By Susan Neul
A more stunning specimen than Japanese Knotweed cannot perhaps be found to grace a carefully curated landscape. Its arching bamboo-like branches, its widely spaced huge ovate leaves and its thousands of tiny wispy white flowers provide a magnificent backdrop to any garden.
At least that’s what botanist Philipp Franz von Siebold thought in the early 1800s when he encountered this ancient plant in Japan where it had been thriving for 150 million years on the slopes of volcanoes. Siebold admired the plant so much he brought cuttings to England for cultivation and sale around the European continent and North America. Knotweed was even awarded a gold medal by the Agricultural and Horticultural Society in Utrecht.
Little did landscapers know that once out of its native habitat, this plant would exercise hegemony and that the battle to contain it would be a very long and arduous one. Knotweed spreads by extending its underground rhizomes in a spoke-like pattern where they take up soil space and nutrients. The aggressive rhizomes combined with knotweed’s large leaves, which densely shade the soil, quickly crowd out other plants. Knotweed loves moist soil and recently disturbed ground so is often found along verges and ponds.
Removing the flowers before they seed can help a little to control the spread of knotweed, but it is more effective to kill the plantaltogether. Digging out the rhizomes has proved to be futile as even a smidgen left behind can regrow.
Carefully injecting glyphosate or triclopyr into the plant stem or spraying the leaves in late summer can successfully kill knotweed,but most homeowners prefer to go the organic route. The usual organic techniques – continual cutting down and/ or smothering with heavy black plastic – can have some success, but it can take up to five years for the roots to finally give up the ghost. In addition, these techniques are both labor intensive and necessitate careful disposal procedures. Recent observations suggest that knotweed’s reaction to the sudden loss of its photosynthesis ability is to speed up its rhizome growth. The roots sometimes can travel up to 60 feet to escape the plastic shroud, where they happily resprout—a phenomenon which means the battle never ends.
The plant has become such a scourge around Center Pond in Becket that two residents researching eradication techniques discovered the Karro Frost starvation method (named for its founder; see the PDF document below), which is used by the Westfield Wild and Scenic River organization to control the huge stands of knotweed found along the river.
A Massachusetts conservation botanist, Karro Frost’s research focused on attempting to stop the rapid rhizome growth following clear cutting. He discovered that the roots do not go into overdrive if they are permitted some limited access to photosynthesis by allowing two leaves to stay on the plant rather than a total denuding. Even with keeping two leaves, the photosynthesis is reduced enough to eventually starve the roots.
For more information: ipm.cahnr.uconn.edu/invasive_plants_japanese_knotweed/
