Japanese knotweed (polygonum cuspidatum) is a non-native herbaceous perennial that grows up to 3 metres in height, forming dense thickets. Colonies in North America originate from one or a few clones.
The leaves are broadly oval and 5 to 15 centimetres long. The stems are large and hollow, like bamboo. White flowers appear in August and September.
Most commonly found in moist open habitats and in areas with disturbed soil.
Recommended Methods of Control
To control the spread of Japanese knotweed in gardens and residential properties, stems must be cut down several times throughout the growing season to deplete the root system. Cut the base of the stalk just before flowering once the plant reaches a height of 5 to 6 feet. This usually occurs around mid to late June in our region. Subsequent cuttings may occur around early August and again in early September. Persistent cutting may be combined with other options such as digging out roots and laying down tarp material for several years in order to successfully control this species.
As with many other invasive plants, Japanese knotweed can regenerate from root fragments, cut material should be disposed for garbage pick-up for landfill to avoid spread to other areas. In forest management or natural resource applications, established colonies should be cut followed by foliar application of glyphosate herbicide to the re-sprouted plants in order to kill the roots, under approved conditions.