April 28, 2011

Japanese knotweed (Fallopia japonica) and giant knotweed (Fallopia sachalinensis), including any of their cultivars and hybrids, are in the process of being designated Nebraska noxious weeds.

Knotweeds form dense monoculture stands that shade out other vegetation, reduce native species diversity, and alter wildlife habitat. Knotweed species were originally introduced to North America from Asia as an ornamental species and for erosion control along stream banks. They can now be found in more than 40 states. These weed species are in the early stages of invasion in Nebraska and eradication efforts are being initiated.

Biology, Identification and Spread

Japanese knotweed and giant knotweed are upright, herbaceous, perennial plants that can grow 6-12 feet tall. Both species develop an extensive network of underground rootstocks called rhizomes that give rise to dense clumps of thick, bamboo-like, hollow stems that are erect and branched at the top. They grow from large rhizome systems and are spread in most areas by rhizome and stem fragments, although occasional reproduction may occur by seed as well.

Japanese knotweed and giant knotweed are similar in appearance and are known to hybridize. The hybrid between these two is known as Bohemian knotweed (Fallopia bohemica). The major differences between Japanese knotweed and giant knotweed species are in the leaf shape and size. Japanese knotweed has fairly flat leaf bases, while giant knotweed has more curved, heart-shaped bases. Japanese knotweed leaves grow to about 6 inches long, while giant knotweed leaves can be up to twice that size. Small white or greenish flowers grow in dense clusters from the leaf joints in July and August. Dormant in winter, the dead reddish-brown stems often remain standing. Knotweed emerges from root crowns in April and reaches full height by June.

Both species of knotweed grow especially well along river and stream banks, preferring moist, unshaded habitats. Spread to new locations is often facilitated by flood water moving fragments of stem and rhizomes downstream and by human interactions. Once established, knotweeds spread aggressively through the expansion of rhizomes, and are difficult to eradicate. Currently, knotweeds are only present in a few Nebraska counties, with the largest infestations in Lancaster and Garfield counties.

Management Options

Eradication of the rhizome system is necessary for successful control of this aggressive invasive species.

Mechanical

Mechanical control methods alone are not very effective. Digging is only effective for small initial populations or environmentally sensitive areas where a herbicide application is not desirable. Large colonies of these species are extremely difficult to dig out due to high density of rhizomes. Digging a large colony for control is not recommended, as it is very labor intensive and it’s unlikely that all rhizomes can be removed.

Juvenile plants can be hand pulled if they are not well established and soil conditions allow for complete rhizome removal. Small portions of the rhizome left in the soil have a potential to re-sprout. Disrupting plant growth by cutting or mowing the stalks at least three times per season can reduce rhizome growth by slowly depleting stored food reserves and removing photosynthetic tissue. Plant stands that are at least five years old must be mowed for several seasons before a population reduction is visible.

Chemical

Chemical control is probably the most widely used method for most weed species, including knotweed. It is also the method that will provide the fastest control.

Stump Cut Application. This method is recommended for small sites and ornamental plantings. Cut the stems about 2 inches above ground level, and then apply a 25% v/v solution of water and glyphosate (e.g., Roundup®, or Rodeo® if applying close to water) or triclopyr (e.g., Crossbow® or Garlon®). A subsequent foliar application of glyphosate may be required to control new seedlings and re-sprouts.

Foliar Application: This method is recommended for controlling larger infestations. Best control is achieved when foliar herbicides are sprayed in late summer and early fall, basically from August to October, but at least two weeks prior to first killing frost. Apply a 2% solution of glyphosate or triclopyr in a water solution with 0.5% of non-ionic surfactant (NIS) in order to penetrate the leaf cuticle.

In most cases, management efforts are more successful when you use a combination of treatment methods. For example, combining methods such as early cutting and later herbicide use allows more options and flexibility. Digging or pulling before spraying also may increase the efficacy of herbicides. Regardless of which control is used, be persistent and remember that if some rhizomes remain in the soil, knotweed will re-sprout and will require repeated treatments.

Avishek Datta, Postdoctoral Research Associate
Stevan Z. Knezevic, Extension Weeds Specialist
Both at the Haskell Agricultural Laboratory, Concord