Japanese knot weed

Shawn Swift asked 1 year ago

Hello, I recently discovered Japanese knotweed on my property in riverside, RI. I’ve tried to dig it out but it keeps coming back. Do you have any tips to fight this invasive plant? I prefer not using heavy herbicides but am open to suggestions. Thank you, 

2 Answers
Annie Klodd answered 1 year ago

Hi Shawn,
The following recommendation is from Mark VanGessel, professor and extension specialist in weed management at the University of Delaware:
Cut the knotweed back, as low to the ground as possible, and dispose of the stems.  Do this now.  Then allow it to regrow to about 2 ft in height and spray with glyphosate/Roundup.  If mixing your own solution (as opposed to use a product ready-to-use) apply at rate recommended for perennial weeds on the product label.  Do not allow the spray to come in contact with desirable plants and tree roots.
Repeat this procedure for 2-3 years, the time required to eliminate Japanese knotweed.
I have an additional note – after you cut it back, dispose of the stems instead of leaving them on the ground to help prevent spread. Here is an article with further info from Penn State Extension: http://extension.psu.edu/natural-resources/forests/private/tools-resources/publications/invasive-plants-and-insects/japanese-and-giant-knotweed
If you can tell us how big of an area is infested, the type of equipment you have access to, and what your land is used for (crop land, pasture, lawn, etc) we will be able to provide you with a more specific recommendation for your particular situation.
Please let us know if you have further questions.

Annie Klodd answered 1 year ago

Shawna – Further info: complete control without using herbicides is difficult, but can sometimes be achieved with diligent mowing and digging of shoots. The tough thing about this plant is that it spreads underground via rhizomes. So even if the aboveground shoots are cut off, the rhizomes survive and will continue to give rise to new shoots. This is why we recommend a spot application of glyphosate to the cut area, following shoot removal as described above, because it allows the herbicide to be transported to the rhizomes to stop them from spreading. However, without herbicide application, you may still be able to keep it in check with digging, careful monitoring, and repeat weekly mowings. Over time, this will weaken the plant enough that it would eventually die out.  But without diligent repeated mowings, it will reinfest quickly.