PlantFiles: Spotted Knapweed, Ballast Waif
Centaurea biebersteinii

This plant is listed on the North Dakota invasive/troublesome list and this information is being distributed in a guide developed by the ND Weed Control Association and other agencies.

Plant Features
Biennial or short-lived perennial, up to 4 feet tall
Basal rosette is formed first year
Bracts on flower heads black-tipped
Branched stems on upper half of plant
Small stiff hairs give plants a grayish look
Leaves 1 to 4 inches long, rosette leaves are deeply lobed
Purple flowers (one or more) on ends of branches, up to 1 inch wide
Blooms early July through August
Taproot
Spreads by seed only

Distribution
Documented in a few areas mostly along roadsides. Grows under most conditions

Interesting Facts
Suppresses growth of other plants by releasing a toxin (allelopathic)
Flowers resemble garden bachelor buttons
Widespread invasive in western U.S.

I've never met a more aggressive weed. It'll grow in any place that has soil - doesn't matter what kind - it just needs to be dirt of any kind, but the better, the more profuse. It has a large tap-root, making it nearly impossible to pull unless you catch it when it is small and the ground is still moist. If it is growing in rocky soil - forget pulling. I've read this plant produces a chemical that actually prohibits other plants from growing around it. Our local paper had an artical describing efferts to isolate the chemicals this plant secretes to use as an herbecide! It might produce something safer than Round-Up, but in the mean-time, that's what I'm using on it. I spot-spray it in the early summer while it is still a rossette of leaves on the ground. Later on it shoots up spikes that flower and the concentation of an effective dose is hard to give without the spray flying everywhere. In addition to the ballast explanation for one name, I might add that the "knap"-in Knapweed reffered to shorn sheepwool that was shipped to various locations, spreading the seed.

Colorado Class B Noxious Weed. Mandatory eradication in all counties except Clear Creek, Elbert, and La Plata.
All locations of this plant in Colorado should be immediately reported to the Colorado Department of Agriculture.

The common name 'Ballast Waif' refers to its introduction to the U.S., via ship's ballast (the name is shared by Lythrum salicaria, aka Purple Loosestrife.)

Like many other species in this genus, the Knapweed is beneficial for foraging honeybees, but it is an aggressive plant that tends to take over other plants in its midst, namely by exhausting the water and nutrients from the soil. A single plant disperses thousands of seeds in a growing season.

One source indicates that one of the major ways this plant is spread is by vehicles picking up seeds when driving through an infested field.