Where it's from

Eastern Asia

What Japanese Knotweed does to the habitat

Fast growing, it forms thickets that shut out native plants. It survives river flooding then quickly takes over areas from native plants.

It can break through asphalt and foundations to cause structural damage.
Where it grows it alters fish and wildlife habitats.

Management

One way to deal with the plant is to cut the stalks periodically. This could be a  drawn out process because of the rhizome and root system food bank under ground. Just keep cutting until you starve the roots and rhizomes and the plant dies.
The plant can regenerate from small sections so be sure to thoroughly dry or burn the remains, or bag and dispose.

Using a contact herbicide is also a  process that requires patience, like cutting. The plant regenerates from food in the roots.  So several applications will be necessary.

Perhaps the best approach is a combination of cutting and then spraying the cut stalks with a quickly degrading systemic herbicide such as Round Up.

DESCRIPTION

Botanical Glossary

Polygonum cuspidatum is an herbaceous perennial that appears woody, and reaches 1-3 m (3-10 ft.) in height. The round stems are hollow and covered with scales. The shoots grow from spreading rhizomes that can reach 20 m (65 ft.) in length. The leaves are broadly oblong-ovate or ovate-lanceolate, 8-15 cm (3-6 in.) long and 5-12 cm (2-4.75 in.) wide. The tips of the leaves are abruptly acuminate, while the bases of the leaves are truncate. The lower leaf surface lacks the minute trichomes that other similar species possess (see below). The numerous, greenish-white flowers of Polygonum cuspidatum are borne in panicles from the upper axils. These panicles measure 8-15 cm (3-6 in.) long. The flowers are functionally unisexual: each of the male and female flowers still have the complementary organs, but they are vestigial. The inflorescences of the male flowers tend to be upright, while those of the female flowers tend to be drooping. Flowers appear from August to September. The fruit are papery and winged, and are 6-10 mm (0.25-0.4 in.) long. These fruits contain black, smooth, shiny, 3-angled achenes that are 3-4 mm (0.2 in.) long. When frost hits this plant, it quickly turns brown and dies back for the year. Page References Bailey 348, Fernald 589, Gleason & Cronquist 139, Holmgren 124, Magee & Ahles 441, Newcomb 190.