Japanese Knotweed

Japanese Knotweed, a highly invasive plant, has appeared in various locations around Center Pond. Found throughout much of the United States, it is a world-wide issue. In order to prevent its expansion, the District researched the plant, the problems it creates and the methods for its eradication. This article shares information about Japanese Knotweed and provides the most useful, accurate guidance discovered. There is much discrepancy, particularly on the effective eradication of this frighteningly prolific invasive, so please follow the recommendations provided here. Note that Japanese Knotweed may take multiple years and multiple approaches to eliminate, so vigilance is important. The primary sources for this summary are Wild & Scenic Westfield River Committee (westfieldriverwildscenic.org) and an article prepared by Victoria Wallace, Alyssa Siegel-Miles, and Klaudia Sowizral, UConn Extension.

This invasive species can be seen all over the northeast, most noticeably along roadways and river banks. It grows up to 15 feet with a deep taproot and rhizomes (underground stems) that can spread the weed unnoticed – up to two thirds of the plant is hidden underground. It also releases chemicals that can choke out the growth of native plant species. It emerges from the ground in early spring and is sometimes mistaken for bamboo. Japanese Knotweed flowers in late August/early September, appearing at the top as 3 to 4 inch clusters of small white or cream colored petals. Its tiny dark brown and glossy seeds can be dispersed by wind, water, birds, insects, etc. The primary source of spread, however, is by rhizomes or rhizome fragments.

The UConn article cites: “Japanese knotweed thrives in disturbed areas, along roadsides, and on stream or river banks, forming dense thickets that pose a significant ecological threat to riparian areas. Its ability to rapidly colonize anarea threatens native vegetation and can greatly alter natural ecosystems. It can grow well in full sun, deep shade, soils of high salinity, and extreme drought. It can also survive severe floods. Its extensive root system has been known to penetrate asphalt and cracks in concrete.” As a result it can adversely affect foundations, other structures and septic fields. In addition, the presence of Japanese Knotweed can deter potential buyers from purchasing homes when up for sale, thus reducing property values.

While herbicides are often used to suppress Knotweed, Center Pond and vicinity forms a sensitive ecosystem that prevents the use near the lake and runoff areas. Therefore, the following effective deterrent methods are recommended:

•     The Karro Frost (non-chemical) method: Cut the tops of the plants so that bacteria can enter and help weaken the Japanese Knotweed. Remove all but one to two upper leaves on one plant within a three-foot area. This tricks the plant into believing all is well, while seriously reducing its strength and ability to grow and extend both above and below ground. Do this twice monthly, starting when the plants start appearing in spring. This process will need to be repeated annually for three to five years. Wild & Scenic has experienced great success utilizing this method to drastically reduce the growth and spread of Japanese Knotweed. It is an effective approach for homeowners with Japanese Knotweed on their properties. Note: District members volunteered at one of the local test sites to learn this approach. It is a simple, non-strenuous process that took very little time and no cash outlay.

 •      Mechanical removal: Having an excavator dig up the entire area is another possible solution, but requires proper disposal and attention for the long term. Note: depending on where you are located, you may need Conservation Commission approval for this work. New growth can be addressed by using one of the other methods.

 •      Smothering/starvation method: After cutting, cover the area entirely with a thick tarp or black plastic, extended a foot or two beyond the patch and weighted down with stone or wood chips. The covering needs to remain in place for at least five years.

 •     Manual removal: Hand-digging and hand-pulling of established plants are NOT recommended approaches as newshoots can easily form on root and rhizome fragments. However, it can be effective for very young plants, making sure the taproot and rhizomes are fully removed.

Place all removed plant material in heavy-duty, black contractor bags. DO NOT dispose of it in regular trash – doing so will just spread the plant to the resulting dump site. Do not place any cut plant material in a compost pile or leave loose on site as rhizome fragments can easily resprout. Note: cut Japanese Knotweed can survive a very long time after cutting. Leave cut material in contractor bags for the season until completely rotted or burn the waste following Becket burn regulations.

After completing one of the above methods, replant the area with native species to minimize Japanese Knotweed re-establishment. There are a number of plants that can impede its growth/establishment. It is best to ask at a local nursery for non-invasive plants that will achieve this goal in our area.

 If you use landscape companies to maintain your property, please share this information with them.

 Further research is being conducted to determine if there is an environmentally-friendly chemical product, approved by the Becket Conservation Commission, for use at Center Pond and the vicinity (wherever runoff to the lake is possible). We will notify you of any such products when the information is available.