Synonyms: Centaurea maculosa
Common Names: Bushy knapweed
Description: This species is listed as a prohibited noxious weed by the Michigan Department of Agriculture. It poses a serious threat to western rangelands; may cause skin reactions in some individuals.
Habit: Short-lived herbaceous biennial or perennial reaching 0.6-1.2 m (2-4 ft).
Leaves: All leaves pale or grayish green with rough fine hairs; basal leaves form a rosette which may persist for up to four years; basal and lower stem leaves up to 15 cm (6 in) long; leaflets deeply divided to irregularly lobed; tapered at both ends; upper stem leaves smaller (2-7 cm in length) with few or no lobes.
Stems: 1-7 upright rough stems that branch towards the upper half of the plant.
Flowers: Numerous, pink-purple, terminal solitary flowers at the end of each stem, phyllaries with dark tips and fringed margins; bloom from July through September.
Fruit and seeds: Small brown wind-dispersed seeds, germinate throughout the growing season; remain viable for up to nine years.
Habitat: Roadsides and right-of-ways, old fields, pastures, undisturbed dry prairies and oak and pine barrens; also appears on dunes and beaches during low water conditions.
Reproduction: By prolific seed production and to a lesser extent by lateral roots.
Similar species: Russian knapweed (
Centaurea repens), diffuse or white-flowered knapweed (
C. diffusa), black knapweed (
C. nigra), and yellow star thistle (
C. solstitialis). The best way to distinguish spotted knapweed is by the dark tips and fringed margins of its phyllaries, although these species are also problems.
Monitoring and rapid response: Monitor well-drained, sunny sites; knapweed is recognizable throughout the growing season. Wear gloves, long pants and sleeves to prevent skin contact. Begin control efforts in highest quality areas; pull or dig plants in small infestations and remove entire root. Remove flower/seed heads from site. Foliar herbicide application of rosettes is most effective before the stem elongates. Continue control efforts until the seed bank is exhausted.
Credits: The
Michigan Natural Features Inventory (MNFI) has partnered with MISIN to provide the information in this fact sheet. Species images and/or information were used with permission from "
A Field Identification Guide to Invasive Plants in Michigan's Natural Communities" and "
A Field Guide to Invasive Plants of Aquatic and Wetland Habitats for Michigan.