https://extension.psu.edu/japanese-knotweed

https://mnfi.anr.msu.edu/invasive-species/JapaneseKnotweedBCP.pdf

https://web.uri.edu/wp-content/uploads/sites/2078/Japanese-Knotweed.pdf

What is the best method/ product for eradication of Japanese knotweed. We have used Tordon RTU with some success, but the plant has returned.

Japanese Knotweed (JK) is one of the more troublesome invasive plants to control due to its very tough root structures; especially once it is established. If the JK you are trying to eradicate is several years old, it will take several years to get it under control and a continued observation to control the plant as it may reemerge.

I gave you 3 links from university and government sources that help with understanding the plant and ways to control the plant. These articles also provide identification, so make sure it is JK. Timing of herbicide use is probably the most critical issue in getting JK under control and keeping it controlled. These articles do not discuss Tordon RTU, but this is the better product than the ones given in the links. However, Arsenal could provide extra help with control if you do not have other woody plants or perennial plants to worry about. Tordon RTU gives you the ability to clip the plant near ground level and then trickle the herbicide on the stem and let it go to the roots and prevent it from getting to other desirable plants. Tordon RTU can move with the soil moisture if it gets into the soil by over applying the stem, so be careful not to get the herbicide on the soil.

The Penn State Article gives you timing and treatment information that you could follow for treatment time. Since you are in the U.P. in early August it could be a suitable time to apply the Tordon.

If you are getting regrowth the following year you can let the JK get a foot or so tall, clip at ground level and dab with Tordon RTU and do so with each sprout that comes up. Come August follow up with more treatment if you see other escapes. It may take several years to get good control and then look for it each month of the growing season to keep it under control.

If you use Arsenal follow the label as it can harm nearby vegetation.

Thank you for reaching out to “ask extension” with your concern of controlling Japanese Knotweed.

The information below is from our invasive plant expert

Dr. Hill

Japanese knotweed (Fallopia japonica) 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. Mowing is not recommended as it can spread the infestation. Mulching with most materials is not an effective means of suppressing Japanese knotweed. However, smothering it for several years with a thick plastic has been shown to be effective. Again, the practicality of this will depend on how large the area is, but you can read some detailed instructions from New Hampshire's Dept. of Agriculture at https://www.agriculture.nh.gov/publications-forms/documents/japanese-knotweed-control.pdf

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 http://mariannaszucs.weebly.com/ 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 or as injections for more effective treatment of small populations, see herbicide labels for details. These methods help prevent movement to other plants/areas. Glyphosate (e.g. Roundup Weed and Grass Killer Super Concentrate, though there are other generics available at stores like Tractor Supply Company, TSC) is usually the best choice for residential users because of its safety, 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.

**Note, if your neighbors also have this weed and they are not actively controlling it; it will continue to be a problem. I have seen that in my 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: https://www.michigan.gov/invasives/take-action/local-resources 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).

Jackson, Lenawee & Washtenaw CISMA: https://www.michiganinvasives.org/jlwcisma/

Erin Hill, PhD

Jay B.

MSU Extension Master Gardener.