Japanese knotweed poses threat to U.S. landscapes

The forest behind a residential property has become overrun with non-native invasive weeds, notably Japanese knotweed. The plants have encircled trees and caused significant damage to the local flora.

Japanese knotweed was introduced to the United States from Japan in the mid-1800s by botanists. It spread rapidly due to the absence of natural predators and is now prevalent across the country, particularly in watery areas with disturbed soil.

This plant resembles a bushy bamboo with white flowers that bloom in late summer. Its rapid growth rate and minimal care requirements initially made it popular among gardeners unaware of its invasive nature. Knotweed can grow up to three feet per week, and its underground rhizomes can extend several feet each growing season, causing structural damage by pushing through retaining walls, walkways, road surfaces, and even concrete foundations.

Robert Naczi, curator of North American botany at the New York Botanical Garden, states that eradicating knotweed is challenging as it requires either complete physical excavation or pesticide application over several years. A new plant can sprout from even a tiny root fragment.

Naczi emphasizes respect for this resilient plant: “You have to respect this plant.” He notes that his neighbor's infestation could potentially breach his property line if not addressed soon.

In the United Kingdom, where knotweed is more rampant, eradication efforts have been deemed unrealistic. U.K. homeowners must disclose the presence of knotweed during property sales and may face prosecution if they cause it to spread into the wild. Knotweed insurance policies are available to cover annual treatment costs aimed at controlling its spread.

In contrast, while U.S. botanists are concerned about knotweed's presence, it has not caused widespread alarm yet. Caitlin and Paul Maher Jr., writers who moved to Lubec, Maine in 2020, discovered an acre-sized patch of Japanese knotweed on their property. Paul Maher describes his initial attempts to control it as arduous and physically taxing but reports some success after three years of repeated treatments.

Dr. Bernd Blossey from Cornell University suggests mowing smaller patches repeatedly or digging them up entirely for effective control. For larger infestations near sensitive environments, herbicide injection into cut stalks is recommended.

A potential biological control method involves releasing Aphalara itadori psyllids from Japan that feed on Japanese knotweed. Jeremy Andersen from the University of Massachusetts reports promising results against giant knotweed but notes mixed outcomes with Japanese knotweed due to climate adaptation issues.

Landscaper Dan Philos-Jensen confirmed that another invasive species—oriental bittersweet—was present in one resident’s yard instead of Japanese knotweed but warned that it might only be a matter of time before knotweed appears there too.