Japanese Knotweed

Outdoors

By:  Eddie Ayers, County Extension Agent

Driving around the area, I have been seeing a plant that has become a problem in both Gilmer and Fannin Counties.  The weed I’m talking about is Japanese knotweed, commonly known as crimson beauty, Mexican bamboo, or Japanese fleece flower.  It was probably introduced to the U.S. as an ornamental and a plant that has flowers that bees love. It’s fairly easy to spot as it has been growing in large patches all over the area.  The leaves are alternate, 6 in. (15.2 cm) long, 3-4 in. (7.6-10 cm) wide, and are broadly-ovate or heart shaped. Flowering occurs in late summer when small, greenish-white flowers develop in long panicles in the axils of the leaves.

This native of Japan was initially useful for erosion control, as an ornamental, and for landscape screening.  It spreads quickly to form dense thickets that can alter natural ecosystems or interfere with landscaping. It is a semi-woody, bushy perennial and a member of the Polygonaceae (Knotweed) family.  Another fact about the plant is that the stem is hollow. Knotweed spreads rapidly from stout long rhizomes. Seeds are distributed by water in floodplains, transported with fill dirt, and to a lesser extent are wind-blown. Populations escaped from neglected gardens, and discarded cuttings are common methods of distribution. Once established, populations are quite persistent and can out-compete existing vegetation.

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, utility rights of way, and around old home sites. It can quickly become an invasive pest in natural areas after escaping from cultivated gardens. It poses a significant threat to riparian areas, where it can survive severe floods. It is rapidly colonizing scoured shores and islands.

Controlling this invasive fast growing plant is very difficult.  One method that is used is grubbing. This method is appropriate for small initial populations or environmentally sensitive areas where herbicides cannot be used. Using a digging tool, remove the entire plant including all roots and runners. Juvenile plants can be hand-pulled.  Any portions of the root system not removed will potentially resprout. All plant parts, including mature fruit, should be bagged and disposed of in a trash dumpster to prevent re-establishment.

There are several herbicides that can be used, but it takes some work for them to be effective.  One treatment method is the cut stump treatment. Use this method in areas where plants are established within or around non-target plants. Cut the stem 2 inches above ground level.  Immediately apply a 20% solution of glyphosate or a 10% solution of Arsenal AC, Polaris AC or Imazapyr 4SL and water to the cross-section of the stem. A subsequent foliar application may be required to control new seedlings and resprouts.

The other spray method is foliar spraying the plants.  Use this method to control large populations. It may be necessary to precede foliar applications with stump treatments to reduce the risk of damaging non-target species.  Apply a 1% solution of glyphosate or 20%Garlon4 and water to thoroughly wet all foliage. Do not apply so heavily that herbicides will drip off leaves. The ideal time to spray is after surrounding vegetation has become dormant (October-November) to avoid affecting non-target species.  A 0.5% non-ionic surfactant is recommended in order to penetrate the leaf cuticle.

For more information, contact me in the Gilmer County UGA Extension office.

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