Idaho Weed Awareness Campaign
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Giant knotweed

The Enemy: Knotweeds (Polygonum spp.) Giant knotweed, Japanese knotweed, and a hybrid of the two known as Bohemian knotweed.

The Strategy: These species all resemble each other. The plant was brought here as an ornamental from Asia as it grows like bamboo. That is, it has large segmented hollow stems that grow to a height of 15 feet. The leaves are palm size and arrow shaped. The plant produces a pink or white flowers on a small red colored stem. It mostly spreads by its underground stems known as rhizomes. The rhizomes migrate outside of the mother plant and as they reach soil surface they send up new shoots. We find these plants at many homes in the area. In some areas we have found them growing wild along roadsides and irrigation ditches.

The Attack: The roots have been known to grow under sidewalks, then enlarge, thus lifting and breaking the concrete. If it gets under the foundation of a house it can lift and crack the foundation. The largest threat is when it gets along waterways. The plant becomes such a monoculture that when the plants die off they will fall into the waterway and prevent proper flow. Also, if the plant is cut down and laid upon the ground it will re-root from the enlarged nodes and will start its own colony.

The Defense: Digging up this plant is not recommended as each fragment of the plants roots left in the soil will resprout. There are no insects or animals that will eat this invader. There are two labor intensive ways to kill it. One is to inject each stem with a specialized tool using Roundup. The other is to cut the stems and pour either Habitat or Renovate into every third hollow stem.

PLEASE NOTE -The proper use and application of herbicides can be an effective way to control and eradicate noxious and invasive plants. Before using herbicides, always carefully follow the label and safety instructions on the label. While we recommend the use of herbicides as one of the effective tools for integrated pest management, the Idaho Weed Awareness Campaign assumes no liability for herbicide applications.

For more information, click on the link below to download the Idaho's Noxious Weeds Control Guidelines publication produced by the University of Idaho Extension.

U of I Idaho's Noxious Weeds Control Guidelines (183 KB PDF download)

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