Giant knapweed

Giant knapweed: how do you recognise it?

Habit

Broad, upright clump 1–1.2 m tall and about 60 cm wide, with sturdy, sparsely branched stems.

Leaves

Large, oblong to lance-shaped, rough to the touch; up to 30 cm long at the base, smaller higher up.

Flowers

One flower head 5–8 cm across per stem, golden yellow and thistle-like, from June to August.

Bud

Round, 3–5 cm, covered in shiny, brown, papery scales; also attractive dried.

Roots

Deep, strong roots: once established it copes well with drought, but prefers not to be moved.

Flowering: June–August · Plants: March–April and September–October · Pruning: March and August–September

Why grow it?

• Striking, large yellow flower heads that catch the eye in any border.
• An excellent cut flower, and the buds and flowers also dry beautifully.
• Bees, bumblebees and butterflies flock to the nectar and pollen.
• Long-lived, hardy and happy in ordinary to dry garden soil.
• The brown, scaly buds are decorative too, even before the flower opens.

Giant knapweed: where does it grow best?

• Full sun; in shade it flowers less and the stems flop.
• Sand, loam or clay, acid to chalky, as long as it drains well.
• Dry to moderately moist; it dislikes wet soil in winter.
• Allow 60 cm per plant: the clump grows wide and the basal leaves are large.
• In the middle or back of the border, in a spot that is not too windy.

Light

Sun

Soil type

Sand, loam or clay

Acidity

Not fussy

Moisture

Dry or normal

Size

1.2 m tall, 0.6 m wide

Hardiness

Hardy (to -35 °C)

Goes well with

• Globe thistle: Blue globes beside the yellow heads, for the same sunny spot.
• Lady’s mantle: A soft green edging at the foot of the sturdy stems.
• Catmint: A low blue haze at the front of the border.
• Sage: Purple spikes that contrast beautifully with the golden yellow.
• Ornamental grasses: Airy plumes that soften the large leaves.

Giant knapweed: when and how to plant it?

Plant in March–April and September–October.

1. Plant in March–April or in September–October.
2. Dig a deep hole and loosen the bottom too, for the deep roots.
3. On heavy clay, mix in some coarse sand or grit to improve drainage.
4. Set the plant at the same depth as it was in the pot and keep 60 cm between plants.
5. Water well straight away and then regularly until the plant is established.
6. Do not mulch too thickly against the base, so the crown does not stay too wet in winter.

Care

Water

In the first year during dry spells. After that rarely needed, except in long, hot droughts.

Feeding

Not needed; rich soil gives floppy stems. At most some compost in March on poor sandy soil.

Support

In a windy spot or rich soil, a plant support put in place in May helps.

Deadheading

Cut spent heads back to the next leaf or bud for a longer display.

Cutting

Cut for the vase as the flower starts to open; for drying you can also cut the round buds and hang them upside down.

Tip: Give it a sunny spot with at least 60 cm of space and no fertiliser: that keeps the stems sturdy. After flowering, cut everything back for a fresh clump of leaves in late summer.

Giant knapweed: when to prune it?

Pruning months: March and August–September.

When: After flowering, in August–September, cut the whole plant back to just above the ground.

How:

• During flowering, cut spent heads back to the next leaf.
• After the last flowers, cut all the stems and tatty leaves back to just above the ground: fresh new leaves often follow.
• If you want seed for the birds, leave a few heads standing and cut them down in March.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Division

In March–April or September, lift an older clump and divide it with a sharp spade into pieces, each with roots and buds. Old clumps are tough: use a spade or two garden forks.

Seed

Sow in spring in pots in a cold frame and plant the young plants out in autumn or the following year. Seedlings usually take two years to flower.

Giant knapweed: how does it get through the winter?

Hardy to −35 °C.

• Very hardy, to about −35 °C: no protection needed in Belgium.
• The top growth dies back and the plant regrows in spring.
• Heavy, wet soil is more dangerous than frost in winter: make sure drainage is good.

Diseases and pests

• Powdery mildew: White powder on the leaves in late summer, especially in dry weather. Cut the plant back after flowering; the new leaves are usually healthy.
• Slugs and snails: They eat the young shoots in spring. Pick them off in the evening.
• Aphids: On stem tips and buds. Spray off with water or leave them to ladybirds.
• Falling stems: In rich soil or in shade. Do not feed, choose a sunny spot and put supports in place in good time.

Through the year

• March: Clear old debris. Plant or divide.
• April: Plant. Watch for slugs.
• May: Put supports in place in a windy spot.
• June: Flowering begins. Cut flowers for the vase.
• July: Full flower. Deadhead. Cut buds for drying.
• August: Flowering ends. Cut back after the last flowers.
• September: Cut back. Plant or divide.
• October: Plant.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Butterflies

Nectar for butterflies.

Did you know?

• Macrocephala means ‘large-headed’, after its big flower heads.
• The dried buds, with their brown scales, keep their shape and colour for a long time in a dried arrangement.
• It is related to the blue cornflower of arable fields, but is much more robust and lives for many years.