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  • Knotweeds are usually introduced as ornamentals

  • An escaped ornamental from eastern Asia forming dense thickets

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Himalayan knotweed
(Polygonum polystachyum)

Quick Identification

  • Bamboo like stalk

  • Unlike other knotweed species, Himalayan knotweed has longer tapered leaves

  • Plumed green to white flowers in spring

Impact
Takes over riparian systems, choking water flow and other pants and damaging habitat.  Knotweeds can grow through concrete meaning that they can cause significant damage to infrastructure.

ManagementCertain herbicides can be effective. Digging and mowing are not effective. Excavating root mass can be effective if entire root system is removed.

Flower
Small, white-green flowers grow in showy, plume-like, branched clusters along the stem and leaf axils (joints).

Leaves & Stems
Leaves are 10 to 20 cm and tapering. Stems, or canes, are hollow with varying thicknesses, upright, and bamboo-like with reddish-brown speckles and thin, papery sheaths. Stems are generally 1-3 m in height and grow in large, dense thickets. Stems may persist through the winter as bare, reddish-brown stalks.

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Roots
Extensive creeping root system that can go down 3 m and spread laterally 20m. Root and stem fragments as small as 1 cm can start new colonies.

Reproduction & Dispersal
Primarily from root (rhizomes) and stem fragments that are dispersed by human activities or by water to downstream areas. It is often spread by movement of soil from infested sites or by gardeners sharing plants.

Seeds
Knotweed seeds are small and have wings which give them a heart shape.

Preferred Habitat
Thrive on freshly disturbed soil in roadside ditches, low-lying areas, irrigation canals, and other water drainage systems. Also found in riparian areas, along stream banks, and in other areas with high soil moisture. Able to grow in partial shade or full sun.

Interesting Facts
Roots are know to have grown through foundations.

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