6. Japanese knotweed (Polygonum cuspidatum)
A herbaceous shrub-like perennial, Japanese knotweed has teardrop-shaped leaves. It reproduces primarily by vigorous creeping roots, but it also spreads by seed. Once established, it’s difficult to remove. Japanese knotweed was introduced to North America in the last 1800s from eastern Asia and is still widely sold at garden centres and nurseries. It prefers open woods and forest edges, but it can tolerate a wide range of habitat.