Understand the Invasive Japanese Knotweed before doing battle
This is the time of year when the invasive plant Japanese knotweed is blooming in large clusters of greenish-white flowers. This perennial plant blooms in August and September and is quite visible along sunny roadsides in long swaths of green from 3 to 10 feet tall. It is also found along riverbanks, adjacent to swamps and in wetland areas. Like many of the invasive plants we have in our region, Japanese knotweed will shade out native plants, outcompeting them for sun and soil.
An excellent source of information about gardening in our region and the plants that thrive here is the Connecticut Gardener. Its September-October 2018 issue included an excellent article on Japanese knotweed by publisher Will Rowlands. Here are some key points from his article. The article can be found at: https://www.conngardener.com/japanese-knotweed/
-“Japanese knotweed (Polygonum cuspidatum, Fallopia japonica) is sometimes called Mexican Bamboo but is neither. It’s native to Eastern Asia and has many common names including Hancock’s curse, Himalayan fleece vine, monkey weed, tiger stick and donkey rhubarb.
-Japanese knotweed was introduced to North America in the late 1800s as an ornamental as well as for erosion control and landscape screening. According to the USDA, it’s now present in 42 states.
-It’s considered invasive in Connecticut because it’s fast growing and forms dense stands that shade out native plants and reduce wildlife habitat and biodiversity. It will also explore weaknesses in structures and underground pipes.
-It’s a threat to riparian areas because it can withstand flooding and colonize islands and stream banks creating a monoculture. It has an extensive root system that can extend 45-60 feet.
-The plants turn brown and die with the onset of frost. The leaves turn a distinctive coppery brown and are easily spotted once you know what they look like.
-Japanese knotweed readily invades disturbed areas, such as roadsides, wetlands and stream banks that get plenty of sun. It’s tolerant of road salt and compacted soil. It prefers a moist environment and sun.
-Reproduction is primarily vegetative through rhizomes and shoot fragments. Female plants produce small (.033 inch) triangular, winged seeds that can be dispersed by wind and water.”
Another excellent source for information about invasive plants is the Connecticut Invasive Plant Working Group at UCONN (CIPWG): https://cipwg.ucon.edu. Its data base and list of invasive plants is extensive and contains a list of states where specific species are banned for sale as well as suggestions for native replacement species.
If you have Japanese knotweed on your property here is some helpful information from CIPWG about how to identify and eradicate it:
“Japanese knotweed is an herbaceous perennial which forms dense clumps 1-3 meters (3-10 feet) high. Its broad leaves are somewhat triangular and pointed at the tip. Knotweed reproduces via seed and by vegetative growth through stout, aggressive rhizomes. It spreads rapidly to form dense thickets that can alter natural ecosystems or interfere with landscaping.
Japanese knotweed can tolerate a variety of adverse conditions including full shade, high temperatures, high salinity, and drought. It is found near water sources, in low-lying areas, waste places, and utility rights of way. It poses a significant threat to riparian areas, where it can survive severe floods.
Grubbing: This method is appropriate for very small populations or in environmentally sensitive areas where herbicides cannot be used. Using a digging tool, remove the entire plant including all roots and runners. Care must be taken not to spread rhizome fragments. Juvenile plants can be hand-pulled depending on soil conditions and root development. Any portions of the root system not removed will potentially re-sprout. All plant parts, including mature fruit, should be bagged and disposed of in the trash to prevent reestablishment.
Cutting: Repeated cutting may be effective in eliminating Japanese knotweed, but this strategy must be carried out for several years to obtain success. Cut the knotweed close to the ground at least 3 times a year. Cutting stems over time results in a significant reduction of rhizomatous reserves. Manual control is labor intensive, but where populations are small and isolated or in environmentally sensitive areas, it may be a good option.
Herbicides: Triclopyr will kill the top growth within a few days, but Japanese knotweed may re-sprout following treatment. Residual effects on emergence and growth the following year are variable. Glyphosate applied in spring or early summer may stunt or yellow growth, but knotweed will generally recover and continue growing. Glyphosate treatments in late summer or early fall are much more effective in preventing regrowth of Japanese knotweed the following year.
Herbicide Strategy: Late June – Cut or mow down stalks. Allow knotweed to regrow. After August 1, spray knotweed with ROUNDUP [glyphosate (41%)] @ 2.5 fl. oz./gal. Established stands of Japanese knotweed are difficult to eradicate even with repeated glyphosate treatments. Adequate control is usually not possible unless the entire stand of knotweed is treated (otherwise, it will re-invade via creeping rootstocks from untreated areas). However, glyphosate treatments will greatly weaken the plant and prevent it from dominating a site.”
Thankfully I don’t have Japanese knotweed on my property – yet. Our neighborhood and part of our property does get most of the woody stem invasive plants including autumn olive, buckthorn, barberry, burning bush and bittersweet. Perhaps our land is too dry for knotweed.
Our natural world faces many challenges and the influx of pernicious species of invasive plants continues to be the most difficult, especially for the homeowner trying to maintain native plants of our region.
Before “doing battle” with invasive plants it is important to take the time to read up and understand these new invaders. The Connecticut Invasive Plant Working Group website and other resources is a key first step. Our climate is warming, and this year’s rainy spring and summertime high temperatures makes for perfect growing conditions for many plants, including the invasive species.
Bill Reid is the chief ranger of The Last Green Valley National Heritage Corridor and has lived in the region for more than 35 years. He can be reached at email@example.com.
Sign up for our newsletter