Extension Viewpoints: Noxious Weed of the Month: spotted and diffuse knapweeds

Photo courtesy Jamie Jones
Spotted and diffuse knapweed.

By Ethan Proud
PREVIEW Columnist

That plant with the purple flower usually means thistle, but knapweeds have purple blooms as well. Russian and spotted knapweed flowers are typically purple and diffuse knapweed blooms may be purple — though they are usually white. 

For this article, we will only be discussing the biennial knapweeds in our area: spotted and diffuse. In the early stages of life, knapweeds can be hard to identify. They have sage-green highly dissected leaves, meaning that the margin flares out and returns to the mid-vein. Being biennials, the first year they will not grow out of the rosette stage and bloom. During their second year, they will bloom small inflorescences that appear similar to Canada thistle. The petals are feathery and subtended by bracts of scales on the flower bud. 

On spotted knapweed these bracts are tipped with black and on diffuse they are yellow and spiny, and stick out between 90 and 45 degrees from the bud. In a perfect world, these plants would die after they bloom, but knapweeds can also act as short-lived perennials and live for three to five years, producing seeds the entire time. After the aboveground portion of the plant dies back, or senesces, the base of the stem becomes brittle and it forms a tumbleweed, dispersing seeds until it becomes stuck in a fence, ditch or some other obstruction. 

Spotted and diffuse knapweeds readily hybridize and hybrids can be expected in areas with both species. Two other knapweed species are toxic Russian knapweed and yellow star-thistle, and the same toxins can be found in the biennial species — just in lower quantities. Livestock will not usually select these plants, but if a pasture is completely infested, they will eat anything green in the absence of desirable forage. Be sure to use rotational grazing practices as knapweeds will readily invade overgrazed and degraded rangeland and pasture.

Archuleta County Weed and Pest is your local resource for managing noxious weed populations and controlling other pests.

This story was posted on July 19, 2020.