Japanese Knotweed

Miller, James H.; Manning, Steven T.; Enloe, Stephen F. 2010. A management guide for invasive plants in southern forests. Gen. Tech. Rep. SRS–131. Asheville, NC: U.S. Department of Agriculture Forest Service, Southern Research Station. 120 p.

Japanese knotweed (Polygonum cuspidatum Siebold & Zucc.) is a tall perennial, canelike shrub 3 to 12 feet (1 to 3.5 m) in height, freely branching in dense, often clonal infestations. Hollow-jointed, reddish stems like bamboo survive only one season while rhizomes survive decades. Alternate leaves appear in spring on new sprouts, ovate with pointed tips and flat bases. In late summer, sprays of tiny white flowers emerge along stalks at leaf axils that yield abundant tiny winged seeds with highly variable viability. Dead tops remain standing during winter. Japanese knotweed spreads along streams by stem and rhizome fragments, and rare seeds to dominate extensive riparian habitat. Also, spreads along highways and roads by similar means through maintenance mowing.

Management strategies

Do not plant. Remove prior plantings, and control sprouts and seedlings. Bag and dispose of fruit in a dumpster or burn.
Minimize disturbance within miles of where this plant occurs, and anticipate wider occupation when plants are present before disturbance.
Repeated cutting and pulling will not control this species unless young.
Burns hot in dormant season to clear tops with rhizomes remaining.

Recommended control procedures

Thoroughly wet all leaves with one of the following herbicides in water with a surfactant: Garlon 3A (or aquatic Renovate) as a 2-percent solution (8 ounces per 3-gallon mix); better results may occur from a mix of Garlon 3A as a 1-percent solution (4 ounces per 3-gallon mix) and a glyphosate herbicide (Rodeo for aquatic sites) as a 2-percent solution (8 ounces per 3-gallon mix). Fall applications are most effective while seed production where it occurs can be stopped by earlier treatments.
On terrestrial sites when safety to nontarget vegetation is not a concern, thoroughly wet all leaves with Arsenal AC* as a 0.25-percent solution (1 ounce per 3-gallon mix), Arsenal PowerLine* as a 0.5-percent solution (2 ounces per 3-gallon mix), Tordon 101* ‡ as a 4-percent solution (1 pint per 3-gallon mix), or Tordon K* ‡ as a 2-percent solution (8 ounces per 3-gallon mix).
On aquatic sites, thoroughly wet all leaves at flower plume stage with Habitat* as a 1-percent solution (4 ounces per 3-gallon mix) mixed with an aquatic surfactant.
For stems too tall for foliar sprays, cut large stems and immediately treat the stump tops with one of the following herbicides: a glyphosate herbicide or Garlon 3A as a 25-percent solution (3 quarts per 3-gallon mix). ORTHO Brush-B-Gon, Enforcer Brush Killer, and Vine-X are effective undiluted for treating cut-stumps and available in retail garden stores (safe to surrounding plants). A subsequent foliar application of glyphosate will be required to control new seedlings and resprouts.

* Nontarget plants may be killed or injured by root uptake.

‡ When using Tordon herbicides, rainfall must occur within 6 days after application for needed soil activation. Tordon herbicides are restricted use pesticides.

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