Spotted knapweed — an invasive plant in pastures

Spotted knapweed is a prohibited noxious weed, meaning a land owner is legally obligated to control it on his/her land.  Spotted knapweed grows well in dry sandy soils.  Spotted knapweed grows 1-3 feet tall. 

It flowers from July to September. Its flowers are pinkish to purple and sit at the ends of branches. Directly underneath the flower is an egg shaped bulge that is covered with bracts. These bracts have dark spots tipped with fringe. The spotted egg-shaped bulge is what spotted knapweed is named for.

 The leaves are somewhat hairy and have a slight silver coloring. The leaves at the base of the stem are deeply divided, whereas the upper leaves are thin and undivided. The stems are silvery green with purple stripes. 

In the first year of growth, the leaves grow out of a basal rosette, meaning they resemble a spindly dandelion. In the second year of growth, the plant bolts to produce 1 or more branching stems. After the plant bolts, spotted knapweed resembles a thistle. 

Spotted knapweed is not a serious concern in cropland because it is controlled by herbicides. In pastures, spotted knapweed is a serious problem. Its early spring growth allows it to out-compete other plants for moisture and nutrients. Spotted knapweed also produces a chemical that prevents other plants from growing in the immediate area. So once spotted knapweed is introduced to a pasture, it can quickly spread and take over. 

If you notice spotted knapweed in your pastures, take action quickly. Spray the infestation with an herbicide that has a long lasting residual effect. Spotted knapweed seeds can remain viable for five years. If the herbicide doesn’t have a long lasting residual effect, knapweed can pop right back up from old seed. Yes, the herbicide will kill the clovers. Frost seeding later is a better option than having an entire pasture of relatively useless forage. Cattle will eat spotted knapweed when there is nothing better. 

After the plant has been eliminated, the invasion site and the area surrounding it should be monitored for many years to make sure spotted knapweed doesn’t reinfest. 

For more information on spotted knapweed, visit:  http://www.ag.ndsu.edu/pubs/plantsci/weeds/w842w.htm