Knotweed Woes

Knotweed Woes

Invasives Invasion


Stems appear reddish with bamboo-like structure.

Invasive species can make habitat conservation, restoration and management incredibly difficult. Once non-native, invasive or noxious plants establish themselves, removal and eradication become very labor intensive, requiring manual removal, possible chemical applications and years of monitoring and maintenance.  Invasive species will compromise a habitat by taking over the niche of native species, native wildlife will no longer be attracted to the area, throwing the biome into disorder.  More and more, roadside botanical surveys consist of non-native species and noxious weeds, picking out the natives is nearly impossible. It will always be difficult to control areas adjacent to roads where traveling cars are able to pick up and deposit pollen and seeds over long distances, however when management isn’t universal the problem only intensifies.  Climate change is exacerbating the issue, realigning hardiness zones and making once uninhabitable areas habitable and ready for non native species to get a foothold.

dense_ japanese_knotweed_stand_alongside_a_river
A monocultural stand of knotweed along this river.

Exotic Exposure

Japanese knotweed (Polygonum cuspidatum) is native to Asian countries and was originally, purposefully,  introduced to the US in the 19th century, for ornamental value and erosion control. After the initial introduction a 50 year lag time was recorded prior to the exponential growth period which the US has been experiencing since 2006.  Knotweed spreads through vegetative and root material sprouting and suckering.  Its success seems to be bolstered by many manual removal tactics, separating plant material from parent plants spawns reproduction.  Japanese knotweed has become a management issue throughout New England and Mid-Atlantic states establishing itself in riparian areas, wetlands, wet meadows, herbaceous and shrub wetlands and forest floodplains. Although it seems to thrive almost anywhere, given the chance.

baby knotweed
Knotweed sprouts have red venation throughout the heart or spade shaped leaves.

Stemming the Spread

Various methods are currently used to attempt to eradicate knotweed from habitat and landscapes; manual pulling, mechanical removal, chemical applications and/or a combination there of.  As mentioned above, disturbing vegetative or root mass tends to spur growth, so if mechanical removal is the method of choice, several feet of soil will need to be removed with the plant material and disposed of properly.  Many projects attempt to “clean” soil and re-use it, which, while an environmentally sound method, may not produce a landscape free from knotweed-any tiny bit of root left over, will take hold and begin a new, vigorous population.  Manually pulling out knotweed plants is not only incredibly, back-breakingly, labor intensive it also spawns additional plants.  Should chemical applications be selected, spray targeted herbicide when the plants are flowering, this is when they are most vulnerable.  After years of dealing with invasive species issues, specifically knotweed as of late, the most effective method seems to be to cut plants down to 1 foot and apply a narrow spectrum herbicide directly to stem, a hand sprayer or paint brush is the ideal manner. Whichever method is deemed most effective and efficient for the problem at hand monitoring and maintenance will be required over the next several growing seasons.  Visit sites every couple of weeks and evaluate growth, re-treat if necessary and continue to monitor, eventually the population will become stressed and growth will diminish.

If you find Japanese knotweed on your property, suspect it is there or have land that seems especially susceptible to invasion, consult your arborist to develop a plan for prevention or eradication.  It may seem to be a futile battle but we can all make a difference by planting natives, being responsible stewards of our property and community and being vigilant.