Invasive Plants, Pacific Northwest
Japanese knotweed has established itself in the Pacific Northwest and now grows on all parts of the coast. It is a perennial originally from Asia, it was introduced to North America in the late 1800’s as an ornamental. It has horrendous invasive potential due to its rapid growth and reproductive capabilities.
This plant flourishes along stream side banks, ditches, wetlands, and disturbed areas. The key problem with this invasive plant is that once established, it displaces virtually all other vegetation. It is difficult if not impossible to eradicate it once it has become established.
It forms dense stands which grow up through the preceding years dead thickets. The dead stems and leaf litter decompose very slowly, forming a deep organic layer which prevents native seeds from germinating. Once present at a site, Japanese knotweed increases in area very rapidly and soon forms huge stands.
It is able to grow through concrete, walls and tarmac. This robust weed quickly spreads via its rhizomes that reach up to 20 m in length. New plants can sprout when these rhizomes travel downstream or are relocated by humans in fill dirt. Even tiny fragments of the plant can form new rhizomes, which highlights the importance to remove all parts of the weed when exercising control methods.
It also reproduces to a lesser extent with seeds that spread primarily by water. This means of reproduction is far inferior to rhizome propagation. This invasive plant grows in a variety of soils including silt, loam, and sand. Additionally, japanese knotweed can tolerate adverse conditions including full shade, high temperatures, high salinity, drought and flooding. Japanese knotweed grows well in disturbed soils and easily invades urban environments where humans routinely disturb and transport soils.