How to grow and care for dahlias

What you need to know about dahlias

Name: dahlia, Dahlia species.

Height: from dwarf 40cm to 1.5–2m giants.

Climate: prefers warm temperate, but can be grown in all climates. In cold areas, plant when threat of frost has passed.

Soil: moist, well-drained soil, improved with well-rotted animal manure, leaf litter or compost.

Position: morning sun and afternoon shade. Dahlias do well under shade cloth. Protect from wind.

Flowering: many delicate petals around a central core in various shades of white, yellow, orange, red, pink and purple, depending on the variety, ranging in size from 5–30cm.

Feeding: regular feeding when plants are 30cm tall with an all-purpose fertiliser. After buds appear, feed with a liquid fertiliser every 10–14 days.

Watering: water after planting in spring, then lightly after plant reaches 20cm in height.

Appearance and characteristics of dahlias

Dahlia is a striking annual plant. Generally grown from dormant tubers, although it’s available in punnets and pots, dahlia is one of the most popular competition flowers among growers. Dahlia types are generally grouped by flower type; some of the more common types include cactus dahlia, anemone dahlia, water lily dahlia, decorative dahlia and ball dahlia. For competition they’re often grouped according to flower size, such as giant flowers with blooms over 250mm in diameter to the small-flowered dahlia, whose flowers have a diameter of up to 155mm, right down to the tiny pompoms, with flowers less than 50mm across.

Uses for dahlias

Dahlia makes a fabulous cut flower, or it’s perfect if you want to try your hand on the flower show competition circuit.

How to plant and grow dahlias

1. Choose an area protected from wind, with morning sun and afternoon shade and moist, well-drained, fertile soil.
2. It’s traditional in cold and warm temperate regions of Australia to plant your dahlia two weeks either side of Melbourne Cup Day. In New Zealand, the tradition is to plant after Labour weekend. In tropical climates, dahlia can be planted in September, provided the area is frost-free. Regardless of where you are, plant when the soil temperature is rising and there’s no danger of frost.
3. Dig over your site with plenty of well-composted animal manure two weeks prior to planting.
4. If you’re planting a tall dahlia, put the stake in first, then lay the tuber down, making sure that the eye (shooting) end of the tuber is raised above the root end.
5. If you live in a wet area, raise the beds to help prevent rotting.

Caring for dahlias

Once your dahlia starts to shoot, it will grow quickly, flowering about 10 weeks after planting. Keep the surrounding area moist and free of weeds. If you’re not picking for the house, deadhead your plants to keep them busy and producing flowers.

How often should you water and feed dahlias?

Water regularly, to keep soil moist as the weather warms up. Weekly feeding with a seaweed-based fertiliser is ideal.

Diseases and pests

Keep an eye out for white fly, two-spotted mites, aphids, thrips, grasshoppers, earwigs, caterpillars and harlequin bugs. Depending on the season, you may have one or all of these annoying and destructive pests. Many of these can be eradicated with an insecticidal soap spray. Resist using anything nasty, as this will only destroy the good insects in your garden, such as bees. Dipel can be used if caterpillars are a problem. Snails and slugs love the juicy green new growth—these can be deterred with pet-friendly snail bait.

How to propagate dahlias

1. Lift your tubers every year after flowering has finished and the foliage has died off, so the tubers don’t rot in the ground.
2. Layer a mix of potting mix and sawdust in a polystyrene box.
3. Place the tubers on top of the mix with the eye pointing up, then cover with about 50mm of the mix. This ensures that the tubers are kept moist, and keeps out the light so that they don’t sprout prematurely.
4. You can use the same method, but in plastic bags instead of a polystyrene box, and hang the bags in the shed in a cool, dark place. Make sure you tag them with the variety name, so you know what you have for next year.

If you like this then try

Peony: cool-climate perennial with large fragrant flowers in shades of pink, mauve, salmon and white.

Chrysanthemum: spectacular blooms in almost every conceivable colour.  

Sunflower: hardy annual with daisy-like flower heads, some growing to 30cm across.

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