From Pitkin County Public Works Land Manager, Crystal Yates-White

Summer is almost over!  And what a wet one it has been!  And our Noxious Weeds --- especially thistles --- have been drinking it up.  As a result, I have been seeing a lot of new thistle growth, even in areas that were treated with herbicide earlier in the season.  The reasons for this are several:

1) Some of these thistles were rosettes earlier in the summer and I missed them when we sprayed.  Now, with all the rain, they have grown tall and visible.

2) In the case of Canada Thistle, these are new shoots of Canada’s extensive underground root system.  It can take several years to completely kill the root system of a Canada Thistle infestation that has become well established.  That means after you spray, there still may be some viable roots under the surface.  When conditions are good (like with all this rain) the plant can gather up enough strength to shoot up some new stalks in late summer/fall.  Much of the new growth I have been seeing in Old Snowmass is Canada Thistle, much of it for this reason.

3) It rained enough right after the weeds were sprayed that the herbicide didn’t "take".  Typically, for Milestone, Telar, or Banvel applications, a 6 hour window between spraying and a light rain is enough.  However, it is possible that the heavy rains we have been getting may have washed off some of your herbicide if it rained the afternoon after you sprayed.  Make sure you are using a good dose of a good surfactant and watching the weather forecast!  Don’t spray on stormy days if you can help it!

Whatever the reason for it, re-growth is happening.  This means it is important to check those areas you thought you took care of in June and July.  You may be surprised to find good healthy stands of thistles, with their purple flowers getting ready to start producing white fluffy seeds.  If your thistles are in late flower or seed, it may be time to start cutting.  Herbicides are much less effective on late-flowering weeds.  Budding and early-flower plants can still be sprayed.

An important part of effective weed management is monitoring and follow-up.  My County Crews will spend most of September going back over their work, cutting or pulling any noxious weed stragglers.  This will help protect all the work and money we invested in chemical control earlier in the summer… I recommend you do the same on your property!

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