DESCRIPTION: Japanese knotweed is a semi-woody perennial with spreading rhizomes (underground stems) and numerous reddish-brown stems. Plants send up new red-splotched green shoots in April and grow quickly, reaching full size by July. The plant can reach four to eight feet in height and is often shrubby. The stems are hollow and jointed, resembling those of bamboo, especially in older plants, and are visible in winter as bare brown stalks with spent flower clusters. The petioled leaves are four to six inches long and somewhat heart shaped. The plant is deciduous, dropping its leaves in the fall. The flowers are small greenish or white and bloom in August and September. The female flowers may produce small 3-angled, shiny black-brown seeds. Rhizome fragments are capable of producing new plants. Knotweed thrives in any moist soil in full sun to part shade. Japanese knotweed is a native of Asia introduced to the United States in the 18th century as an ornamental.
CONTROL: Once established, Japanese knotweed is extremely hard to eradicate. Control of knotweed relies on the death of the extensive rhizome system, which is an intensive project. Some non-chemical methods of control include regular cutting or pulling. The plant will initially re-sprout quickly after the first cutting. However, continued frequent cutting of all stems at ground level for several years will weaken the rhizomes and eventually kill the plant. Cut at least every 4-6 weeks through the summer until the plant loses its leaves in the fall. The multiple cuttings will reduce the plant’s ability to send reserves to its roots for the winter months.
Hand pulling or digging knotweed is another means of control if the plants are small. Remove all of the root and dispose of it in a way that will not allow re-sprout. Do not throw the stems into a waterway or a compost pile. Check to see if the plant returns, as a small piece of root or rhizome can start new plants.
Herbicides are another alternative if cutting or digging is not possible. Dense stands can be treated with a glyphosate-based herbicide, such as ‘Roundup’. If the Japanese Knotweed is sparsely distributed, 2,4-D amine may be applied with decent results. If spraying near water, be sure to use an herbicide such as Garlon3A or Rodeo, which are labeled for that use. Use adequate spray volume to thoroughly cover each plant. Native or desirable plants sprayed by herbicides may also be damaged or killed.
The following timing of treatment and combination of herbicides has proven to be most effective so far: In spring when the plants are 1-2 feet tall, spray with Garlon3A (if near water) or Garlon4. In summer, just before flowering, cut stems back and apply full-strength Roundup directly to the cut stems. Some root system disruption can speed herbicide uptake.
REMARKS: Japanese Knotweed is difficult to control due to its asexual and sexual reproduction and rapid deep-rooting infestation. It can quickly overtake areas and choke out native or favorable species. Always replant sprayed areas with competitive vegetation to avoid weed re-infestation.
Ø When using herbicides, READ AND FOLLOW MANUFACTURER’S LABELS and take caution to avoid the drifting of chemicals. Mention of Trade Names does not imply endorsement of products.