Before and After: Blessed Milkthistle Management One Year Later

What a difference a year makes

WeedWise Specialist, Jeff Lesh removing milkthistle seed heads from isolated plants

It was one year ago that we reported a large infestation of milkthistle (Silybum marianum) growing in a twenty-acre horse pasture outside of Wilsonville.  At that time, the WeedWise program was approached by the resident for assistance in helping her to control the milkthistle.  As a priority invasive weed in Clackamas County, we have resources available to assist with control of this invasive weed on behalf of the landowner.  Upon visiting the site, we realized we were dealing with the largest milkthistle patch in the region. WeedWise program specialist, Jeff Lesh immediately began working with the landowner to control the infestation.

A rapid response

In developing a response to this infestation, the WeedWise program developed the Best Management Practices: Blessed Milkthistle document that outlines effective control methods for treating milkthistle.  At the time of the year that the infestation was reported to us, many of the plants had already flowered and were starting to set seed.  As a result, WeedWise program specialist, Jeff Lesh prescribed hand removal of seed heads on all isolated milkthistle plants within the field.  This seed head removal effectively eliminated seed production in areas of the field that were not heavily infested.  Although this practice would have been much too labor-intensive for the entire infestation, it certainly helped to protect relatively intact portions of the pasture.

Milkthistle seedlings emerging after the onset of fall rains.

These plants, as well as the heavily infested areas of the field, were then spot treated with an herbicide application of 2,4-D and dicamba (Weedmaster) at the recommended label rate.  This herbicide is labeled as safe in pastures and was prescribed in an effort to reduce seed production for the season.

Working with Jeff, the landowner then carried out a field mowing operation later in the summer to knock down the standing material.  This greatly increased access to the site and helped to protect the animals present from the physical injury posed by the milkthistle on site.

Proper timing

A boom spray was carried out to target milkthistle seedlings

After the initial response to the milkthistle infestation, we then waited for the ideal timing and conditions for treatment the next crop of milkthistle lying dormant in the soil.   In October, after the onset of fall rains, we began to see seedlings germinating throughout the field.  The germination of seedlings was widespread across the site.  The field mowing carried out by the landowner provided us with ideal access for a targeted herbicide application.

Jeff then prescribed a foliar application of aminopyralid (Milestone) to treat the carpet of seedlings. The WeedWise program hired a local contractor to broadcast spray the field.  The spray boom provided even coverage to target the multitudes of small seedlings spread across the site.   The contractor also used a tracked sprayer to avoid damaging the fields.  This was of particular concern especially when the ground becomes soft following the fall rains.

With the herbicide application completed, it was time to wait for spring to see how well our treatments worked.

WeedWise Program Manager, Sam Leininger, removing a few remnant milkthistle plants

A welcomed surprise

This past spring, the WeedWise program was back out on site to see how well our treatments had worked.  To our great surprise, the milkthistle was nearly completed suppressed.  Although we were confident that our treatments would be successful, we couldn’t have hoped for the level of control we achieved.  In late spring, WeedWise Specialist Jeff Lesh and WeedWise Manager, Sam Leininger spent a little bit of time removing the handful of milkthistle plants that had survived from the previous year. Most of the plants removed were clustered around a turnout that hadn’t been treated the previous year.

The resident was also excited to see her pasture transformed.  After battling this infestation for over a decade she was ecstatic to see the results.


Before and After

Nothing tells the story better than these before (left) and after (right) photos below.

We are here to help

If you are dealing with a similar situation on property, be sure to contact us for assistance.  Although we can’t treat all of your weeds for you, we can certainly arm you with the information you need to make your efforts more successful!

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WeedWise Program