Wildflower (forb) · Polygonaceae
Water knotweed
Polygonum amphibium · includes its varieties and subspecies
Water knotweed is recorded as native to 36 states and territories in Kew’s World Checklist of Vascular Plants and USDA PLANTS, and recorded in 1,104 counties of those states by USDA PLANTS.
Key facts · USDA PLANTS
Water knotweed at a glance
- Native in
- 1,104 counties, across 36 states and territories
- Estimated cold hardiness
- Zone 5a PLANTS minimum temperature −23°F
- Mature height
- 1 in PLANTS mature height
- Light and water
- Full sun (shade-intolerant) High water use
Estimated cold hardiness: zone 5a. PLANTS gives a minimum temperature of −23°F; zone 5a winters bottom out at −20°F, the coldest half-zone that stays within that limit.
Water knotweed is recorded as native to 36 states and territories in Kew’s World Checklist of Vascular Plants and USDA PLANTS, and recorded in 1,104 counties of those states by USDA PLANTS.
Counties where it is native
The 15 counties with the most ZIP codes among the 1,104 where it is listed.
Native counties by state
- Alaska 13 counties
- Arizona 6 counties
- California 46 counties
- Colorado 20 counties
- Connecticut statewide record
- Delaware 2 counties
- District of Columbia 1 county
- Idaho 22 counties
- Illinois 92 counties
- Indiana 42 counties
- Iowa 23 counties
- Kansas 90 counties
- Maine 16 counties
- Massachusetts 13 counties
- Michigan 71 counties
- Minnesota 69 counties
- Missouri 62 counties
- Montana 23 counties
- Nebraska 45 counties
- Nevada 9 counties
- New Hampshire 8 counties
- New Jersey 15 counties
- New Mexico 8 counties
- New York 56 counties
- North Dakota 47 counties
- Ohio 69 counties
- Oklahoma 12 counties
- Oregon 28 counties
- Pennsylvania 47 counties
- Rhode Island 1 county
- South Dakota 37 counties
- Utah 18 counties
- Vermont 13 counties
- West Virginia 1 county
- Wisconsin 63 counties
- Wyoming 16 counties
Growing traits
Values from the PLANTS characteristics record. “Not recorded” means PLANTS has no value; nothing here is estimated except the zone.
Growth
- Growth habit
- Forb/herb
- Duration
- Perennial
- Mature height
- 1 in
- Growth rate
- Moderate
- Growth form
- Rhizomatous
- Shape
- Prostrate
- Lifespan
- Moderate
- Active growth period
- Summer
Light and water
- Shade tolerance
- Intolerant (full sun (shade-intolerant))
- Moisture use
- High water use
- Drought tolerance
- None
- Tolerance of waterlogged soil
- High
- Precipitation range
- 14 to 60 in per year
Soil
- Soil textures
- Medium and fine
- pH range
- 4 to 7
- Lime (CaCO₃) tolerance
- Medium
- Salinity tolerance
- None
- Fertility requirement
- Medium
- Minimum root depth
- 6 in
Climate
- Minimum temperature
- −23°F
- Estimated cold hardiness
- Zone 5a and warmer
- Minimum frost-free days
- 110
- Fire tolerance
- Medium
Flowers, leaves and fruit
- Flower color
- Yellow
- Showy flowers
- No
- Bloom period
- Late Spring
- Foliage color
- Green
- Showy in fall
- No
- Keeps leaves
- No
- Fruit color
- Brown
Other
- Toxicity
- None
- Nitrogen fixation
- None
- Nursery availability
- Routinely Available
- Family
- Polygonaceae
- PLANTS symbol
- POAM8
Source & data date
- Source
- USDA NRCS PLANTS Database: county distribution records, native status and plant characteristics. CC0 (public domain dedication, per the data.gov catalog record).
- Kew’s World Checklist of Vascular Plants (WCVP): native or introduced status by state. CC BY 3.0.
- USGS US-RIIS (ver. 2.0): species listed as introduced are excluded. CC0.
- Citation
- USDA, NRCS. 2026. The PLANTS Database (https://plants.usda.gov, 11/04/2026). National Plant Data Team, Greensboro, NC USA.
- Govaerts R. (ed.) (2026). WCVP: World Checklist of Vascular Plants. Facilitated by the Royal Botanic Gardens, Kew. Data file dated 2026-06-04, http://sftp.kew.org/pub/data-repositories/WCVP/ (retrieved 2026-11-04). Paper: Govaerts R. et al. (2021) The World Checklist of Vascular Plants, a continuously updated resource for exploring global plant diversity. Scientific Data 8, 215.
- Data date
- Retrieved from PLANTS on November 4, 2026; WCVP data file dated June 4, 2026.
- Scope
- Polygonum amphibium (PLANTS symbol POAM8), including 2 varieties and subspecies folded into the species.
- Method
- A species is listed when it is recorded as native to the state in Kew’s World Checklist of Vascular Plants and USDA PLANTS, and recorded in the county by USDA PLANTS. Species PLANTS labels both native and introduced are listed separately. Full method · All sources