If I cut my Giant knotweed off, paint the stem with Roundup to kill the plant, does the root etc which is underground become poisonous? (i have never used poison in the garden, but have recently learned that may be the only control method available for this infestation) Thank you.
Hennepin County Minnesota
I assume giant knotweed is the same as Japanese knotweed. Some of the glyphosate may be excreted from the roots into the soil; however, the glyphosate will be tightly bound to the soil and should not harm other plants in the area. The advantage of the cut-stem method is that you will apply a fraction of the herbicide than if you sprayed the foliage. This will minimize any unintended effects of the herbicide.
Thank you Bob. I am primarily concerned with the effect on burrowing mammals and insects. Will the residual poison in the soil hurt them? Maureen
The acute toxicity of glyphosate is relatively low and would have little direct affect on organisms in the treated area. There have been numerous studies looking at non-target effects of glyphosate on insects, mammals and other animals in treated areas. The primary effect is due to the change of the vegetation caused by the herbicide. In forest clear cuts they see a dramatic reduction in animals due to loss of cover and food, not due to the toxicity of glyphosate. In your situation you will be using a selective application and thus there should be habitat present to support the organisms residing in the area.