joined in the autumn by fellow gardening expert Val Bourne to host Dahlia Days at the gardens , and discuss how they have evolved to become a firm favourite.

Miller offers the following tips on how to achieve success with your dahlias…

Plant them in spring

Buy your tubers and start them off in a pot in compost under cover, in a cool greenhouse or under a porch, in spring, covering the crown.

If you’ve only room to place the pots next to the house, cover them with straw or horticultural fleece if there’s a cold snap. Plant them out in June, when danger of frost has passed.

“Plant tubers with just a small bit of the shoot underground, but almost level with your soil surface,” he advises. They prefer full sun, although they will tolerate a little shade.

Feed them

“We feed our dahlias three times between June and September, I’d say once a month. You can use a granular feed or a watering can with a multi-purpose feed,” Miller suggests.

Water carefully

“Don’t water them at all during the dormant season (after they have died back but before new growth appears in spring). Once shoots appear, start to water more regularly,” he advises.

“There’s a lot of leaf growth on dahlias, so when it gets hot the leaves transpire, losing a lot of water through their leaves – and they can start to wilt. In hot weather, water every day or even twice a day in extreme weather.”

If the leaves have wilted, they should recover after you’ve given them a good soaking.

Deter slugs

Miller does not recommend slug pellets. He says at Hever, they use spent coffee grounds, spreading them round the base of the plant, which he says is more effective than crushed eggshells.

Deadhead regularly

“Like roses, deadheading will promote new buds. Once the flowers have started to droop, cut them off,” he says.

Dahlias make wonderful cut flowers which will last a long time in a vase, so don’t be afraid to cut some for indoor use, because that will encourage further blooms, he adds.

Create bigger blooms

“For really big blooms, cut new buds back to the stem, so all the goodness of the plant is concentrated into one or two blooms, which will get bigger. You have to sacrifice quite a few smaller blooms and new growth to encourage a few bigger blooms.”

Be bold with colour

Dahlias in Hever beds and borders come in a rainbow of different colours and look spectacular.

“We’ve always done a mixed match, giving a kaleidoscope look,” says Miller. “One year we did it colour-coordinated, planting all whites together and all reds together, but it didn’t look good. It wasn’t good to photograph and we prefer to mix and match for a stunning display. There are many tri-colour and bi-colour types, which do work if you mix them up.”

Succeed with pots

If you’re planting dahlias in pots, you will need to adopt a stricter watering regime, he advises, as they are likely to need more watering in a pot than they do in the ground. Go for a single colour, he suggests, and choose dwarf varieties such as the Bishop series, which grows to around 60cm tall.

Go for wildlife-friendly

“Many dahlias are tightly compact, so there’s no way that bees and other wildlife can get the nectar. But the collarette type has open, flat, daisy-like flowers, which bees love,” he says. “They are one of the last-flowering plants of the season for the bees to see them through the winter.”

Which types need staking?

Miller says: “Generally, all of them apart from the dwarf ones. At Hever we stake every one, as some of them grow to 6ft. But if you’re planting them in a herbaceous border, the rest of the planting around them should help to prop them up.”

To lift or not to lift?

“I don’t think you need to life dahlia tubers, although if you have a really cold snap there’s a chance some will die off,” he warns. “If you are in a mild area, dahlias can overwinter in a pot. Move the pot to a sheltered spot, cover it with some mulch, either compost or straw, just to protect the crown in a cold snap, then leave it. Don’t water it at all during the winter.”

Dahlias in borders are tough and can also survive the winter, again by covering crowns with mulch, although some people prefer to lift them annually, replanting them the following spring.

“If you are doing this, let the frost get to them, which will make the plant go black and slimy. Then, cut the stems down to about 10cm,” he says. Don’t cut them right down because you can open a wound in the tuber and it can get infected.”

If you lift your dahlias, give the tubers a good wash under a hosepipe to get rid of all the soil and make sure any weeds are removed, then leave in a light, cool place to dry out, he advises. Store them in a shed in a cool place and start the process again in the spring.

Hever Castle ’s Dahlia Days run on October 8 and 9 with Neil Miller and Val Bourne.