When few flowers are blooming through autumn, there’s one that will continue to shine on: the dahlia. Once considered old-fashioned, dahlias (Dahlia sp.) are making a comeback to the gardening world – it’s a wonder they ever left! Their flamboyant blooms burst onto the scene in late summer, lasting well through the autumn months, and come in a spectacular mix of yellows, reds, oranges, pinks and purples. They’re also available in many forms and different sizes, ranging from dwarf 40cm to 1.5-2m giants.
You can grow dahlias from seed or tubers – both will happily thrive in a pot or planted in the garden. And, with reasonable care, they will produce a mass of colourful blooms for years to come.
Dahlias prefer warm areas but can be grown outdoors in all climates. In colder climates, tubers should be planted only when the last frost has passed.
These plants are sun-lovers, so choose a sunny position in the garden where they’ll get at least six hours of sunlight. Ensure they are protected from strong winds.
Dahlias grow best in moist, well-drained soil. Before planting, improve the soil by digging in well-rotted organic matter such as compost, leaf mould and manure.
The easiest way to grow dahlias is from tubers, which are sold packaged (like bulbs) in late winter and spring. They look great planted en masse, so prepare an area of garden well in advance. And if you have friends with dahlias, maybe you could encourage them to share! The tubers gradually multiply beneath the soil, and if you dig up the plant when it’s dormant over winter, you can divide the tuberous roots and replant the divisions (see How to divide dahlias).
Plant tubers at a depth of 10cm and about 30-90cm apart, using closer spacing for smaller varieties. Ensure the growing tips are facing up. Tall-growing dahlias will need staking, so it’s a good idea to put the stake in at planting time, when there’s little chance of damaging the tubers.
You can also grow dahlias from seed, which is usually sown in spring, though in tropical and sub-tropical zones you can sow from mid-winter. Seeds can be started indoors or sown directly into the ground. To start seeds indoors, use a well-drained seed-raising mix and position in a warm spot, away from direct sunlight. Once the plants have two or more sets of leaves, transplant into pots or garden beds.
When planting in spring, water generously at first, then only lightly until the plant reaches about 15cm high.
When the plants are about 30cm tall, apply a general all-purpose fertiliser. Spread it in a circle around the plant, then fork it in and water thoroughly. When buds appear, give liquid feeds of soluble fertiliser every 10-14 days to promote large blooms and longer flowering.
As dahlias grow, pinch out the growing tips to encourage bushy growth. Regularly cut flowers to promote flowering – they make great cut flowers to display in vases.
After flowering has finished, leave plants to die down. If your soil doesn’t remain wet or cold for too long over winter, leave tubers in the ground and they will re-shoot the following year. For colder climates, you’ll need to lift and store the tubers (see How to divide dahlias). Or, if your dahlia is in a pot, leave it to die back naturally and pop the pot in a shady spot or, in frosty areas, in a garden shed. Replant tubers once the weather warms up, in late winter to mid-spring.