Centaurea nigra aka Spanish Buttons

Centaurea nigra is a perennial in the daisy family, Asteraceae. Common names include common knapweed, black knapweed, and hardhead. It is native to Europe, including Britain and Ireland. It grows in temperate regions, in grasslands, along roadsides, and on cliffs. Records place it at elevations up to 1,970 feet [600 metres]. In gardens, it suits meadows, wildlife plots, and woodland edges.

This plant forms clumps and has branching stems. Plants often reach 2.5–3 feet [0.75–0.9 metres] tall. The listed garden range reaches 3.3 feet [1 metre]. Clumps can spread to 1.6 feet [0.5 metres] across. Plants take two to five years to reach full size. The leaves are simple or pinnately lobed. The plant loses its foliage rather than keeping leaves year-round. Purple flowerheads look like those of a thistle. Each head can span about 1.6 inches [0.04 metres]. Dark, almost black bracts stand out around the heads.

Choose a sunny site with fertile soil that drains well. Partial shade is also suitable, according to the RHS. The plant grows in chalk, clay, loam, or sand. It accepts acid, neutral, and alkaline soils. Moist soil with good drainage suits it, but dry ground is tolerated. Established plants can cope with long periods of little care. The RHS gives it a hardiness rating of H6. Plants For A Future lists USDA zones 4–8. These ratings describe a plant suited to cold outdoor gardens.

Raise new plants from seed; routine pruning is not needed. It seeds freely and can become troublesome without control. This makes its spread worth considering before planting cultivated beds. Its meadow habit suits less formal planting schemes. The RHS reports few pest problems and resistance to deer browsing. Powdery mildew can still affect the plant. Easy care does not mean it will stay within bounds.

Plants For A Future lists raw flower petals as edible. The petals can serve as an addition to salads. That source reports no known hazards, rather than proving universal safety. It also records past medicinal uses of roots and seeds. These include use for wounds and to promote urination or sweating. Such records describe traditional uses, not proof of treatment benefits.