Centaurea maculosa

Spotted Knapweed is a weed that was introduced to North America in the 1890's from Asia Minor, probably in alfalfa seed. It is an aggressive species that rapidly invades pastures, rangeland and fallow land, and causes a serious decline in forage and crop production.

Spotted Knapweed is a short-lived perennial that reproduces only by seed. Each year the plant produces 1 to 20 tough, wiry shoots from a thick taproot. The shoots are branched and grow one to three feet tall. The lavender flower is soft and thistle-like and produces seeds that can be viable for 8 years. The weed is spread readily in hay, through birds and wildlife, and on the undercarriage of vehicles.

Spotted Knapweed can be manually removed and destroyed from small well-defined areas and subsequently treated with an herbicide to prevent re-infestation from seeds.
It is not economically feasible to treat large infestation areas and re-establishment often occurs. There are some biological controls for use on Spotted Knapweed, including insects and spiders, and other plants.

more info: http://www.aphis.usda.gov/lpa/pubs/invasive/9knapwee.html