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What is Japanese Knotweed?

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  • Written By: K T Solis
  • Edited By: Bronwyn Harris
  • Last Modified Date: 18 September 2016
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Japanese knotweed is a type of plant that is an herbaceous perennial, meaning that it only grows seasonally. It does not thrive during the fall and winter. The Japanese knotweed is similar in shape to a shrub and can grow more than 10 feet (3.05 m.) tall. The plant's six-inch (15.24 cm.) leaves are usually oval shaped or similar to a triangle in appearance. Its hollow, jointed stems boast a reddish color. During the summer, greenish-white flowers bloom on the plant. Soon, small fruits develop on the plant as well.

Japanese knotweed is a swift spreading plant that quickly interferes with native plant growth. It grows in 36 mainland states in the U.S. The Japanese knotweed is a hearty plant that can overcome harsh weather conditions, including drought, high salinity, hot temperatures, and shady areas.

It is often found growing along water sources, including rivers and streams, but can also be found along the road and in fields. Since the plant grows so aggressively, it can create unstable banks near rivers and creeks. This makes the banks more prone to break off into the water during flooding. This, in turn, causes more sediment and plant fragments in the water. These fragments are then transported by water to other sites.

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The plant is native to China, Japan, Korea, and Taiwan. Japan first introduced the plant to England in 1825, and it was first introduced to the U.S. in the late 19th century. At that time, the plant was used for both ornamental and erosion control purposes. Today, this quickly growing plant has become a threat to natural ecosystems, threatening the lives of native plants.

Japanese knotweed spreads by using its long rhizomes, the horizontal stems of the plant. The species is spread when its plant parts or seeds are carried by water or the wind to a new site. Untended gardens or discarded plant cuttings are common sources of the plant parts or seeds that are dispersed to other areas.

To control the growth of Japanese knotweed, homeowners can dig out the rhizomes or cut the plant's stalks, being careful to remove the entire plant. Unfortunately, digging can cause the spread of rhizome parts, leading to the spread of Japanese knotweed. If deciding to dig up the plants, it's imperative that all discarded plant parts are placed in a sealed garbage bag and taken to the local dump. Otherwise, the plant parts can spread and grow in other areas. A better way to eliminate the plant from a garden is to use glyphosate and triclopyr, pesticides that can be purchased at the local garden and lawn care store.

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