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Invasive Species Sheet - Spotted Knapweed

Invasive Species Identification Sheet

Spotted Knapweed Centaurea biebersteinii DC. (or C. maculosa auct. non Lam.)

  • biennial or short-lived perennial, herbaceous plant; 8" - 32" (or more) tall
  • numerous, showy, pink-purple (occasionally white) flowers grow at the ends of stems
  • plant is open in appearance because of highly dissected leaves and squarish, slender, stems
  • no sharp spines on leaves or in the rows of dark-tipped phyllaries found below the flowers
  • thick taproot lacks the remains of old leaf bases at the ground’s surface

In its first year, Spotted Knapweed produces only a basal rosette (circular spoke-like pattern of leaves on the
ground). These leaves may be 6" long and not very lobed. In the second year, the plant shoots up 1-20 rough
stems that branch repeatedly in their upper halves. The alternately arranged stem leaves are 1"-4" long and
highly dissected into narrow, linear lobes. Leaves close to the flowers may be less dissected or even unlobed.
The end of each stem branch has a flower head that opens mid-late summer. The basal rosette may stay green after
the flowering stalks turn brown. The dead stalks may persist over winter.

The showy “flowers” of Spotted Knapweed are really flowerheads. In the Knapweeds (genus Centaurea), the each
flowerhead is composed of many tiny, tubular flowers that have long, thin lobes. The flowers at the edge of the
flowerhead are larger than those in the center and have longer and more deeply-cut lobes making the flowerhead
appear to have strap-like ray flowers.

Below the flowers in the flowerhead are several rows of tiny, highly modified leaves called phyllaries (or
bracts). In Spotted Knapweed, the outer phyllaries are strongly, vertically ribbed and have dark, fringed tips
(best seen with a handlens). The side edges of the phyllaries have a white fringe (use handlens). From a distance,
the dark phyllary tips create a spotted pattern below the flowers. Taken as a unit, the rows of phyllaries supporting
the flowers are about 1/2" in height while the showy flowerheads may be 5/8" to 1"across. Spotted Knapweed is an
aggressive weed on dry sites, likely to be first noticed on roadsides.