Giant knotweed, scientific name Polygonum sachalinensis, is an introduced invasive species to North America and Europe that is quickly causing ecological and economical damage. Not only does it spread quickly, its destructive capabilities often render the soil unfit for construction until the plant has been totally eradicated. This rapid growth rate has also lead to knotweed being considered a candidate for future biofuel generation.
Giant knotweed is a bamboo-like shrub with canes that grow up to 12 feet tall with roundish heart-shaped leaves about 1 foot long. The canes are up to 2 inches in diameter and are separated into nodes similar to bamboo. It forms large clumps that send out underground rhizomes up to 25 feet that sprout more canes. The small flowers arise sparsely in clumps around the nodes on the stem and are a greenish yellow-white color.
Giant knotweed is native to eastern Asia where it thrives in rich moist soil. In other parts of the world where it has been introduced by the ornamental plant trade, it grows in many different types of soils and prefers moist low areas and stream banks. It survives cold winter areas by dying back to the ground level and resprouting as the temperature warms. It requires at least 120 days a year above freezing to survive.
Giant knotweed will reproduce by seed, and the flowers are self-fertile. It will hybridize with the closely related Japanese knotweed, creating Bohemian knotweed, which is very common in the United States. The primary method of reproduction is by division or cuttings. A small piece of cane or root less than an inch long is enough to sprout a new plant, making it difficult or impossible to eradicate.
Because of its growth habit of spreading by runners and its rapid growth rate, knotweed can quickly take over a riverbank or moist area, choking off all of the native plant life and the animal life that depends on those plants. The roots invade underground sewer lines, water lines and foundations. Canes can grow through asphalt so often every single piece of the plant needs to be removed to lay a road or parking lot.
Many different eradication methods have been tried. Giant knotweed will respond to sprayed-on herbicides, but they are not always practical around aquatic settings, where it tends to colonize.
Mowing or manual removal is the most effective, but every piece of the plant needs to be removed and destroyed. It can not be composted. After three years of cutting all the new shoots down twice a month, the rootstock may die off, depleted it of energy reserves.
Recent trials of injecting concentrated systemic herbicide directly into the base of a growing cane has led to some success, but it is very labor intensive.