Skip to Content
Weed control, management, ecology, and minutia
Comments:
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:  
 
https://wric.ucdavis.edu/information/natural%20areas/wr_C/Centaurea_solstitialis.pdf
by Brad Hanson
on February 27, 2019 at 8:38 AM
Bob,  
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
by Guy B Kyser
on February 27, 2019 at 9:24 AM
Here's another good publication available on line. https://www.cal-ipc.org/resources/library/publications/yst/  
 
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.
 
Leave a Reply:

You are currently not signed in. If you have an account, then sign in now!
Anonymous users messages may be delayed.
 

Security Code:
GDHELL