If you live in the Ranchos, Chambers Field, Country Lane or Centerville areas you have probably run across this weed. It is Diffuse Knapweed and is a noxious weed as defined by Nevada Revised Statutes 555 and as such, must be controlled by residents.
Diffuse Knapweed is currently growing at the rosette stage (see photos).
The rosettes can frequently be found under dried up plants (see photo) from last year's growing season, but it is a highly competitive and aggressive plant that can form dense colonies wherever there is dry, well drained soil - pastures, overgrazed rangelands, croplands, uncultivated lands, waste areas, roadsides and along riverbanks.
The main stem arises from a deep, stout taproot, can be up to 2 feet tall, and branches freely.
The branches are covered with short, stiff hairs and each will develop a single white to light purple flower that will be vase shaped, about 1⁄2 inch long and have yellow comb-like bracts tipped with a spine beneath the petals. The leaves are alternately arranged on the stem, with the lower leaves being pinnately divided (see rosette photo) while the upper leaves are more strap-like with smooth edges. Diffuse Knapweed is typically a biennial, but can be an annual or short-lived perennial and reproduces by seed. Once the plant matures and dries out, It can break off and travel with the wind like a tumbleweed spreading seeds as it travels. Since each plant can produce up to 18,000 seeds it is important to control it before it sets seed as the seeds can remain dormant in the soil for several years.
Mowing or cutting plants in the bud to flower stage can reduce seed production, and since it is a short-lived plant, repeated hand pulling can be effective. Chemical control, when applied at the proper time, is the most effective and can be achieved by using 2,4-D or dicamba (Banvel) in the rosette stage or clopyralid (Curtail) or aminopyralid (Milestone) between the rosette and the mid-bolt stages. If you come across this weed anywhere in Douglas County or need assistance controlling it on your property, please call the Douglas County Weed Control office at 782-9835.
Craig Burnside is Douglas County Parks Superintendent.