Centaurea macrocephala aka Bighead Knapweed

Centaurea macrocephala belongs to the daisy family, Asteraceae. It is a perennial, so it lives for several years. Its native range runs from northeastern Türkiye to northwestern Iran. Common names include bighead knapweed and globe knapweed. Gardeners also call it giant knapweed. It has another name, Grossheimia macrocephala, which is a synonym. It suits temperate gardens and forms a large, upright clump. It is not native to Britain or Ireland.

Garden plants grow about 3.3–5 feet (1–1.5 metres) tall. They spread about 1.6–2 feet (0.5–0.61 metres) wide. RHS gives a taller, narrower range than Missouri Botanical Garden. Plants reach full size in two to five years. Leaves may be simple, with no lobes, or have lobes. The plant is deciduous, so it sheds its leaves. Strong, stiff stems give it a bold, coarse outline. Its upright shape works well with other plants in borders.

Brown buds open to form yellow flower heads in summer. The heads look like thistles. Glossy, light brown bracts, or leaf-like parts, grow around them. Sources give widths of about two to four inches. Missouri Botanical Garden lists blooms from June to July. It notes a midsummer show of two to three weeks. Chicago Botanic Garden gives a wider span: May through August. Use the heads as cut flowers or dry them. The blooms draw butterflies, and the plant draws birds.

Most sources say to grow this plant in full sun. RHS also accepts a spot with some shade. Both sources call for soil that drains well. RHS favors moist soil. Missouri Botanical Garden accepts dry to moderately moist soil. The plant copes with drought and dry ground. It grows in chalk, clay, loam, or sand. Acid, neutral, and alkaline soils all suit it. Missouri Botanical Garden lists USDA zones 3–8. RHS gives it a hardiness rating of H7.

Grow single plants or groups in beds and perennial borders. Larger groups also suit meadows and gardens for wildlife. The stiff shape gives these plantings a strong outline. The plant needs little care and rarely has serious problems. RHS notes few pest problems, but powdery mildew can occur. Cut off spent heads to help prolong the flower show. Sow seed in pots in a cold frame in spring. Divide well-grown clumps in spring or autumn.