Can you recommend a product to kill thistle in a suburban lawn in Denver. - Ask Extension
Our lawn has an increasing amount of thistle which we mow, and we pull by hand, but it gets worse every year. I believe there’s a large network of r...
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Can you recommend a product to kill thistle in a suburban lawn in Denver. #848221
Asked September 01, 2023, 1:45 PM EDT
Our lawn has an increasing amount of thistle which we mow, and we pull by hand, but it gets worse every year. I believe there’s a large network of roots underground that I can’t see. We do have bunnies in our yard and squirrels and generally don’t use herbicides or pesticides. However, I need to get rid of this before it gets any worse.
We’ve tried strong vinegar and other such methods, but it seems like we are going to need a chemical of some sort to kill this plant. It’s really bad.
Can you please recommend a product and/or a particular brand of herbicide, which is best for thistle?
I’ve been told fall might be the best time because it gives the main root a chance to absorb the herbicide and die during the winter. Do you think that’s correct?
Denver County Colorado
Expert Response
Hi David,
Because Canada thistle is connected via underground roots (rhizomes), chemistry will be best to help control it. Products that are "multi-way" will work best - essentially you'll buy one product and it will contain 3-4 herbicides that are all different and effective on weeds. I would not recommend you use vinegar because it's non-selective and can kill your surrounding turf.
These products include Roundup for Lawns (Roundup is a brand name and this product doesn't contain any Roundup), Ortho Weed Be Gon Max + Crabgrass Control, and Weed Out with Q. There are many others. These are labeled for use for lawns - so they will be selective herbicides that will kill broadleaf weeds, but not kill your existing turf.
Fall is a great time to control Canada thistle. Start now and plan on doing two, more likely three, applications of product. Read the label of the product in its entirety and spray as directed.
I would then focus on boosting cultural health of the lawn - getting the lawn healthy will help combat weeds. So proper water, fertilizer, and mowing will be essential. You can seed this fall (aerate the lawn heavily making lots and lots of holes), but perhaps start with fertilizer to help boost overall turf health. Apply your favorite fertilizer with a push-type spreader and then water in with at least 1/2" to 1" of water or rain. A lawn that is healthy and competitive will be the best defense against weed growth, including Canada thistle.
But you'll need to first control the thistle, fertilize this fall, and then you can also consider seeding. Any seed will need regular water (2-4 times per day) to get established before frost. I would seed before September 15.
Because Canada thistle is connected via underground roots (rhizomes), chemistry will be best to help control it. Products that are "multi-way" will work best - essentially you'll buy one product and it will contain 3-4 herbicides that are all different and effective on weeds. I would not recommend you use vinegar because it's non-selective and can kill your surrounding turf.
These products include Roundup for Lawns (Roundup is a brand name and this product doesn't contain any Roundup), Ortho Weed Be Gon Max + Crabgrass Control, and Weed Out with Q. There are many others. These are labeled for use for lawns - so they will be selective herbicides that will kill broadleaf weeds, but not kill your existing turf.
Fall is a great time to control Canada thistle. Start now and plan on doing two, more likely three, applications of product. Read the label of the product in its entirety and spray as directed.
I would then focus on boosting cultural health of the lawn - getting the lawn healthy will help combat weeds. So proper water, fertilizer, and mowing will be essential. You can seed this fall (aerate the lawn heavily making lots and lots of holes), but perhaps start with fertilizer to help boost overall turf health. Apply your favorite fertilizer with a push-type spreader and then water in with at least 1/2" to 1" of water or rain. A lawn that is healthy and competitive will be the best defense against weed growth, including Canada thistle.
But you'll need to first control the thistle, fertilize this fall, and then you can also consider seeding. Any seed will need regular water (2-4 times per day) to get established before frost. I would seed before September 15.
Dr. O’Connor,
Thank you so much for your informative reply! Very helpful information.
I have just one follow up question: when you say, “weed out with Q” …. Does “Q” = Quinorac?
Is this picture the product that you’re referring to… I believe so, but just wanted to validate before I open the product and use it!
Thanks so much!!
Hi David,
Yes, Q = quinclorac! Very good observation. Remember to mix according to the directions on the label (more is not better) and spray when it's calm and not above 90 degrees.
It will take two, possibly three, applications. Time them according to the instructions on the label.
Yes, Q = quinclorac! Very good observation. Remember to mix according to the directions on the label (more is not better) and spray when it's calm and not above 90 degrees.
It will take two, possibly three, applications. Time them according to the instructions on the label.
