Dahlias add height and dimension to borders and containers
Image: Phillippa Lambert

After languishing for a while outside garden fashion, the dahlia’s return to popularity is long-overdue and much deserved! Long flowering (June to early December in a sheltered spot), easy to grow, invaluable in the summer border, and a desirable cut flower — the list goes on! They offer a wide range of flower types, some small enough for terrace containers, as well as a rainbow of warm, vibrant colours. Here’s why every garden should have some dahlias and how to propagate your favourite varieties.

Browse our full range of dahlia tubers for even more inspiration.

Are dahlias hardy?

Officially, the dahlia is a tender, tuberous-rooted perennial, growing from scratch each year from chubby, finger-like roots. In colder areas these roots are lifted in the autumn to protect them from frosts, before being planted out again in the spring when the danger has passed. However, on the Isle of Wight, where I garden, they can be regarded as generally winter hardy provided they get a little root protection against any unusually penetrating frosts.

Where to plant your dahlias

‘Autumn Shades Mix’ is a sumptuous blend of colours and flower shapes
Image: Dahlia ‘Autumn Shades Mix’ from Thompson & Morgan

Dahlias tolerate a wide range of soil types, but will do best in well-prepared, fertile soil, with good drainage, and positioned in full light. For super results, it’s worth incorporating some well rotted manure into the soil, plus a handful of slow release organic fertiliser containing trace elements. Once the flowers start to appear, a high potash feed (I use tomato fertiliser) is very beneficial, given every couple of weeks through to early September.

Lifting dahlias for winter

Dahlia ‘Totally Tangerine’ brings pots and borders to life
Image: Dahlia ‘Totally Tangerine’ from Thompson & Morgan

Playing safe with dahlias for the winter involves lifting them when the first light frost has taken the foliage down. This is also the best procedure if you want more of that variety in the following year as you can split them in the spring to create further plants.

After digging up the tubers, trim the stems back to 6 – 8in, remove the excess soil and allow any surface moisture to dry before transferring them to boxes containing old potting compost, moist sand or vermiculite. These boxes can then be overwintered in a frost-free place, covered with an old blanket or fleece for extra protection until the spring when the growing cycle starts again. It’s wise to check the tubers from time to time while in storage to make sure that none are rotting, or drying out.

Splitting dahlia tubers to propagate them

Dahlia ‘Munchen’ is a semi-cactus variety with vibrant yellow flowers
Image: Dahlia ‘Munchen’ from Thompson & Morgan

In early spring, simply split the tubers ensuring that each piece has at least a couple of good initial shoots. Pot up in general purpose compost and grow them on in a frost-free greenhouse. It’s important to give minimal water until growth starts in earnest. For sturdy plants, five shoots are the maximum you should allow to grow from each tuber; any excess should be pinched out. The main shoot produced from these five will also benefit from the tip being removed after planting out, just as the first flower buds are formed.

If you don’t have space to grow on your split dahlias under cover, then the split pieces (all possessing shoots and roots) can simply be planted out into their final positions in late May. Although this involves less work, these plants will flower several weeks behind those brought on in a greenhouse.

Supporting and caring for dahlias

Potting up overwintered tubers, planting out greenhouse-raised plants and providing a support frame
Image: Phillippa Lambert

Larger dahlia varieties will need some support to prevent gusty summer winds damaging the sappy hollow stems when the plant is heavy with flowers. This support can either be a short stake plus some twine, or make, as I do, a wire cylinder of stock fencing held onto a metal upright by cable ties. These last well from year to year, and can also be used to enclose a little root protection compost in colder gardens to benefit plants being overwintered in the ground.

Besides supporting the plants, other routine care involves deadheading. Having a regular round of deadheading keeps the plant smart and encourages a very long season of bloom. Unfortunately dahlias are beloved of slugs, and so to avoid any disappointment they should be protected with organic slug pellets, nematodes, or a physical barrier right from the outset.

Five favourite dahlias to try in your garden

The main reason dahlias were banished to the horticultural wilderness for so long was the perception that their flowers could be coarse and their colours unsubtle. However recent breeding has brought forth delicate cultivars, intriguing new types, and a much more attractive palette — there are now dahlias for every scheme and every garden style. I particularly value the very dark shades that range from deepest midnight purple to velvety chocolate brown. These lusciously deep tones are a wonderful contrast to the reds, vermillion and apricot shades of the cannas they share space with in my borders. Here are five to try:

Thompson & Morgan

Dahlia ‘Creme de Cassis’

• Two-tone water lily-shaped petals
• Stunning focal point in borders and containers
• Fabulous cut flowers
• Half-hardy variety suitable for overwintering

Thompson & Morgan

Dahlia ‘Nathalie G’

• Perfectly formed pompom shape
• Gorgeous soft salmon petals
• Long stems for vases and bouquets

Thompson & Morgan

Dahlia ‘Cafe au Lait’

• Large dinner plate blooms
• Subtle and unusual colour changes over time
• Luxurious flowers deliver real wow-factor

Thompson & Morgan

Dahlia ‘Nuit d’Ete’

• Easy to grow and very long flowering season
• Dark velvety blooms are elegant and contemporary
• Looks great planted with cannas

Thompson & Morgan

Dahlia ‘Fire Pot’

• Fun, tropical pink and coral petals
• Flowers seem to glow from within
• Blazes with colour until the first frosts

If you want to learn more about growing these beautiful blooms, head over to our dahlia hub page for a wealth of information and advice.

Phillippa Lambert

Phillippa Lambert is a landscape designer based on the Isle of Wight at a unique site in the Undercliff of the Island — a favoured microclimate sheltered by enormous south facing cliffs. In 2002 Phillippa and Stephen Lambert came across the ‘lost’ gardens of a Victorian mansion dating back to the 1820s, managed to acquire part of the site, including the walled garden and ornamental lake, and have since worked on their restoration. The result is not an ‘expert’ garden and does not try for technical perfection in any sense. ‘Make do and mend’ is the keynote — most plants being raised from seed or cuttings— and self-sufficiency is the motivation for all the growing in the walled garden. In essence, this site goes back to the philosophy of ancient gardens in sustaining the body as well as the soul. Read more at Lakehouse Design.