Garden dahlia

How to recognise it

Habit

Tuberous perennial with upright, branching stems of 0.3–2 m, depending on the variety.

Leaf

Opposite, once or twice divided into oval, toothed leaflets; dark green, almost black or bronze in some varieties.

Flower

Flower heads from 5 to more than 20 cm, single, semi-double or double; in white, yellow, orange, pink, red, purple and bicoloured, from July to October.

Stem

Hollow, sappy and rather brittle; tall varieties snap easily in wind or rain.

Roots

A cluster of thick tuberous roots around the base of the stem. The new eyes are only at the top, at the base of the stem, not on the tubers themselves.

Fruit/Seed

Narrow, flat, black seeds in faded flower heads; seedlings rarely resemble the parent plant.

Flowering: July–October · Pruning: June–November

Why grow it?

• Flowers for months, from July to the first night frost, when many other plants are past their best.
• Single and semi-double varieties with a visible yellow centre are rich in pollen and nectar for bees, bumblebees and butterflies in late summer.
• One of the best cut flowers: the more you cut, the more flowers follow.
• A huge choice of shape, height and colour, for borders, pots and cutting gardens.
• You can keep the tubers for years and divide them every spring, so your collection grows.

Garden dahlia: where does it grow best?

• Full sun, at least 6 hours a day; in partial shade you get fewer flowers and floppier stems.
• Fertile, humus-rich, well-drained soil that does not dry out in summer. In wet, cold soil the tubers rot.
• Sheltered from strong wind, or provide sturdy support.
• In a border, vegetable plot or cutting garden, or in a large pot of at least 30 litres on a sunny patio.

Goes well with

• Cosmos: Airy flowers in the same colours, and another bee plant.
• Purpletop vervain: Purple pompons on tall, wiry stems between the dahlias.
• Ornamental grasses: Soft plumes as a contrast to the big flowers.
• Sunflower: A tall yellow backdrop in the cutting garden.
• Michaelmas daisy: Flowers alongside into October, for late butterflies.

Garden dahlia: when and how to plant it?

1. In March–April, pot up the tubers indoors in moist compost in a light, frost-free place so they start into growth. This gives earlier flowers.
2. Plant them out after mid-May, when no more night frost is expected. Tubers that have not been started indoors can go into the ground from early May, 10 cm deep.
3. Space them 50–80 cm apart, depending on height, with the base of the stem just below the surface.
4. For tall varieties, put in a sturdy stake at planting time, so you do not damage the tuber later.
5. Water after planting, but do not keep tubers without leaves too wet: until they are growing, they rot easily.
6. Protect the young shoots from slugs.

Care

Watering

Once flowering starts, water generously in dry weather, preferably in the morning at the base; in pots almost daily in summer.

Feeding

Compost at planting; from June, an organic feed for flowering plants every few weeks, not too much nitrogen.

Pinching out

Pinch out the tip at 30–40 cm: the plant branches and produces more flowers.

Support

Tie the stems to stakes regularly or use a plant ring.

Deadheading

Remove faded flowers every week, back to a leaf. A spent flower head is pointed, a new bud is round.

Tip: For bees and butterflies, choose single or semi-double varieties with a visible yellow centre: they are just as beautiful and far more useful than fully double flowers.

Pruning and mowing

Pruning months: June–November.

When: Pinch out in June, deadhead from July to October, and cut back after the first frost in October–November.

How:

• In June, pinch or cut out the tip above the third or fourth pair of leaves.
• Cut off faded flowers just above a pair of leaves, with a long stem.
• Cut flowers for the vase in the morning, with long stems: more side shoots will then develop.
• After the first night frost, when the leaves have blackened, cut all stems back to 10–15 cm and lift the tubers.

Caution: Water can collect in the hollow stem stumps and make the tuber rot. After lifting, first let the tubers dry upside down for a few days.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Dividing tubers

In April, when the eyes become visible: cut the clump with a sharp knife into pieces, each with a tuber and a piece of stem base with at least one eye.

Cuttings

In March–April, start tubers into growth indoors and cut off shoots of 8–10 cm with a small heel; in cutting compost under plastic they root in 2–4 weeks.

Seed

Sow indoors in March–April; this gives surprising new plants that do not resemble the parent.

Garden dahlia: how does it get through the winter?

Hardy to −5 °C: protect in harsh winters.

• Not hardy: the leaves die at the first frost and the tubers freeze in harder frost.
• Lift the tubers after the first frost, let them dry for a few days, remove the soil and store them dry and frost-free at 5–8 °C, in a box with dry sand, compost or newspaper.
• In mild areas and on very free-draining soil they sometimes survive in the ground under a thick mulch, but that remains a gamble.
• Check the tubers during winter and throw away any that rot.

Diseases and pests

• Slugs: The worst enemy of young shoots. Pick them off in the evening, and start plants indoors so they are sturdier when they go outside.
• Earwigs: They eat holes in the petals. Hang an upturned pot filled with straw on a cane and empty it elsewhere in the garden in the morning: earwigs also eat aphids.
• Aphids: On young tips and buds. Spray off with water; natural predators will clear them up.
• Powdery mildew: White powder on the leaves in late summer. Leave space, remove lower leaves, water at the base.
• Rotting tubers: Stored too wet or too warm. Dry the tubers thoroughly and keep them cool and frost-free.

Through the year

• January: Check stored tubers for rot.
• March: Pot up tubers indoors. Take cuttings.
• April: Divide tubers. Keep started plants light and cool.
• May: Plant out after mid-May with a stake. Keep slugs away.
• June: Pinch out at 30–40 cm. Water and feed.
• July: Flowering starts. Tie in the stems.
• August: Full flower. Deadhead and cut flowers weekly.
• September: Flowering. Deadhead. Water in dry weather.
• October: After the first frost, cut back the stems and lift the tubers.
• November: Dry the tubers and store them frost-free.

Did you know?

• The dahlia is named after the Swedish botanist Anders Dahl, a pupil of Linnaeus.
• The dahlia is the national flower of Mexico.
• Despite more than two centuries of breeding, there is still no truly blue dahlia.