After languishing for a while outside garden fashion, dahlias are suddenly being recognised again for their very many useful attributes. Long in flower (June to early December in a sheltered spot), easy to grow, invaluable in the summer border, 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. The dahlia’s return to popularity is long-overdue and much deserved!
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 from frosts, the plants then started out again in the spring when the danger of frost has passed. However on the Isle of Wight, where I garden, they can be regarded as generally winter hardy with a little root protection against any unusually penetrating frosts.
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 is 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.
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 more of that variety are required for the following year, as they can be split in the spring as growth starts from the tuber, to create further plants. After digging the tubers, trim the stems back to 6 – 8in, remove 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 is wise to check the tubers from time to time while in storage to make sure that none are rotting, or drying out.
In early spring simply split the tubers ensuring each piece has at least a couple of good initial shoots, pot up in general purpose compost and grow on in a frost free greenhouse. It is important to give minimal water until growth starts in earnest. For sturdy plants, five shoots are the maximum to 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 nowhere under cover is available to grow on dahlias that are being split to propagate, then they can simply be planted out in late May in their final positions, each split piece possessing shoots and root — although involving less work, these plants will start to flower several weeks behind those brought on in a greenhouse.
The larger varieties will need some support to prevent any gusty summer winds damaging the sappy hollow stems when the plant is heavy with flower. 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, so to avoid any disappointment they should be protected with organic slug pellets, nematodes, or a physical barrier right from the outset.
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 through 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.
Potting up an overwintered tuber in spring
Planting out after growing on in the greenhouse
Placing the wire support that will be quickly covered by the growing plant
Using the wire support to enclose compost for protection of a dahlia left in the ground over winter
Some Favourite Varieties:
If you enjoyed this article, and want to learn more about growing dahlias from tubers or seeds and how to care for these gorgeous blooms, head to our dahlia hub page.
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.