Greater knapweed

Greater knapweed: how do you recognise it?

Habit

Loose clump 60–80 cm tall and about 45 cm wide, with stiff, branched stems.

Leaves

Deeply divided into narrow lobes, rough and dark green; the largest leaves form a rosette at the base.

Flower

Purple heads 3–5 cm across, with large, ragged outer florets around a centre of smaller tube florets, from June to September.

Bracts

The knob below the flower is covered with green scales with a dark brown, fringed, horseshoe-shaped edge.

Roots

A deep, sturdy taproot that copes well with drought.

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

Why grow it?

• Large purple flower heads from June to September, showier than those of most wild flowers.
• A top plant for bumblebees, other bees and butterflies.
• The seed heads feed goldfinches and other seed-eaters in autumn and winter.
• Drought-tolerant thanks to its deep taproot: no watering needed after the first year.
• Native and ideal for a flower meadow or a dry, chalky verge.

Greater knapweed: where does it grow best?

• Full sun; in shade it flowers little and flops.
• Sand, loam or clay, neutral to chalky. It grows poorly on acid soil.
• Dry to normal, free-draining soil. It cannot stand wet feet in winter.
• Plant 40–45 cm apart, or sow it on a poor, open verge.

Light

Sun

Soil type

Sand, loam or clay

Acidity

Neutral or chalky

Moisture

Dry or normal

Size

0.8 m tall, 0.45 m wide

Hardiness

Hardy (to -35 °C)

Goes well with

• Field scabious: Lilac flower heads in the same summer, also loved by bees.
• Wild marjoram: A low, pink bee plant on the same dry, chalky soil.
• Lady’s bedstraw: Fine, scented yellow plumes among the purple heads.
• Salad burnet: Fine leaves and small round heads as low filling at the front.
• Common mallow: Purplish-pink flowers that carry on into October.

Greater knapweed: when and how to plant it?

Plant in March–April and September–October.

1. Plant in March–April or in September–October.
2. Loosen the soil and, on heavy clay, mix in coarse sand. Do not add fertiliser or compost: poor soil is better.
3. Dig a hole slightly larger than the pot and set the plant at the same depth as in the pot, without damaging the taproot.
4. Leave 40–45 cm between plants. In grassland, plant them in open patches in the turf.
5. Firm in and water well. In the first year water during long dry spells.

Care

Water

Only in the first year during dry spells. After that never needed.

Feeding

Not needed. On rich soil it becomes floppy and grasses crowd it out.

Spent flowers

Leave them: the seed heads feed goldfinches and give structure in winter.

Mowing

In a flower meadow, mow once a year late in the season and remove the cuttings. Leave a strip uncut for birds and insects.

Support

Usually not needed on poor soil. On rich soil, plant it among other plants.

Tip: Sow or plant it on dry, chalky soil in full sun and do not feed. Leave the seed heads standing in winter: goldfinches love to come and eat the seeds.

Greater knapweed: when to prune it?

Pruning months: March.

When: From late February to late March, before the new rosettes start into growth.

How:

• Leave the spent stems and seed heads standing all winter.
• In March, cut the old stems back to just above the rosette of leaves.
• If you want fewer seedlings, cut off some of the seed heads once the birds have finished with them.

Caution: Do not cut back in autumn: that way goldfinches keep their winter food and insects can overwinter in the stems.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Sowing

Outdoors in March–April or September, where it is to grow on bare soil or in small pots. Cover the seed with a thin layer of soil and keep moist; it germinates readily.

Seedlings

Move self-sown seedlings while young, with a good root ball, because of the taproot.

Division

On older clumps, in March–April, cut off a piece with rosette and root using a sharp spade. This does not always succeed; sowing is easier.

Greater knapweed: how does it get through the winter?

Hardy to −35 °C.

• Very hardy, to about −35 °C: no protection is needed in Belgium.
• In winter a small rosette of leaves remains at the base.
• Leave the seed heads for the birds; wet soil is a greater danger than frost.

Diseases and pests

• Powdery mildew: White powder on the leaves in dry late summers. Harmless; give the plant room and cut off the worst leaves.
• Flopping: On rich soil or in shade the stems become weak. Do not feed, and plant it in full sun.
• Slugs: Eat seedlings and young rosettes. Sow in small pots and plant out only when the plants are sturdy.
• Root rot: On wet, heavy soil the taproot rots in winter. Plant in a dry, free-draining spot.

Through the year

• March: Cut off old stems. Sow and plant.
• April: Sow and plant.
• June: Flowering starts. Bumblebees and butterflies on the flowers.
• July: Full flower. No watering needed.
• August: Flowering. The first seed heads ripen.
• September: Last flowers. Sow. Leave the seed heads for goldfinches.
• October: Plant.
• December: Goldfinches eat the seeds from the heads.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Butterflies

Nectar for butterflies.

Birds

Berries, seeds or insects as food, or a place to nest.

Did you know?

• The large, ragged outer florets are sterile. They only serve to attract insects; the seed comes from the small tube florets in the centre.
• Greater knapweed is the host of knapweed broomrape, a leafless parasitic plant that lives on its roots.
• You can tell it from common knapweed by the leaves: deeply cut in greater knapweed, entire or barely lobed in common knapweed.