We did a roaring trade in dahlia tubers at our local flower show at Sheffield in Tasmania in October. Club president Paul Robinson arrived with a picture of a burgundy and white Collarette dahlia and a rubbish bin full of damp compost and sprouting dahlia tubers. They were the result of lifting and dividing several clumps from his garden.
At just a dollar a tuber and the promise of gorgeous red and cream flowers, I spent a lot of the day up to my elbows in compost finding tubers for eager gardeners.
Dahlias are flowers of summer and autumn but, like tomatoes, are planted in spring or early summer after all threat of frost has past. According to gardening lore this magic date for most of southern Australia is after Melbourne Cup Day (the first Tuesday in November), but tubers can be planted from mid October until early December or from late spring to early summer.
How to plant dahlia tubers
Dahlias grow from elongated rather ugly looking tubers. At one end of the tuber (known as the ‘neck’ or ‘nose’) shoots appear from ‘eyes’. These are the growth points that form at the base of last season’s stem.
Tubers are planted in a sunny, sheltered spot. Full morning sun and afternoon shade is ideal. Show-quality dahlias are grown under 50-per-cent white shadecloth to prevent the flowers burning. To enrich the soil to fuel all that furious growth, dig in well-rotted cow manure before planting.
Lay the tuber horizontally with the shoots upwards. The tubers should be buried 10cm deep. As these are tall plants (some varieties are 2−3m high) with brittle stems, the combination of sun and shelter is vital for good growth. Even with shelter, staking is recommended. Use a tomato stake, trellis or a perennial support frame. Put the stake into position then planting the tuber beside it with the neck close to the stake.
For gardeners who don’t like the idea of a garden full of stakes, dahlias can be trained on a trellis or supported with perennial frames. Alternatively grow dahlias as cut flowers in the vegetable garden where a row of stakes doesn’t look out of place. Space them 30cm apart. In the garden, dahlias mix well with roses and herbaceous perennials.
As the dahlias shoot and grow, apply a complete fertiliser. As the stem forms, tie it carefully to the stake and keep adjusting the ties as the stem grows. Tip pruning now keeps the plant more compact. Keep plants well watered right through summer.
The first frost brings these cold-sensitive plants to a halt. Cut down old growth in late autumn. Tubers are lifted and divided in early spring. Left undivided, plants re-grow each year but the clump becomes large and flowers get smaller.
Dahlia history and hybrids
Dahlias were introduced to horticulture from Mexico in the late 18th century and were growing in gardens in Tasmania by 1838 when the first dahlia was exhibited at Launceston.
Early dahlias had single flowers but after centuries of breeding and selection, modern dahlias have large, full flowers in vibrant colours including yellow, orange, red, brown, pink and white. Some are multi-coloured.
There are 10 categories for hybrid dahlias but most are doubles some with broad flat or spiky, tubular petals. Not all the flowers are huge. The Pompon group has small balls of flowers on stout stems.
Popular in gardens today are single-flowered varieties with red, yellow or orange flowers. ‘Bishop of Llandaff’ is a single-flowered form that helped bring dahlias back into general popularity. It has bold red single flowers and red-tinted leaves.
Another variety that fits with modern gardens is ‘Yellow Hammer’, a tall single yellow dahlia, while the variety ‘Le Coco’ has yellow single flowers with a vibrant orange centre.
Pots of these popular single varieties may be available at garden centres in summer in bloom for those who miss planting time. Dahlias will also be on show and for sale at late summer and autumn flower and dahlia shows.
[This article first appeared in TasWeekend (October 29-30, 2016) in The Mercury.]