- Posted By: Brad Hanson
- Written by: Rebecca Miller-Cripps
Article originally posted on the UC ANR Green Blog. Written by Rebecca Miller-Cripps and reposted with permission. Brad
Weeds, weeds, weeds! Have you noticed? This has been a banner year for weeds. Puncturevine where I’ve never seen it before. Garden soil covered with common purslane (at least it’s good in salads). And solid stands of yellow starthistle everywhere!
What can be done? First of all, identify your weeds. Different weeds require different treatments. Is it an annual or perennial? Does it propagate by wind-blown seeds or by runners? The University of California Integrated Pest Management website, has weed-identification guides that are fun and easy to use. The website also offers treatment guidelines.
In the California foothills, yellow starthistle (YST) is perhaps the most common weed of concern. It impacts much of our open space - agricultural and rangeland - and intrudes into our neighborhood landscapes. Yellow starthistle currently infests more than 15 million acres of land in California. Not only does it prevent recreational use, like walking or hiking, but it chokes out native grasses and wildflowers. It is also poisonous to horses, causing a neurological disorder called "chewing disease” which can be fatal once symptoms develop.
That said, yellow starthistle can be hand-pulled at any time in its lifespan. In its present dry and spiny stage, pulling the weeds can inflict pain, so be sure to wear gloves. Double-bag the plants and burn them later in the fall.
There is a fairly new herbicide (2009) on the market from Monterey Chemical called Star Thistle Killer.
Local pest control companies are also available to provide a one-time herbicide application for yellow starthistle. For more information, go to the Central Sierra Cooperative Extension website or call the Yellow Starthistle Leading Edge Project in the UC Cooperative Extension office at (530) 621-5533 or (209) 533-6993.
Information adapted from the University of California Integrated Pest Management Program and from “Yellow Starthistle: Brief Homeowner Information Sheet” by John E. Otto, Amador County Master Gardener.
YST Homeowner Handout
The active ingredient in Star Thistle Killer is clopyralid which has postemergence activity and some preemergence activity.
Remember that starthistle is an annual plant (grows from seed every year) so those dead plants from 2018 won't regrow or green back up. You're after the next generation that will come from seed produced in 2018.
So, one of the key considerations is the density of last year's skeletons. You need to be able to get good spray coverage down to the small seedlings (I assume they are mostly a couple inches in diameter this time if year). If you have really thick thistle and desirable vegetation, mowing might create a thatch that actually blocks or creates shadows in your spray application. On the other hand, if the density is moderate, mowing the vegetation might help.
There's some useful info at this excerpt from the Weed Research and Information Center publication "Weed Control in Natural Areas in the Western US" here:
https://wric.ucdavis.edu/information/natural%20areas/wr_C/Centaurea_solstitialis.pdf
I forgot to add that the link in my comment above also has some really useful info on timing of mowing starthistle to reduce seed set (and, thus next year's problems). Starthistle seed is not really long lived in soil so you can really reduce it after a few seasons of diligent effort at the scale you are managing (although tough to do economically at ranching/natural resource scale).
Brad
In the past, we've found that clopyralid can be applied right over the top of old starthistle skeletons. It will get down to the soil as long as there's some rainfall after the application. Best of luck.
Ginny
Yellow starthistle is an annual (completes it's lifecycle within one growing season) so it is best to try to control the existing plants now before they set seed.
Fall control strategies are sometimes recommended for perennial species (live for several years) because there is often a timeperiod where the plant is reallocating a lot of it's resources to belowground storage organs before winter. Sometimes translocated herbicides used at this time of the year are more effective than in the spring when the plant is mostly pulling resources from those storage organs and growing above-ground tissues. Again, that is NOT the case for yellow starthistle but is for plants like Canada thistle and field bindweed, among others.
Here's a link to a Yellow Starthistle Management Guide (DiTomaso, Kyser, and Pitcairn) published by the California Invasive Plant Council. You download for free or purchase hard copies at: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiY-pPvjb3xAhVFGDQIHbJKA-MQFjAJegQIBRAD&url=https%3A%2F%2Fwww.cal-ipc.org%2Fproduct%2Fyellow-starthistle-management-guide%2F&usg=AOvVaw3joICzCEU53xgk3Qvs4aCh