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Native Plant Database

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Polygonum amphibium (Water knotweed)
Bruso, George H.

Polygonum amphibium L.

Water knotweed, Water Smartweed

Polygonaceae (Buckwheat Family)

USDA Symbol: POAM8

USDA Native Status: Native to U.S.

Freely rooting, long, prostrate stems growing across mud or in water and topped with erect, dense, narrowly egg-shaped, pink flower clusters.

This aquatic or wetland plant is rather showy when growing in colonies. The genus name is from the Greek poly (many) and gona (knee or joint), as is the family name, and refers to the thickened joints of the stem where a sheath often surrounds the stem at the leaf axil. The genus includes the smartweeds, with tiny flowers in terminal spikes, and the knotweeds, with flower clusters in the leaf axils. This species also occurs in Eurasia. Its seeds provide food for waterfowl. Some authorities recognize two intergrading, environmentally variable varieties in the West. The variety stipulaceum has short oval flower clusters and is not especially aggressive; the pink flower masses of this variety are very attractive, but since the plants grow quickly, they can become an unwelcome weed in decorative ponds. The variety emersum has lanceolate leaves with narrowly tapering tips and slender flower clusters at least 1 1/2 (4 cm) long; once called P. coccineum, a separate species, this variety is aggressive to the point of being classified as a noxious weed.

 

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Plant Characteristics

Duration: Perennial
Habit: Herb
Leaf: Green
Fruit:

Bloom Information

Bloom Color: Yellow
Bloom Time: Jun , Jul , Aug , Sep

Distribution

USA: AK , AR , AZ , CA , CO , CT , DC , DE , IA , ID , IL , IN , KS , KY , LA , MA , MD , ME , MI , MN , MO , MS , MT , NC , ND , NE , NH , NJ , NM , NV , NY , OH , OK , OR , PA , RI , SC , SD , TN , TX , UT , VA , VT , WA , WI , WV , WY
Canada: NB , NS , PE
Native Distribution: Throughout North America, except Georgia, Alabama, Florida, and the Arctic.
Native Habitat: Shorelines, wet prairies, swamps, ponds, and quiet streams, in mud or floating on still fresh water.
USDA Native Status: L48(N), AK(N), CAN(N), SPM(N)

Growing Conditions

Water Use: High
Light Requirement: Sun
CaCO3 Tolerance: Medium

Recommended Species Lists

Find native plant species by state. Each list contains commercially available species suitable for gardens and planned landscapes. Once you have selected a collection, you can browse the collection or search within it using the combination search.

View Recommended Species page

Additional resources

USDA: Find Polygonum amphibium in USDA Plants
FNA: Find Polygonum amphibium in the Flora of North America (if available)
Google: Search Google for Polygonum amphibium

Metadata

Record Modified: 2010-12-17
Research By: TWC Staff

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