Knotweed, Japanese

Fact SheetFallopia japonica

Images of this species:

Common look-alikes:

Identification

Appearance

Japanese knotweed is an upright, shrublike, herbaceous perennial that can grow to over 10 feet in height. As with all members of this family, the base of the stem above each joint is surrounded by a membranous sheath.

Foliage

Stems of Japanese knotweed are smooth, stout and swollen at joints where the leaf meets the stem. Although leaf size may vary, they are normally about 6 inches long by 3 to 4 inches wide, broadly oval to somewhat triangular and pointed at the tip.

Flowers/ Fruit

The minute greenish-white flowers occur in attractive, branched sprays in summer and are followed soon after by small winged fruits. Seeds are triangular, shiny, and very small, about 1/10 inch long.

Check out the downloadable fact sheet above.

Check out this video by the OutSmart Project on how to identify Japanese knotweed.

Biology

Ecological Threat

Knotweeds are capable of quickly forming dense stands where they can crowd out native vegetation. Thickets can clog small waterways and displace streamside vegetation, increasing bank erosion and lowering the quality of riparian habitat for fish and wildlife. Once established, these stands are very difficult to eradicate.

ORIGIN

Both species of knotweed were introduced into North America for ornamental use and for forage and erosion control in the late 1800s.

HABITAT

Forest edges, meadows, fields, floodplains, disturbed areas

Life Cycle

The stems can reach heights of up to 10 feet tall, with some records indicating they can grow to 13 feet tall. The older shoots tend to get woody near the base as they age. Flowers emerge in late summer as small white to off-white racemes / panicles. Pollination is by insects, primarily by bees. The three-winged seeds (Calyx) were often thought to be sterile; however, a basic germination test in NH showed that 95% of seeds collected from various populations were viable, but not seen as a significant vector for its spread.

Seedlings often succumb to frost, desiccation, shade, predation and smothering. The rooting system, which is composed of numerous intertwined rhizomes that can grow up to 3 inches in diameter, is the primary reproductive propagule that enables it to quickly spread to new locations. The rhizomes have the potential to spread laterally 23 to 65 feet away from the crown. Most also have a deep taproot.

Based on the extensive rooting system, the majority (2/3) of Japanese knotweed plants occurs below ground. It helps to ensure the plant will rebound if damage to the shoots occurs. In addition, perennating buds found on the root crown and along the rhizomes will also react to shoot damage, i.e. mowing/cutting, by sending up additional shoots along the root. This typically results in radial/clonal spread of the plant and increases its shoot density.

Management Options

This species is Quarantined: Class B Noxious Weed

Treatment Concerns:

Up to 2/3 two thirds of the mature plant’s biomass is stored underground in its system of rhizomes, which can reach 3 in (7.5 cm) in diameter and dive down over 6.5 ft (2 m).
Japanese knotweed spreads by rhizome and aboveground plant material. Seed viability of North American populations is currently debated, and examination of populations across the northeast may show an increased presence of hybrid knotweed (Fallopia x bohemica) which can be a pollen source for Japanese knotweed.
When stems are cut, both the cut portion and the portion still in the ground can resprout and continue to grow, and fragments of both stem and root material can sprout.
It is unclear how long seeds persist in the seed bank.
Japanese knotweed is allelopathic and inhibits the growth of competitor plants through chemicals secreted by its roots/rhizomes and retained in its leaves where they are released into the soil during decomposition.

General Guidance:

Most methodologies alone will not suffice and should be part of an integrated pest management plan.
Which methods you choose also depend on the purpose of control: eradication (removal of all plants, plant parts, and seed bank), or containment (reducing seed production, reducing spread, reducing growth, removing individual plants near site of concern).
All action promotes disturbance - mechanical, chemical, and even just doing nothing. Our work is in figuring out how to minimize disturbance and maximize positive benefits.
The intent of mechanical or chemical treatment is to kill target plants.
Management options listed below are representative of research by institutions like The Nature Conservancy, Minesota Department of Agriculture, Michigan Department of Natural Resources, and the New Hampshire Department of Agriculture, Markets & Food.
Here are additional guidance documents on best management practices for Japanese knotweed.
Minnesota Department of Agriculture Summary of Japanese Knotweed
Michigan Department of Natural Resources Best Control Practices for Japanese Knotweed
New Hampshire Department of Agriculture, Markets & Food BMPs for Japanese Knotweed 2018

Summary of Mechanical Treatment Options

Mechanical methods alone will not eradicate established Japanese knotweed patches, and may worsen the infestation.

text in a table describing mechanical treatments for Japanese knotweed

Summary of Chemical Treatment Options

Be careful not to damage or kill nearby native plants when conducting management work.
Always read and follow pesticide label directions. Application of pesticides may require a certification from the Agency of Agriculture, Food and Markets. The Agency website provides information on what applicator certification is needed.
If a treatment for a specific species isn’t listed on a given product, don’t use it.
Product labels may require you to check Endangered Species Protection Bulletins. Learn more about protection bulletins here.
Understand the risk of, and how to avoid, drift.
The timing of some chemical treatments may overlap with when certain plants are flowering, and, in order to protect pollinators, herbicides should not be applied when plants are flowering. To mitigate the risk, consider utilizing an integrated pest management plan, such as cutting the plants to set back flowering time, and then applying pesticides in the lowest effective volume.
Chemical treatments pose a risk to plants, animals, and humans, but can be used in ways that greatly reduce this risk, and provide a solution to otherwise hopeless scenarios.
For questions regarding the appropriate chemical to use for a particular situation, or general information on pesticide safety, ingredients, and more, contact the Vermont Agency of Agriculture, Food & Markets’ Pesticide Program.
For questions about certification and continuing education credit opportunities, contact the Vermont Agency of Agriculture, Food & Markets’ Pesticide Program.
For questions about additional training opportunities, contact the UVM Extension Pesticide Safety Education Program.
Special permits are required to apply herbicides in a wetland. Contact Vermont Department of Environmental Conservation’s Wetlands Section for more information.
Herbicides cannot be applied within 200 feet of a Public Water Source Protection Area unless the Water Supply Division is notified. Call 1-800-823-6500 for more information. Similar requirements may be in place for proximity to private water sources.
Forests that are certified organic or adjacent to organically certified agricultural lands may carry restrictions regarding chemical application. Check with the Northeast Organic Farming Agency of Vermont for more information.
The National Pesticide Information Center provides objective, science-based information about pesticides. Check out their website for more information or call 1-800-858-7378.

text in a table describing chemical treatments for Japanese knotweed

Biocontrol

There are currently no known insect biocontrol agents for Japanese knotweed, though studies are underway to explore the potential of a leaf-eating psyllid that may reduce the growth rate of Japanese knotweed.
Although they will not eradicate Japanese knotweed, livestock biocontrol agents can reduce the severity of impacts from an infestation. Most livestock find knotweed palatable, but are not an appropriate treatment at all sites.