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Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

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Monday - July 28, 2008

From: Chicago, IL
Region: Midwest
Topic: Invasive Plants
Title: Getting rid of Polygonum punctatum (dotted smartweed)
Answered by: Nan Hampton

QUESTION:

Dear Mr. Smarty Plants, After much online research, my property has been invaded by what I believe to be Persicaria polygonum punctata, more commonly known as dotted smartweed. I have tried horticultural vinegar but it only curled the leaves. Additionally, I have read that horticultural vinegar detrimentally changes the soil. These plants become at least three feet tall so it's hard to spray all parts of it. I have thought of covering them with clear plastic sheeting but don't know if that is enough heat to kill the roots. I feel my last resort is glyphosate, of which I have heard pros and cons. Digging them out is as horrendous as trying to comb out knotted hair and I do not have the time nor energy as they are everywhere. As I have said, I have been all over the Web looking for an answer as to how to kill these plants. Do you have any suggestions? I would really appreciate any advice you can provide. Jo

ANSWER:

I assume you mean Polygonum punctatum var. punctatum (syn.=Persicaria punctata)(dotted smartweed), a plant native to North America. Another common name is water smartweed. Or, are you talking about Polygonum persicaria (spotted ladysthumb). This is an introduced species. Whichever one it is, the treatment will be the same. The most environmental friendly way, of course, would be to dig them out or pull them up. Alternatively, instead of spraying them you can cut them off near the ground and carefully paint or inject the cut stalk with herbicide. This would put the herbicide where it would do the most good and would be the least harmful to other plants and to the environment. If you spray them, you run the risk of harming other plants nearby that you want to save. Consult with the Cook County Extension Service or visit a reputable garden nursery for advice on the best herbicide to use. Please read carefully all instructions that come with the chemical and follow the safety guidelines to protect yourself and the environment.
 

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