Giant knotweed is an herbaceous perennial that is woody in appearance and can grow over 10 feet tall. Root extend deeply into the soil creating a dense impenetrable mat.
Alternate, simple, dark green. Leaves are 6-14 inches long and have a heart-shaped base coming narrow to a point.
Numerous small, greenish-white flowers appear in the leaf axils of the upper stems. Giant knotweed blooms have both male and female parts in the same flower.
Fruits are papery and broadly winged. Each fruit contains a 3-sided achene that is small, shiny and brown. Small amounts of seed are viable and have no dormancy requirement.
Knotweeds are capable of quickly forming dense stands where they can crowd out native vegetation. Thickets can clog small waterways and displace streamside vegetation, increasing bank erosion and lowering the quality of riparian habitat for fish and wildlife. Once established, these stands are very difficult to eradicate.
Introduced to North America as an ornamental plant in the late 1800s.
Meadows, fields, river banks, lake shore.
Early in the season, new shoots can grow three to four inches per day. Knotweed grows three to 12 feet tall. The plant’s greenish white flowers are functionally unisexual... Blooms are up to 4” long and occur during August-October.
Giant Knotweed, 5447672, Leslie J. Mehrhoff, University of Connecticut, CC 3.0
Giant Knotweed flower, 2137039, Barbara Tokarska-Guzik, University of Silesia, CC 3.0
Giant Knotweed, 2137058, Barbara Tokarska-Guzik, University of Silesia, CC 3.0
Giant Knotweed and Japanese knotweed, 2136091, Barbara Tokarska-Guzik, University of Silesia, CC 3.0
Giant Knotweed Leaf, 5447656, Leslie J. Mehrhoff, University of Connecticut, Bugwood.org