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The war on Japanese Knotweed


July 24, 2012

A battle is being waged against an invader that has taken over river banks, riparian zones and roadsides in the Upper Delaware region, and the public is being educated on the most effective means of controlling Japanese knotweed.

Using grant funds provided by the Pennsylvania Association of Conservation Districts, several organizations have partnered in a demonstration project that targets management of a large plot of Japanese knotweed along the Lackawaxen River, behind the Stourbridge Complex in Honesdale. The multi-phase endeavor is aimed at removing the invasive plant by various means and stabilizing the riverbank with native plants afterward.

The organizations have been using the process to educate the public about management of the aggressive plant. The key to controlling knotweed is to control the rhizome system of the plant, which can spread up to 60 feet, according to National Park Service (NPS) biologist Jamie Myers, who is licensed to apply pesticides and herbicides in both Pennsylvania and New York.

At the most recent demo, on July 11, Myers was preparing to apply a glyphosate-based herbicide to an area where the plant had already been depleted by cutting. Applying herbicide to the plant’s leaves typically results in a 90% kill rate, while injecting individual stalks with herbicide kills 99% of the plants. The aquatic-approved version of the herbicide is commonly known as Rodeo, and the terrestrial version can be found under such names as Roundup and Accord XRT. They are considered to be non-selective and will kill all plants in the spray zone.

The best approach to managing knotweed is to use as many control methods as possible so that each adds to the effect of the others. Successful management will require a multiple-step control phase and an ongoing maintenance phase in following seasons to make sure the knotweed does not reestablish.

Managing Japanese knotweed without herbicides is labor intensive, requiring cutting the plants twice monthly during the primary growth period, from April to September, and employing other practices, such as plastic sheeting.
Any piece of the plant has the potential to spread. After cutting, stalks should be dried and then burned since fragmentation is the greatest source of spread. Studies using plastics applied over patches of ground where knotweed had been cut down show that the solarization effect is greater under clear plastic rather than black. A thickness of at least six- to eight-millimeter plastic is recommended.