Japanese Knotweed, showing alternate leaves with general tear-drop shape and flattened base, and reddish twigs.
Name: Japanese Knotweed (Fallopia japonica)
General Information: Japanese Knotweed is a semi-woody perennial plant that is often mistaken for bamboo. It is native to Eastern Asia, and was first introduced to North America as a horticultural plant in the 1800’s. Japanese Knotweed is a particularly persistent invasive species due in part to its vigorous root (rhizome) system. Japanese Knotweed can grow in a range of habitats, including open sunny sites, ditches or disturbed areas, riparian areas and in the forest understory. In Algonquin Park’s mixed and deciduous forests, large stands of Knotweed could outcompete most forest wildflowers, shrubs, and tree seedlings. Typically native plants cannot regenerate underneath established stands of Knotweed. Over time, the lack of tree regeneration could result in loss of tree canopy and a more open forest; in the short-term, knotweed will exclude any native shrubs or herbs from growing, thereby decreasing habitat capacity for wildlife.
Japanese Knotweed can easily be spread along ditches or waterways, where pieces of rhizome or stem can be dispersed by moving water. More typically in Algonquin Park, Knotweed could be spread by people moving plant material or improperly disposing of plants, and by heavy equipment that has not been properly cleaned and contains plant parts that then become established.
Identification: Leaves are alternate, typically bright green, tear-drop shaped with a flattened base. Twigs can be green or reddish in a zig-zag pattern. Stems are round, light green with reddish-purple markings (during growth) or brown and brittle (previous year’s growth), smooth, and resemble bamboo. Knotweed typically flowers in late summer/early fall, and has many sprays of tiny greenish-white flowers clustered on a stalk. There are no native species that look similar to Japanese Knotweed.
Control: A variety of means can be used to control Japanese Knotweed, but the initial effort should always be towards limiting the spread of established populations and eliminating satellite populations. The rhizomes of this plant are extremely persistent, and cutting of aboveground vegetation will likely result in regrowth. However, repeated cutting throughout the growing season can be used to control the species, and cutting over several years can significantly weaken the rhizome system. Digging up and removing the rhizomes along with aboveground vegetation can be effective, but follow-up treatments will likely be required. Ultimately both the aboveground vegetation and rhizomes need to be controlled. Some plants may require herbicide treatment for successful eradication. Upon discovering Japanese Knotweed, notify Ontario Parks at firstname.lastname@example.org or 705-645-7436 so that a plan for control can be made.
Japanese Knotweed is regulated as a restricted species under the Ontario Invasive Species Act, 2015 (https://www.ontario.ca/laws/statute/s15022). Under the act, a restricted species is prohibited from being brought into a provincial park, and prohibited from being deposited or released in Ontario.
Ontario’s Invading Species Awareness Program: Japanese Knotweed
Best Management Practices in Ontario: Japanese Knotweed
Japanese Knotweed growing along a forest edge in Algonquin Provincial Park.