How to grow Dahlia in New Zealand

Where to plant

Plant dahlias in full sun in rich, well-drained soil enriched with compost or blood and bone. They are frost tender and tubers rot in cold, wet soil, so wait until the soil warms. Tall varieties need a sheltered spot or staking.

• Northland and Auckland: plant tubers from September to December. In mild northern gardens tubers can stay in the ground over winter if drainage is good.
• Waikato and Bay of Plenty: plant from October, once inland frosts are over; warm coastal Bay of Plenty gardens can start a few weeks earlier under cover.
• Wellington and Nelson and Marlborough: plant from October to December.
• Central North Island, Canterbury, and Otago and Southland: plant in November or December. Lift and store the tubers dry over winter.
• Central Otago: plant in December and lift tubers before winter frosts.

Choosing a variety

• Dinner-plate types such as Café au Lait: huge creamy blush blooms, a florist favourite; tall and needs staking.
• Bishop of Llandaff: dark bronze-red foliage and single scarlet flowers, an heirloom that bees can easily reach.
• Pompon dahlias: small, perfectly round blooms with a long vase life. Pompon Mix from seed grows to 80 to 120 cm.
• Dwarf bedding dahlias such as Cinderella: compact plants for massed colour and pots, easily grown from seed.

How to plant tubers

1. Wait until stored tubers start to show eyes or shoots and the soil has warmed.
2. Dig blood and bone or compost into the bed.
3. Put in a stake for tall varieties before planting, so you do not spear the tuber later.
4. Plant each tuber whole with the eyes or shoots facing up, with the root end covered by at least 10 cm of soil.
5. Do not water until shoots appear above the soil, as watering too early can rot the tuber.

From seed, sow in trays of seed raising mix in a warm spot from October, lightly covered, and plant out when seedlings are about 5 cm tall and frosts are over.

Watering and feeding

Once shoots appear, water deeply and keep the soil moderately moist, especially in hot weather and while flowering. Mulch 5 to 8 cm deep, keeping it off the stems.

Feed with a flower fertiliser or liquid feed through the growing season.

Pruning and care through the year

• Pinch out the growing tips when plants are about 30 cm tall to make them bushier with more flowers.
• Tie tall stems to stakes as they grow.
• Deadhead regularly to keep flowers coming into autumn.
• In frosty areas, after the first frost blackens the foliage, cut stems down, lift the tubers, let them dry, and store them in a cool, dry, frost-free place until spring.

Harvesting

Cut flowers for the vase in the cool of the morning when they are almost fully open, and put them straight into water. Regular cutting encourages more flowers.

Growing in pots

Compact dahlias grow well in pots at least 20 cm wide with good drainage; tall types need much larger pots and staking. Use quality potting mix, keep it moderately moist once shoots appear, and feed regularly.

Propagation and saving seed

Divide clumps of tubers in spring as they begin to shoot, making sure each piece has part of the old stem with an eye. Dahlias also grow easily from seed, though seedlings vary from their parents.