Knapweed
Centaurea nigra
Thistle-like purple heads on a tough meadow plant — it wants poor ground and resents being fed.
Light Happy in full sun, and untroubled by light shade.
Water Drought comes and goes without troubling it; no extra watering needed.
Height 60 cm
Spread 11.8 in
Spacing 30 cm
Root depth 3–6 ft
- Type
- Flower · Perennial
- Botanical family
- Mixed / other
- Foliage color
- Green
- Flower color
- Purple and violet
- Sunlight
- Happy in full sun, and untroubled by light shade.
- Watering
- Drought comes and goes without troubling it; no extra watering needed.
- Feeding
- Never feed it.
- Frost tolerant
- Yes
- Flowering
- June
- Pruning
- The only cut it needs is dead stems taken down in late winter.
- Overwintering
- Fully hardy; leave it to the winter.
- Mar–Jun Bud Break
- Jun–Sep Flowering
- Sep–Nov Seed Set
- Nov–Dec Dormancy
- Jan–Mar Dormancy
- Plant spacing
- 11.8 in
- In-row spacing
- 11.8 in
- Between rows
- 15.7 in
- Square-foot density
- Not suited to SFG
- Root system
- A deep taproot
- Working depth
- 3–6 ft
- Can reach
- 10 ft down where the soil allows
- Spread
- 9–15 in across, about 0.75–1.25× the width of the plant
- Shallowest pot
- 24 in of soil
It mines water and minerals from below anything growing around it, which is what makes it drought-proof and what makes it so hard to be rid of: most of these regrow from any length of root left in the ground.
The habit is Knapweed’s own; the figures are what that habit usually comes to at this size, not a measurement of this species. Roots follow the soil either way: a compacted pan, a high water table or heavy clay stops them well short of any of it, and deep open ground lets them go further.
- Powdery mildew Powdery mildew through dry summers — seldom serious, though better air circulation helps if it persists
- Knapweed gall fly larvae (Urophora), which do the plant no harm and point to a healthy patch of ground
Green-leaved flowers, like this one.
The Cottage Garden Companion — the gardener's herbarium at cottagegardencompanion.com