Description and Variation:   Bighead knapweed, Centaurea macrocephala, is a member of the thistle tribe (Cynareae) in the sunflower family. This perennial species is the tallest knapweed growing in the Pacific Northwest, ranging from two to five feet tall, depending on the habitat. The plant stems are upright and unbranched, terminating in a single flower head. The leaves are broadly lance shaped with toothed edges and pointed tips, and they have a rough surface. Basal or rosette leaves are stalked and they can reach 15 inches long and three inches wide. The leaves and leaf stalks are progressively smaller upward on the plant stem, with the top leaves being stalkless. The solitary flower heads are globe shaped, and one to three inches in diameter. The bracts beneath the flower head have thin, papery, fringed margins. The lower bracts show evidence of spines. The flowers are yellow. The seeds are medium brown and ridged, with a ring of light-colored bristles. C. macrocephala has a taprooted woody crown.

Economic Importance: Detrimental: C. macrocephala has escaped cultivation to establish in an abandoned homestead in Pend Orielle, County, Washington. Beneficial: Bighead knapweed is sold as a garden ornamental and in seed packets. C. macrocephala is also found in dried flower arrangements. This species is sold under a variety of common names - including Lemon Fluff and Globe Centaury.

Habitat:  In its native habitat, C. macrocephala is found in high elevation grassy fields and subalpine meadows. In our region, the established field location of bighead knapweed is in grassy meadows, in loam and silt loam soils with precipitation of 20 inches or more at a elevations ranging from 2,560 to 2,680 feet. 

Geographical Distribution::  Bighead knapweed is native to Armenia and Romania. It is found in fields and grassy areas of the upper mountainous region of the Caucasus Mountains. In Romania it is found in pastures and glades. In Turkey it is found in subalpine meadows at elevations ranging from 6,600 to 7,590 feet.

History:  There is a 1918 collection from the garden of Wilhelm Suksdorff near Bingen, Klickitat County, at an elevation of 1,900 feet. There are two known field locations in Washington. All sites appear to have been originally cultivated as an ornamental species. The site in Pend Oreille County, was discovered in 1981. This site is an abandoned homestead, and C. macrocephala has grown from a few plants to over 10 acres. The site is actively controlled and regularly monitored.

Growth and Development:  C. macrocephala is a perennial that reproduces by seed. The rosette (usually) does not produce a flowering stalk the first year. Flowers are present from mid-July to August. The seeds are dislodged from the mature cup-shaped seed heads by wind or by direct contact. The large, heavy seeds are not wind dispersed. This species can overwinter as a mature plant, seedling, or seed. Seed viability in the soil is not known.

Reproduction:  Bighead knapweed spreads by seed, and it can be propagated by root division.

Response to Herbicide:  Not known.

Response to Cultural Methods:  Not known.

Response to Mechanical Methods:  C. macrocephala is a taprooted perennial, and pulling as a control option is impractical for large plants. When the flowering stem is broken off, a new stem will grow from the woody crown, producing another flower head later in the season. Repeated mowing will reduce seed production and will eventually diminish root reserves. Small plants can be dug, but the site needs to be monitored until the seed banks are depleted.

Biocontrol Potentials:  None known.

Rationale for Listing:  In Washington state, this species has escaped garden cultivation to establish in an abandoned homestead in high elevation rangeland. Once established, this species is very difficult to control, and the threat exists to our natural areas, including subalpine meadows. This species is still offered for sale in nurseries and in seed packets. Centaurea macrocephala is a Class A noxious weed in Washington, and eradication is still possible at this time.

References:

Bohlmann, F. and J. Laser. 1970. Constituents of Centaurea macrocephala Puschk. Chem Ber 103 (7): 2100-2104. (German, with an English summary).

D.P. 1986. The use of perennial plants as cut flowers. (Het gebruik van vaste planten als snijbloem (2). Verbondenieuws voor de Belgische Sierteelt. 30(5): 271, 273-275.

German Federal Republic. 1978. Annual reports of the German Plant Protection Service.

Braunschweig. P. 167-168: Weed Control in Ornamentals.

Klokov, M.B., D.I. Sonsovskii, N.N. Tsvelev and C.K. Cherepanov. 1963. Centaurea. In Flora USSR XXVIII:370-579. Moscow and Leningrad.

Roche’, B.F., Jr. and C.J. Talbott. 1986. The Collection History of Centaureas Found in Washington State. Research Bulletin XB 0978. Washington State University, Pullman, WA

Roche’ B.F., Jr. and G.L. Piper and C.J. Talbott. 1986. Knapweeds of Washington. Washington State Cooperative Extension and USDA. EB1393. P. 28-9.

Roche’, C. T. (no date) Identification of Knapweeds and Starthistles in the Pacific Northwest. Pacific Northwest Extension Publication 432. Washington State University. P. 17.

Roche’, C.T. 1991. Bighead knapweed. Pacific Northwest Extension Publication 396. Washington State University, Pullman, Washington

Roche’, C.T. 1989. Survival of the fittest, knapweed style. In Proc. Washington State Weed Conference, Yakima, WA. P. 47-55.

Roche’, C.T. 1989. Less known knapweeds: potential threats? In Proc. Knapweed Symposium, Montana State Univ. Coop. Ext. Bull. 45, Boseman, MT, pp. 47-53.

Wagenitz, G. 1975. Centaurea. In Flora of Turkey and the east Aegean Islands. P.H. Davis, Ed. 5:465-585. Edinburgh Univ. Press, Edinburgh. p. 522.