JoAnne Skelly: Stop yellow starthistle from spreading

Yellow starthistle (Photo Wendy Hanson-Mazet)

Yellow starthistle (Photo Wendy Hanson-Mazet)

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I have been seeing a nasty noxious (that means it’s against the law) weed in bloom recently, yellow starthistle (YST).
It is a 2- to 4-foot-tall gray-green plant covered in whitish hairs with wicked ½- to 1-inch spines at the base of each yellow flower. This annual weed sprouts, grows, blooms, produces seed and dies in one year. Despite its short life, it is remarkably invasive. It is highly competitive and often develops impenetrable stands, displacing desirable vegetation. YST is a prolific seed producer, with a single flower producing 30 to 80 seeds. It flowers until frosts kill it. A large plant with many blooms can produce thousands of seeds that can remain viable in the soil for up to 10 years. At maturity it has a deep taproot, up to 6-feet long, which enables it to access soil moisture most other annuals can’t reach.
This plant is poisonous to horses and causes a nervous system disorder called "chewing disease." This disease damages the area of the brain that controls fine motor movements, particularly of the mouth, resulting in starvation or dehydration.
Since YST only reproduces by seed, preventing seed production is essential for control. Grazing, mowing, pulling, digging and cultivation can be effective management strategies if done prior to seed production. Animals like goats, sheep and cattle, which are not susceptible to “chewing disease,” can graze it before the spiny flower heads form. While grazing will not eliminate the weed, it can be used as part of an integrated management plan. Pulling or digging can be effective, but you must remove the aboveground portion of the plant. Leaving a portion of the stem and leaves at the base of the plant allows regrowth.
Tilling is an effective control method for cultivated fields. It will destroy the root and so destroy the plant. Disturbing areas like roadsides or rangeland with cultivation, however, may increase the infestation of yellow starthistle or other weeds present in the seed bank.
Mowing just as a few flowers have formed can be an effective way to prevent seed production. If you mow too early, repeat mowing may be required. If you mow after flowers have formed, you need to bag up and remove clippings. The time from flower to seed production is only eight days, so leaving mowed flowers on-site may contribute to the seedbank.
Thanks to my colleague Melody Hefner for this information, available at
JoAnne Skelly is associate professor and extension educator emerita at
University of Nevada Cooperative Extension.


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