Conservation Committee Tip of the Month

Yellow Star Thistle

Centaurea solstitialis

This looks to be a banner year for Yellow Star thistle, one of our very worst invasive weeds.  Please do what you can to remove this pest from your property.  Your neighbors will thank you.

• Member of the Aster Family and Knapweed Genus, arrived in CA in 1850s from the Mediterranean.
• Prefers compacted or damaged soil, spreading rapidly along roadsides via cars and equipment.
• It has a very deep and vigorous tap root, but if you get the crown it will not grow back.
• YST chokes out most other vegetation and depletes the soil of moisture.
• The seed bank for YST can last ten years
• It is readily browsed by sheep and goats, but can be lethal to horses.

Control

1. Hand pull or mow (or weed-whack) before <2% of plants are blooming (the first day you see flowers).This significantly reduces the reseeding rate.The goal is to get rid of the plants before any set seed. This may need to be repeated to get resprouting plants.
2. Goats effectively graze on YST, even in the spiny stage - allowing struggling native plants to receive more light and moisture.
3. Replant area with fast growing natives such as native Salvias or Eriogonums (Buckwheats).
4. Yearly weeding, mowing or grazing can keep YST under control afterwards, but it needs persistence for the first few years and ten years of no new plants before it can be considered gone.

More information on C. solstitialis can be found here:

UC Integrated Pest Management Pest Note:

https://ipm.ucanr.edu/PMG/PESTNOTES/pn7402.html

California Invasive Plant Council (Cal-IPC) webpage:

https://www.cal-ipc.org/plants/profile/centaurea-solstitialis-profile/