Yellow Starthistle, a Noxious Weed You Need to Control

Yellow starthistle in bloom.
Got yellow starthistle? It is a 2- to 4-foot tall plant with bright yellow flowers and wicked ¾-inch spines. Photo by Wendy Hanson Mazet.

Have you noticed a 2- to 4-foot tall gray-green plant covered in whitish hairs with bright yellow flowers and wicked ¾-inch spines at the base of the flower? This is yellow starthistle, a state-listed noxious weed.

This state legal designation requires landowners to control and prevent the weed’s spread. Whoever owns the land is responsible for controlling noxious weeds. On public lands, the federal, state, county or city municipality responsible for caring for the land is responsible for controlling noxious weeds. For private land, it is the landowner.

Yellow starthistle often infests rangelands, pastures, cultivated fields, waste areas and roadsides. It has infested areas in Washoe and Douglas counties and Carson City. It is presently infesting areas around Verdi, Bartley Ranch and Rancho San Rafael.

It contains a chemical that causes chewing disease in horses, damaging the area of the brain that controls fine motor movement.

Yellow starthistle is an annual weed, which means it sprouts, grows, blooms, produces seed and dies in one year. Despite its short life, it is remarkably invasive. At maturity it has a deep taproot, up to 6-feet long, which enables it to access soil moisture most annuals can’t reach. It is a prolific seed producer, with a single flower able to produce 30 to 80 seeds. A large plant with many blooms can produce thousands of seeds. Seeds can remain viable in the soil for up to 10 years, awaiting the right conditions to sprout and grow.

Yellow starthistle only reproduces by seed, so preventing seed production is essential for control.

While it is poisonous to horses, other animals like goats, sheep and cattle can graze it before the spiny flower heads form. Grazing will not eliminate the weed, but 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 can allow for 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.

Broadleaf-selective herbicides can be used prior to flowering. Once the weed flowers, it is getting ready to die and herbicides are not effective.

Wondering if you have yellow starthistle? Visit www.unce.unr.edu/programs/sites/ipm/gallery/noxious for pictures and information. Want to report an infestation? Visit agri.nv.gov/NoxiousWeeds and fill out Nevada Department of Agriculture’s noxious weed notification form. You can also contact Sean Gephart, Nevada Department of Agriculture’s noxious weed coordinator, at 775-353-3640.

Yellow starthistle spines catching a child's sock as he walks on a park path.
Yellow starthistle’s vicious spines negatively impact recreationists, livestock and wildlife. Photo by Wendy Hanson Mazet.

Melody Hefner is the Urban IPM and Pesticide Safety Program Coordinator for University of Nevada Cooperative Extension. Have a gardening question? Contact a master gardener at 775-336-0265 or mastergardeners@unce.unr.edu, or visit www.growyourownnevada.com.

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