Yellow Star Thistle

The Strategy: Yellow Star Thistle (centaurea solstitialis) is a silver-green winter annual knapweed (like Spotted, Russian, and Diffuse) with bright yellow flowers and grows to a height of 36 inches. The bracts (outer side of bud) on this plant are modified into needle-like-spines which makes it almost impossible to walk through. Livestock will rarely travel through the sites, and is in-fact poisonous to them as a food source.

The Attack: Yellow Star Thistle has single handedly destroyed the River of No Return area of the Salmon River. Less than two acres were discovered years ago and now has grown to over 100,000 acres. This plant is found in Malad and Burley (coming from Utah in both cases) which puts Eastern Idaho on full alert for. This plant spreads by wind as well as by simply dropping seeds on the ground to sustain local populations. This also makes the plant the fastest spreading plant of all the noxious weeds and as with all annual weeds the most prolific in management.

The Defense: Mechanical control is very effective if you can tolerate the spines on the seed heads. Bio-Control is not very effective due to the plants ability to spread faster than the insects. Many herbicides are effective. 2,4-D is adequate early in the season, but once they get larger one needs to switch to Tordon 22K, Milestone, or Opensight. Treatment is most effective in the fall or early spring. Once established you need to attack the plants with any of the other products listed. After treatment, replanting of useful species is a must. Grasses and clovers are great competitors and desirable for foods for livestock and wildlife. If you suspect that you have seen this plant in our area please call your local weed authority for help and proper identification.

PLEASE NOTE – The proper use and application of herbicides can be an effective way to control and eradicate noxious and invasive plants. Before using herbicides, always carefully follow the label and safety instructions on the label. While we recommend the use of herbicides as one of the effective tools for integrated pest management, the Idaho Weed Awareness Campaign assumes no liability for herbicide applications.