There are two types of Knapweed plaguing North America: diffuse (Centaurea diffusa) and spotted (Centaurea maculosa). If you live in the western United States or Canada, you don't want to find either one in your yard. You don't want to, but you probably will.

Diffuse Knapweed occurs in a wide range of habitats, especially dry valley bottoms and the kind of hillsides where you find Ponderosa Pine and Douglas-Fir. It has a single upright stem, 8 to 40 inches tall (20 to 100 centimeters if you're in Canada), and numerous spreading branches. The plants start out as rosettes, close to the ground. They bolt in early May, and produce numerous white (or sometimes pink or purple) flowers in July and August. The flowers are urn shaped, like thistles, and surrounded by yellow-green bracts with short, stiff spines. The leaves are pinnately divided. It is a biennial to short-lived perennial weed. It produces over 900 seeds per plant under the driest of conditions. If irrigated, a single plant may produce over 18,000 seeds! Diffuse Knapweed contains volatile oils and is bitter but not poisonous.

Spotted Knapweed is found in more or less the same places as Diffuse Knapweed - primarily low to mid eleveations, with the Douglas-Firs and Ponderosa Pines. Like Diffuse Knapweed, it's a biennial or short-lived perennial weed. Like Diffuse Knapweed, the plants start close to the ground, as rosettes. The mature plants are 20 to 120 centimeters tall (8 to 48 inches if you're in the States). They have long, fibrous taproots. Overwintering rosettes bolt in early May. They produce one to fifteen stems, which are somewhat hairy when young (as are its young leaves) and highly branched. Thistle-like pink to purple flowers, 1 to 1.5 centimeters long, bloom from July through to October (which is truly hellish if you're allergic to them). The bracts of the flower head are easily recognized, having a black-tipped fringe which give the flower heads a spotted appearance. Each flower is capable of producing 400 seeds under dry conditions and over 25,000 seeds when irrigated. The plant contains volatile oils with a distinctive smell. It is extremely bitter, but not poisonous. Skin contact with the plants can cause hives if you're allergic.

Knapweed was introduced from Eastern Europe in the early 1900s. Some sources say it was a contaminant in crop seed. Others say it was brought by beekeepers. Bees love Knapweed. The plant has no naturally indigenous enemies or parasites in North America, and so it spreads rapidly. It is easily distributed great distances on the wheels and undercarriages of cars, trucks, trains, small airplanes that land at infested airstrips, logging trucks, and heavy machinery. It is also spread by florists, who use it in dried arrangements. Hay farmers moving from infested to non-infested areas can transport the weed with them, and animals may pick up the weed and disperse it. Rodents and small birds (such as chickadees and sparrows), eat the seeds and disperse them in their feces. Wind can also spread the seeds. Sometimes the wind spreads whole plants full of seeds, as they blow over easily when they are mature and roll like tumbleweed over the landscape.

Both Knapweed species are highly competitive and are capable of invading grassland sites and choking out all native vegetation. They have shown that they may be alleopathic - they produce their own herbicide to reduce the growth of other plants. This eventually results in a knapweed monoculture. Knapweed encroachment can destroy the forage base in deer and elk habitat, resulting in significant declines in populations. Knapweed now infests several million acres of grazing land in the northwestern United States and Canada.

Knapweed confined to small, well defined areas should be treated as soon as detected to avoid spread of the weed. First, all visible knapweed plants should be removed and destroyed. Then the area should be treated with a herbicide to prevent reinfestation from seedlings.

Careful followup is necessary to control missed plants and seedlings. Many attempts to control knapweed have failed because followup treatments were not applied.

and very annoying personal experience