Blue Mountains National Resources Institute

Wanted: DEAD!

Yellow Starthistle

Alias: St. Barnaby's thistle, cotton tip thistle

• Stems: Rigid and winged by the continuation of the leaf bases. Grows up to 6 feet tall.
• Leaves: First-year's growth is a rosette of 8-inch deeply lobed leaves. Stem leaves grow the second year, and are 4 inches long; lower leaves deeply divided; upper leaves small, narrow, entire and sharply pointed.
• Flowers: Yellow, head about 1 inch long; solitary at the end of each branch. Flowering period is from July to September. Middle and lower bracts have sharp stiff spines up to 3/4 inch long.
• Fruits: Seeds are tan with white and brown mottling. Fruit of the marginal flowers dark colored and without bristles, inner fruits lighter in color with a terminal tuft of white-silvery bristles.

Yellow starthistle is a bushy annual or sometimes biennial. It is covered with loose white cottony wool. It can grow wherever cheatgrass grows; it infests roadsides, waste areas, and rangelands. It spreads by seeds that can remain dormant for 10 years. Spines are a deterrent to grazing, and the plant causes "chewing disease" in horses which renders them unable to eat or drink.

Control: Effective control requires both weed suppression, and reestablishment of desirable species.

• Mechanical: Small infestations can be hand pulled, and disturbed sites should be reseeded with perennial grasses.
• Biological: Seedflies and weevils are being used, but an effective control is many years away. Cultivation effectively controls yellow starthistle. Repeated cultivation is required to control each new seed flush. Repeated mowing can also be effective, if properly timed. Waiting until the early flowering stage leads to less regrowth.
• Chemical: Yellow starthistle is becoming resistant to some herbicides. Contact you local weed board for information about the ongoing war against this invasive pest.

Report all sightings to your local Weed Board