Japanese and giant knotweed (Polyganum cuspidatum and P. sachalinense) are difficult plants to control under the best of circumstances. In their home range of Asia, they specialize in re-colonizing and re-sprouting after volcanic lava flows. In order to do this, knotweeds store energy in underground stems called rhizomes. This adaptation not only makes them hardy to fire, it allows them to spring up over and over again when cut. Knotweeds, like mint and bamboo, can also sprout from pieces of stem as small as a few inches long, making tilling and mowing into ways to spread rather than methods of control.
For all of these reasons, chemical control becomes the best bet for killing Japanese and giant knotweeds. However, their hardy nature means that it may take several years to “conquer” these invasive plants. The Northwest Michigan Invasive Species Network has been treating knotweeds in high-quality (near waterways) and high-spread-risk (roadsides) areas for three years now, and we’re finally seeing some positive results.
This spring, when the knotweed was about 3 feet tall, we sprayed stands throughout our 4-county service area with Milestone (aminopyralid). Stands without trees nearby were chosen, as Milestone can harm some species. Most stands died very soon after, though some sent up a few shoots that were sprayed again in the fall.
In August, we sprayed the remaining knotweed stands with a mixture of Clearcast (imazamox) and glyphosate (the active ingredient in Round-Up) at least 60 days before frost, as recommended by the manufacturer. As the plants will die back in fall anyway, we’ll have to wait ’til spring to see how effective our treatments were.
Finally, a word of caution.The Northwest Michigan Invasive Species Network state-certifies and trains its crew in appropriate herbicide handling and application. Though all the herbicides we use are the lowest-risk to human health while still being effective at control and are not restricted-use chemicals, we encourage anyone considering chemical treatment to seek help from a professional in choosing and applying herbicide. At the very least, reading and following the label and instructions that accompany any herbicide will reduce risk of environmental and health impacts.
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