Short-fringed Knapweed (Centaurea nigrescens Willd., C. dubia Suter in Flora of the Pacific Northwest)

Introduction

      Short-fringed knapweed, native to south central and eastern Europe, has naturalized in southern Australia, the northern United States and southern Canada.  First discovered in eastern Canada in 1878, extensive stands have developed in southern Ontario.  The only population in British Columbia was found in 1966 on Vancouver Island.  Short-fringed knapweed was found near Odell, Hood River County, Oregon, in 1919 and still persists near Hood River.  In Washington, it currently grows near Trout Lake, Klickitat County, and near Sullivan Lake, Pend Oreille County.  Previous populations in Wahkiakum County, at Bingen, Klickitat County and Manchester, Kitsap County, Washington, and Idaho County, Idaho, apparently have not persisted.

      Short-fringed knapweed, also called Vochin knapweed, is a Class A weed in Washington, but has not been classified as a noxious weed in Oregon and Idaho.

 Identification

      Short-fringed knapweed, with the other knapweeds, is a member of the thistle tribe (Cynareae) in the sunflower family (Asteraceae).  It grows to 4 feet tall from a woody root crown, with multiple upright flower stalks branched from above the middle.  Lower leaves are oblong lance-shaped, entire or lobed, and taper to a stalked base.  In lobed leaves, the rounded terminal lobe is often much larger than the other lobes.  Upper leaves are smaller, less lobed and merely tapered the base, without a stalk.  Leaf margins are lined with minute stiff hairs.

     The flower heads are single at the tips of the branches, more or less flat across the tip of the branches, with the outer flower heads opening first.  Flower heads are about ½ to ¾ inch tall.  The outer ring of flowers are sometimes larger than the central flowers in the head.  The lower portion of the head, made up of rows of bracts, is oblong, compared to the urn shape of diffuse and spotted knapweed heads.  Bracts have small dark triangular tips, with 6 to 8 small fringes on each side which do not taper along the side of the long slender green base of the bract.  Flowers are rose-purple and some of this color may tinge the center of the bracts.  Seeds are achenes about 1/8 inch long, light brown or ivory with lengthwise lines.  If present, the plume consists of a few short stiff hairs opposite the oblique scar where the seed was attached.

     Short-fringed knapweed is distinguished from spotted and diffuse knapweed by its entire or coarsely lobed leaves and from meadow knapweed by the dark triangular tip on its bracts.

Biology and Ecology

      Short-fringed knapweed is a perennial that reproduces by seed and can be propagated by root division.  It flowers from July to September, releasing seed from August through winter.  Seeds are free to fall from the cup-shaped heads when the dried flowers that formed a plug are dislodged.  The unplumbed seeds are probably not moved by wind and fall near the parent plant unless carried by flowing water, vehicles or animals.  In addition to unintentional seed movement, short-fringed knapweed has been transplanted by people as an ornamental flower.  It was first found in Australia in an old flower garden.

     In North America it grows along roadsides and irrigation ditches and in pastures, orchards and waste ground.  So far, all of the short-fringed knapweed in the Pacific Northwest has been found in cleared areas in forested regions, indicating that it is best adapted to moist conditions but requires full sunlight.  Relatively coarse when mature, short-fringed knapweed probably ranks about the same as meadow knapweed and bighead knapweed in palatability to livestock.  Thus, while livestock will graze it, it is not particularly desired as a forage species in pastures.

Control

     Small infestations can be eliminated by repeatedly digging all knapweed plants until seed reserves in the soil are exhausted.  How long the seeds remain viable in the soil is not known.  Although the woody crown of short-fringed knapweed is too stout for hand pulling, it may be successfully removed using hand tools such as shovels or pulaskis.  Dig out as much of the root as possible and plan to return to remove plants that regrow from missed fragments.  For larger populations on tillable land, control short-fringed knapweed with repeated cultivation, then reseed to desired pasture species.  If short-fringed knapweed responds like other perennial knapweeds, mowing will not control it nor prevent seed production.

     For chemical control recommendations, refer to the Pacific Northwest Weed Control Handbook, an annually revised extension publication available from the extension bulletin offices of Oregon State University, Washington State University and the University of Idaho.

     The author acknowledges the support of the Washington State Department of Agriculture and the Washington State Noxious Weed Control Board in preparation of this bulletin.

By Cindy Roche, M.S.,. Extension Coordinator Department of Natural Resource Sciences.  Photo by author.

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