Getting rid of thistles - Knowledgebase Question

Aurora, Il
Question by mbtelephoneq
April 19, 2010
I picked up a bottle of Poison Ivy and Oak killer that is Systemic. I wanted to spray it on the thistles and let it work a little and then pull the weeds. I just wanted to be sure that it will not effect the perenials as long as I don't spray it on them. This is a large area and not my own. We pulled them all last year but they just keep coming. Any help would be great.


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Answer from NGA
April 19, 2010

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Your systemic is a broad spectrum, nonselective, systemic brush and weed killer and it won't travel in the soil so it won't affect nearby plants - as long as you don't get the product on them.

Thistle is probably one of the worst weeds to try to control! Canada thistle, a perennial, multiplies both by seed and through spreading horizontal roots called rhizomes. Most other thistles, such as bull and musk thistles, are biennials. Their first year, they produce 4- to 18-inch-wide rosettes, but must first go through a cold winter period before they flower, set seed and die the following season. Unlike Canada thistle, these biennials can be knocked out by keeping them from setting seed. As long as you keep cutting them down when they appear, the roots will eventually run out of stored energy and the entire root system will die out. 2,4-D is widely used for thistle control, but one application is seldom enough to kill the plants. I think the best time to spray thistles is in the early fall, when temperatures are still in the 70s F. Nearly all of the plants will be in the rosette stage and actively growing. At this point in the season, the thistles are transporting sugars and starches to their roots for winter, and so more effectively carry the herbicide down along with these nutrients than in spring. Go ahead and use your systemic and keep hacking and hoeing this summer if necessary. Then spot treat with 2,4-D or your systemic again this fall. Best wishes with your landscape!

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