Japanese Knotweed

NEGATIVE IMPACTS

 A major trait of invasive plants, including Japanese knotweed, is the fact that they grow prolifically and usually outcompete native plant species for space, water and nutrients.

 Japanese knotweed is a persistent, fast-growing invasive plant which degrades the soil and can cause damage to septic systems, foundations and roads. The presence of Japanese knotweed can even reduce home values due to the potential damage and difficulty of removal.

 Mass Audubon has information on invasive plants commonly found in Carlisle, including Japanese knotweed.

HOW TO IDENTIFY JAPANESE KNOTWEED

Leaves:  leaves are spade shaped, with a flat base and pointed tip. They are alternate and can grow up to 6 inches long and 4 inches wide.

Stems: stems grow in a zig-zag growth pattern, up to 1 inch diameter and can resemble bamboo because of joints. They are hollow and green to purple in color. Stems remain standing in winter, turning reddish-brown and woody. Young shoots can have reddish-purple specks and resemble asparagus spears. 

Flowers: small cream or white flowers bloom in late summer and grow in branched clusters from leaf axils near the end of the stem.

Roots: the tap root grows over 6 feet deep and roots spread widely, sometimes in excess of 40 feet. They have a brown exterior and orange interior.

Size: mature plants grow 6 - 10 feet tall (sometimes even more) and can form dense, wide thickets.

Growing conditions: Japanese knotweed is commonly found in disturbed areas and along roadsides and streams. It tolerates a wide range of light, moisture and soil conditions but favors full sun

Growing cycle: Japanese knotweed sends out fleshy straight shoots that resemble asparagus stalks in April. It can grow up to 8 inches a day. Blooms appear in August and last about three weeks. The leaves will die and fall off with the first frost and the stems become woody and remain erect through the winter.

A plant-identifying app may also be of assistance in identification.

Note the spade-like leaves, reddish to purple stem nodes and purplish speckling on the stem

Image courtesy of Ancatdubh43

Note the spade leaf shape, alternating leaf position and zig-zag stem

Image courtesy of Michael Gasperl

Note the color, position and shape of the flowers which bloom in summer

Image courtesy of HOerwin56

Note the straight new stalks in spring and the reddish stems             

Image courtesy of Anneli Salo

Note the dried, segmented, woody stems in winter

Image courtesy of Charlène

A new, young Japanese knotweed plant - ideal time for removal, before it has a chance to fully establish

Image courtesy of James H. Miller, USDA Forest Service, Bugwood.org

You can watch videos about how to identify Japanese knotweed here and here.

HOW JAPANESE KNOTWEED SPREADS

Japanese knotweed primarily spreads through underground rhizomes, which can remain dormant for up to twenty years,  and stem fragments. A new plant can establish from any piece of rhizome as small as 1/2 inch or any stem joint that comes into contact with moist soil. Spreading via seed dispersal is also viable, yet, less likely.

NATIVE PLANTS THAT CAN BE MISTAKEN FOR JAPANESE KNOTWEED

 There are a number of native plants that share some attributes and could be mistaken for Japanese knotweed at various seasons and stages of life. For example, fringed bindweed (Fallopia ciliinodis) and climbing bindweed (Fallopia scandens) could be mistaken for Japanese knotweed due to the shape of the leaves, color of the stem and flowers and growth pattern; spreading dogbane (Apocynum androsaemifolium) can have a stem color that is similar. It is important to ensure proper identification before removal.

BEST REMOVAL OPTIONS

 Any invasive plant removal planned for an area within a jurisdictional wetland or wetland buffer zone needs to first go before the Conservation Commission.

 Japanese knotweed is not poisonous but handling it can cause skin irritation for some people.

 A - HAND REMOVAL

The most environmentally sensitive removal method for any invasive plant is by hand or using hand tools, ensuring the roots are completely removed as well. Unfortunately, in the case of Japanese knotweed, this approach only works for very young plants since the roots of mature plants grow extensively and very deep. 

Grasping the stem near the base and gently pulling can remove small Japanese knotweed plants. If resistance is encountered, a trowel or spade can be gently used to loosen the soil, with extra care given to ensure the roots are not severed.

 Identifying and hand removing young plants before they have the chance to establish fully can save a lot of future time, effort and money. 

 Monitoring and repeated removal will usually be necessary. 

 Local companies are available to identify and hand pull invasive plants.

 B - MECHANICAL REMOVAL

 Mechanical removal is the next best option for reduced negative impacts. However, this method can fail to remove all the roots, so monitoring and repeated removal will most likely be necessary. 

 Local companies can be found who have the equipment needed for mechanical removal. Mowing, weed-whacking and chipping are to be AVOIDED since these practices can spread the invasive through on-site scattering from equipment, transportation of pieces on or in equipment and breaking dormancy of buds on rhizomes.

 Cutting alone will not deter the plant since it will send up new shoots, resulting in an even denser thicket.

A wetland permit is required for any work done within a wetland jurisdictional area. Permits are valid for at least three years (depending on the type of permit) and are obtained through the Conservation Commission.