Invasive Watch:Knapweeds (spotted and Russian) (CENTAUREA STOEBE L. SSP.
MICRANTHOS [FORMERLY C. MACULOSA] AND ACROPTILON REPENS)
WHAT TO LOOK FOR.
Spotted knapweed is an aggressive biennial or, occasionally, a short-lived perennial that reproduces from seed. In the first year, it appears as a basal rosette of deeply lobed leaves, which grow up to 6 inches long. The mature plants grow 1 to 4 feet tall and have alternate pale green leaves 1 to 3 inches long. Stems are erect and rough with slender branches. Pink to light purple flowers with highly dissected petals are produced during the early summer and appear on the tips of terminal stems. The flower petals are surrounded by stiff black-tipped bracts, giving the flower head a spotted appearance for which it’s named.
Russian knapweed is a long-lived creeping perennial that reproduces from roots and seeds. Leaves of newly emerging plants are toothed and covered with fine hairs, later bolting, and flowering through summer into fall. The stems are erect, very branched and reach heights of 2 to 4 feet. The lower leaves are deeply lobed, 2 to 6 inches near the base. The middle leaves have toothlike edges, are smaller and grow narrow near the top of the plant. The light pink to purple cone-shaped flowers appear from summer to fall and are 1/4 to 1/2 inch in diameter and solitary at the tip of leafy branches.
While the two are similar, spotted knapweed can be distinguished from Russian knapweed by their different floral characteristics. Russian knapweed flowers are smaller and do not have black tips on the flower bracts.
WHERE THEY ARE FOUND.
Both of these noxious weeds were introduced from Eurasia in the late 1800s and have been rapidly spreading from the western to the eastern U.S. Seed dispersal is primarily by birds, animals and humans, as well as by hay that has been harvested and moved from infested areas.
Spotted knapweed appears in every contiguous state except for Texas, Oklahoma and Georgia. It’s recognized by many states as a significant invasive plant of natural areas. It commonly invades open habitats, and is able to tolerate soils with poor nutrient levels, and harsh and dry conditions.
Russian knapweed is found throughout the Midwestern and western U.S.; commonly along roadsides, riverbanks, irrigation ditches, clearcuts, pastures and croplands, and especially in areas with high water tables.
HOW TO TREAT THEM.
The most effective treatment for knapweed is Milestone® specialty herbicide at 5 to 7 fluid ounces per acre in a foliar broadcast or high-volume hand-spraying application. Milestone will provide residual control of plants that try to regrow after the initial application and of newly germinating seedlings.
Use 1 to 1 and 1/3 pints per acre of Transline® herbicide for selective control of knapweed around and under trees where the possibility for tree root absorption exists. A nonionic surfactant containing 80 percent or greater active ingredient, at 1/4 to 1/2 percent v/v is often added to improve wetting and penetration.
For optimum control of spotted knapweed, apply Milestone or Transline when plants are at the rosette to midbolting stage, or during fall regrowth up to early November, before the soil freezes. Optimum timings for treating Russian knapweed with applications of herbicides range from bud to midflower growth stages.
Successful treatments also can be made in the fall or winter when plants are dormant, even where the foliage is dead and brown. The herbicide is absorbed through the root crown for root death and control.
Successfully managing knapweed requires long-term commitment because of the life of the plants and the longevity of the numerous seeds in the soil. It’s key to control the plants while also preventing seed production until the seeds remaining in the soil are no longer viable.
Check treated areas each year and treat new plants before they flower and produce seeds. Also consider establishing a thick cover of desirable plants, which helps discourage knapweed invasion.