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Weed control, management, ecology, and minutia
by Bob Hall
on February 26, 2019 at 11:10 PM
I've got 5 acres with star thistle. I've got the Star Thistle Killer. Should I cut down the field first before applying it or just spray over the field as it stands. The dried out thistle is about 1' tall and covers the majority of the 5 acres.
by Brad Hanson
on February 27, 2019 at 8:35 AM
Hi Bob,  
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:
by Brad Hanson
on February 27, 2019 at 8:38 AM
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).  
by Guy B Kyser
on February 27, 2019 at 9:24 AM
Here's another good publication available on line.  
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.
by Ginny
on June 28, 2021 at 11:51 AM
Thank you so much for the info on the yellow starthistle. They are nasty and our neighbors have horse so I really need to kill them. I would like to start now in hot summer. Or should I wait til fall? Thank you  
Reply by Bradley Hanson
on June 29, 2021 at 8:12 AM
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:
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