If you have them in your garden, spend the autumn and winter months deciding what dahlia colours and shapes will turn our patch into a party in summer.
How to grow dahlias
You can grow these shrubs from seeds, but you end up with a random result. For flowers that meet your colour and shape demands, buy and plant tubers instead.
Aspect
Give them plenty of sun and they will reward you with stunning blooms. But make sure to keep them sheltered from winds, as stems can snap when it’s breezy.
Climate
Cool to temperate. 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.
Soil
Dahlias grow best in moist, rich and well-drained soil with full morning sun, afternoon shade and protection from winds. Before planting, improve the soil by digging in well-rotted organic matter such as compost, leaf mould and manure. Stake tall growers at planting so you don't damage the tuber. They thrive in moist, cool and warm climates. If drainage is a problem, plant in a raised bed or container.
Position
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.
Water
When planting in spring, water generously at first, then only lightly until the plant reaches about 15cm high.
After planting tubers, don’t water until foliage is about 15cm high, or you risk rotting the tuber unless the soil is really dry. Also dig out tubers if you have very wet winters or they’ll rot. Once established, water about twice a week.
Fertiliser
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. Protect soil from hot summer sun with organic mulch and water regularly.
Give low-nitrogen fertiliser in spring. Too much nitrogen encourages leafy growth at the expense of flowers. And that’s the point of growing them!
Pruning
As dahlias grow, pinch out the growing tips to encourage lateral growth and a fuller bush. Regularly cut flowers to promote flowering – they make great cut flowers to display in vases. Flowering lasts 3-5 months. When flowers start to fade in late autumn, cut bush back by half, and when leaves start to fade, cut the shrub right back to ground level.
Planting the tubers
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. 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.
Mass plant small tubers 30cm apart, and larger 75-100cm. Water well, then leave them alone until growth is visible.
Make sure the tubers have eyes – this is the growing point. Lay them horizontally with eyes up, about 15cm into the soil. Plant in late spring. It takes about 6-8 weeks from planting to flowering, which will last well into autumn. Once established, in cool climates dig up tubers in winter and store in a cool, dry, dark place before replanting in spring. In temperate climates they can stay in the ground.
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.
Dividing the tubers
- Dig up the tubers in winter with a garden fork. Throw out the squishy ones. Divide the firm ones so you can double your plants.
- Divide healthy tubers with clean secateurs, making sure each section has a 'neck' and 'eye' (a bit like a potato) where the buds are.
- If you don't get frosts, you can replant them immediately.
- If you get frosts, store in a polystyrene or cardboard box on a layer of damp – not wet – mulch. Cover with humus for 3-4 months until buds emerge. Replant in spring.
Planting seeds
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.
Post-flowering
After flowering has finished, cut back to a third, then by another as it starts to yellow and die down. When bush has finished, cut back to a stub. 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. 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.
Once blooms emerge you can cut them for indoors. This encourages more flowers.
Here’s how
- Cut flowers in the cool of the morning.
- Snip horizontally with clean, sharp bypass secateurs (see page 104), just above leaf nodes.
- Choose flowers that are nearly or fully open as buds won’t open once cut.
- Once harvested, make another horizontal cut at the bottom of the stem and put cut ends in about 10cm of hot water for about an hour. This encourages the blooms to last longer – up to 6 days.
- Strip off leaves that will be below the water line and cut ends at an angle to enable greater water uptake.
- Change vase water every 2-3 days.
Bring them indoors
Get the most from your dahlias
These tips will have your brilliant garden favourites thriving for months on end.
Always a cut above
Cutting dahlia stems just above the leaf node means you’ll get more growth. Nodes are where most of the plant’s best healing and growing tissue lives.
Double the delight
In winter you can lift and divide dahlia tubers. If you get frosts or your soil is very cold and wet during winter, the tubers can easily rot, so store in a cool, dark, dry spot until spring.
High-stakes gardening
Dahlia bush heights range from dwarfs – about 20cm – to 1.5-2m. Taller varieties need to be staked so they don’t fall over. Put in stakes as you plant so you don’t pierce the tuber later on.
Pick your shape
1. Cactus
The typically narrow, very pointed, almost spiky petals are seen with 'Jessica'.
2. Dark leaf single
The simple open flowers come in dynamic colours, such as this 'Clarion'. Bees love the easy access to this pollen.
3. Single or butterfly
'Bonnie Skye' is a dwarf dahlia that is perfectly pert and pretty for pots.
4. Dinner plate decorative
If you want to go big, then also go bold with the 30cm wide blooms of 'Kotara'.
5. Orchid
6. Decorative
The broad petals of 'Winkie Cavalier' hide the central disc.
For more growing information and to find out about what dahlia events are on in your state, go to the Dahlia Society of Australia.
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