Dahlia merckii

Dahlia merckii: how do you recognise it?

Habit

Tuberous perennial with slender, branching stems about 1.2 m tall and 60 cm wide.

Leaves

Finely cut, pinnate and opposite; green to grey-green and much lighter than on garden dahlias.

Flower

Single blooms about 5 cm across, with eight lilac to pale pink ray petals around a yellow disc, from July to October.

Tubers

Elongated tubers in which the plant stores its reserves. They rot in cold, wet soil.

Family

A daisy relative: what looks like one flower is a compound head of ray and disc florets.

Flowering: July–October · Sowing: March–April · Plants: May · Pruning: November

Why grow it?

• Flowers for a long time: from July until the first frost in October or November.
• An airy, see-through plant that calms down heavy flowers and grasses.
• The single flowers with an open centre give nectar and pollen to bees and butterflies.
• Finer and often hardier than the big garden dahlias: on free-draining soil it can stay outside under mulch.
• Little work: the light flowers do not flop and the plant needs hardly any support.

Dahlia merckii: where does it grow best?

• Full sun in a warm, open spot.
• Free-draining soil: sandy or loamy, neutral to chalky. Humus-rich soil is fine, but it must not stay wet in winter.
• Normal to fairly dry soil; it tolerates short dry spells but flowers more freely with regular water.
• Shelter from strong wind, as the slender stems bend easily. Space plants 50–60 cm apart.

Light

Sun

Soil type

Sand or loam

Acidity

Neutral or chalky

Moisture

Dry or normal

Size

1.2 m tall, 0.6 m wide

Hardiness

Half hardy (to -5 °C): protect it in harsh winters

Goes well with

• Ornamental grasses: They support the slender stems and hide the bare base; purple moor grass and feather grass suit it well.
• Argentinian vervain: Also airy, with small purple flowers in the same period.
• Coneflower: Sturdy pink-red flowers that echo the lilac of the dahlia.
• Autumn aster: Takes over in September–October in the same colour range.
• Sage: Violet spikes at the front of the border, in the same sunny spot.

Dahlia merckii: when and how to plant it?

Plant in May.

1. Plant tubers or pre-grown plants in May, after the ice saints, when night frost is no longer expected.
2. You can also sow: sow indoors in March–April at about 18–20 °C, prick out the seedlings, harden them off and plant out after mid-May.
3. Dig a planting hole 25–30 cm wide; on heavy soil, mix in some sand or compost.
4. Lay the tuber flat with the eye facing up, 10–15 cm deep. Put a sturdy stake in next to it straight away.
5. Water thoroughly once, then wait until the shoots show before watering again.
6. Once it has sprouted, spread a layer of mulch around the plant to prevent drying out.

Care

Water

In dry summers, water generously at the base once a week. An established plant tolerates short dry spells.

Feeding

In June, a little organic fertiliser. Too much nitrogen gives lots of leaves and few flowers.

Pinching out

On young plants, pinch out the tip above the third or fourth pair of leaves for a bushier plant with more flowers.

Spent flowers

Remove faded flowers regularly and the plant keeps blooming until frost.

Support

Among grasses it needs little support; in an open border a stake or pea sticks help.

Tip: Plant it among ornamental grasses: they support the slender stems and hide the bare base. Mulch 10–15 cm deep in November and the tuber will often survive the winter outdoors.

Dahlia merckii: when to prune it?

Pruning months: November.

When: In November, once the first night frost has blackened the foliage.

How:

• Cut all frosted stems back to 10–15 cm above the ground.
• Cover the stump with a thick layer of mulch so rainwater cannot run down the hollow stems to the tuber.
• During the season: cut off faded flowers down to a pair of leaves.

Caution: Do not cut before the first frost: the foliage feeds the tubers until it dies back.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Seed

Sow indoors in March–April at 18–20 °C. Young plants flower in the same summer.

Dividing tubers

In April, when the eyes swell: cut the clump so that each piece has part of the stem base with at least one eye.

Cuttings

Start a tuber into growth indoors in March. Take 5–8 cm shoots and insert them in sandy potting compost; they root in a few weeks.

Dahlia merckii: how does it get through the winter?

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

• Tender below about −5 °C. On free-draining soil the tuber survives under 10–15 cm of mulch (leaf mould, straw).
• On heavy or wet soil tubers often rot: lift them after the first frost, let them dry and store them dry in sand or peat at 5–10 °C, frost-free.
• In a pot: keep the pot dry and frost-free and give hardly any water.

Diseases and pests

• Slugs: They eat the young shoots in April–May. Collect them in the evening and protect the shoots with a ring of sharp sand or grit.
• Rotting tubers: Common after wet winters and on clay. Improve drainage, or lift the tubers and store them dry.
• Aphids: On young tips in spring. Rinse them off with water; ladybirds help.
• Earwigs: They nibble flowers and buds. Hang an upturned pot filled with straw on a cane in the plant and empty it in the morning.
• Powdery mildew: White powder on the leaves in late summer. Give the plant space and air, and cut off affected leaves.

Through the year

• March: Start tubers into growth indoors or sow at 18–20 °C.
• April: Prick out and harden off seedlings. Divide tubers. Watch for slugs.
• May: Plant out after the ice saints, 10–15 cm deep. Put in a stake.
• June: Mulch. Give a little organic fertiliser. Pinch out the tips of young plants.
• July: Flowering starts. Remove faded flowers. Water in dry weather.
• August: Full flower. Water in dry weather. Check for earwigs.
• September: Flowering. Watch for powdery mildew.
• October: Flowering until the first night frost.
• November: After the first frost cut back to 10–15 cm and mulch thickly, or lift the tubers.
• December: Rest. Check stored tubers now and then for rot.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Butterflies

Nectar for butterflies.

Did you know?

• The dahlia is named after the Swedish botanist Anders Dahl, a pupil of Linnaeus.
• In Mexico, where most dahlias come from, the dahlia is the national flower.
• A dahlia flower is really a compound head: the ‘petals’ are ray florets and the yellow centre is made up of dozens of tiny disc florets.