Centaurea nigra
black knapweed or common knapweed
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- Position: Full sun or partial shade
- Soil: Moist but well-drained soil
- Rate of growth: Average
- Flowering period: June to September
- Hardiness: Fully hardy
This native grassland perennial can be found colonising meadows, scrub, and woodland edges. A tough (and often variable wildflower), it will typically produce a whorl of brown/black bracts, which is topped with a light purple-pink (occasionally white) inflorescence. Definitely a worthy contender for a wildlife-friendly garden where birds and bees are welcomed, it will freely self-seed once settled into a good spot.
These wild flowers prefer poorer soils, so the use of fertiliser should be avoided. Once plants start to bloom, removing the spent flowers regularly will help to prolong flowering by encouraging more to form. When flowering has finished, cut the faded flowerheads down to the ground and compost the dead stems.
Surface-sow seeds in trays filled with good quality compost, water and place in a cold frame or greenhouse. Pot on when they have put on their first true leaves and harden off before planting out after the frosts have passed. Alternatively sow directly in late spring or early summer, and thin out to 30cm as they grow. Seeds sown in autumn and over-wintered in a cold frame will produce plants that will flower earlier in the year than those sown in spring.