Tuberous perennials of the daisy family (Asteraceae) from Mexico and Central America, Dahlias are perfectly at home in Sydney. Flamboyant and showy, they bloom throughout summer and autumn and give a tropical, exotic touch to our gardens, as well as in vases!
The single-flowered ones seem to fit best into garden borders and they come in almost every imaginable hue, thanks to the work of plant breeders such as Dr Keith Hammett. Single-flowered ones also attract bees and butterflies. More compact heights of recent hybrids also make them easier to fit into modern gardens. Although those with hot-coloured flowers of red, orange or yellow are ideal for tropical-style gardens, there are also pastel colours of pinks, purples and whites which mingle in well with summer cottage-style plantings of perennial Phlox, Aster, Echinacea purpurea and Pentas.
Some have stunning dark leaves. Dahlia 'Bishop of Llandaff' was one of the first of these, discovered as a chance seedling in the 1920s; many others have followed. Dahlia 'Mt Noddy' is a particularly beautiful form with rich burgundy flowers amidst its chocolate-coloured foliage, and Dahlia 'Moonfire' is a favourite, with single orange blooms flushed with a red at their centres.
They like a full sun position, with rich, moist (but well-drained) soil. Taller dahlias may need some support from cradle stakes, but pinching out the young growth of dahlias can help to produce more sturdy plants. They benefit from being deadheaded regularly during the flowering season, as this will stimulate the production of more blooms. Cut the spent flower and its stem back to the main stem, rather than leaving stalks on the plant. They love regular applications of fertiliser throughout the warmer months, being greedy feeders. Some gardeners recommend the use of tomato food as a fertiliser for dahlias. During very hot periods in summer, the flowers may become smaller. Applications of Seasol may help counteract this. They are excellent cut flowers: seal the ends of the stems by dipping in boiling water. The more compact types can apparently be grown in large pots, perhaps using lower-growing plants around the base to provide a 'skirt' to balance the composition.
Cut them back to the ground around June. They need to be dug up and divided every few years around September (or in autumn after they have died down) and replanted in soil which has been amended with plenty of compost and decayed manure. In cold climates, they need to be stored over winter in a garage otherwise they will be killed by frosts; in most Sydney gardens this is not necessary. However, if you want to free up garden space for some winter/spring annuals, it is perfectly all right to dig them up around the end of May and store them in a shallow box, covered with cane mulch or potting soil until planting them out in September. Alternatively, they could be wrapped in newspaper. Check them every so often to make sure they haven't rotted. If you are leaving the tubers in the ground over winter, it is possible to grow winter/spring annuals or salad plants such as rocket over the top of them - simply scatter some seeds over once the dahlias have been cut down.
When the tubers are divided, each portion needs to have part of last year's stalk attached, or it will not regrow. Dahlias can also be propagated by taking cuttings of the young shoots in spring, which will grow quickly to form new plants. Alternatively, take tip cuttings in February or March. They will grow easily from seed, often self-seeding in the garden, but seedlings will not necessarily produce the same coloured flowers as the parent plants. Snails and slugs enjoy the fresh shoots of dahlias in spring, so these need to be watched out for then. Sometimes mites will cause leaf distortion. I try to turn a blind eye to this and often the problem goes away - as I don't want to spray with strong sprays. If the plants are well fed and watered, there should be few other problems.