numerous, showy, pink-purple (occasionally white) flowers grow at the ends
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.