How do we eliminate Giant Knotweed? #832073

Asked May 25, 2023, 4:08 PM EDT

We believe we have an infestation of Giant Knotweed beside our pole barn. What herbicides, chemicals or mechanical strategies do you recommend to eliminate this plant?

Clinton County Michigan

Expert Response

This appears to be the invasive species, Japanese knotweed (Fallopia japonica). Japanese knotweed is an aggressive perennial that reproduces both by rhizomes (underground stems) and by seed. It is often found in shaded to partially shaded habitats on moist, nutrient-rich soils, though it can thrive in gardens and on roadsides as well.

Physical control.

Repeated chopping or digging may eventually deplete underground reserves thus reducing the competitive edge of Japanese knotweed, however it is expected to take several growing seasons to achieve eradication. Both root and stem fragments can root and result in new plants, therefore any plant material removed for control should be placed in the trash headed to the landfill and not in yard waste or compost. Mulching is not an effective means of suppressing Japanese knotweed. Mowing is not recommended as it can spread the infestation.

Biological control.

At MSU Dr. Marianna Szucs laboratory group is working on a testing a biological control agent for Japanese knotweed, however this is still in the preliminary stages of testing for the state. More information Dr. Szucs program can be found at in the Resources section.

Chemical control.

Repeated herbicide applications of glyphosate or various brush killers, such as triclopyr (Garlon, Pathfinder, Brush-B-Gon) or triclopyr + 2,4-D (Crossbow) should help keep this plant under control, but eradication is difficult. Chemical applications can be made to cut stems for more effective treatment of small populations, see herbicide labels for details. Glyphosate (e.g. Roundup Weed and Grass Killer concentrate) is usually the best choice for residential users because of its ease of application and lack of residual activity in the soil, however, in a lawn triclopyr or triclopyr + 2,4-D are better options because they will not harm the grass. Be sure to read and follow all labeled instructions. Repeat applications will likely be necessary. Also note that early fall applications (when temperatures are still above 50F) are often the best time to target herbaceous perennial weeds, like Japanese knotweed, due to better movement of the herbicides to the root/rhizome system with the natural fall movement of carbohydrates to those systems at the time.

There are more potent herbicides available that may provide eradication, but they are very persistent and can impact trees and shrubs with roots in the area. If you would like more information on these herbicides please contact me via email at <personal data hidden>.

**Note, if your neighbors also have this weed and they are not actively controlling it; it will continue to be a problem. We have seen that in our own yard.

Cooperative Invasive Species Management Areas (CISMAS).

In Michigan there are several groups working to eradicate invasive plant species. Each group covers a cluster of counties and is funded by varying sources, some volunteer efforts, and sometimes property owners themselves. Groups have varying priority species and their abilities to help vary. If they are not able to help with a particular species of interest they still may be able to point you toward local, reputable companies that can help. You can find your CISMA at this site: Usually there is a coordinators email and phone number listed and sometimes there is also a web address (or you can do an internet search for the group’s name).

Erin Hill, PhD Replied May 25, 2023, 11:11 PM EDT

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