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Illinois Native Plant Guide - Water Smartweed - Pages 108-109

Illinois Native Plant Guide

Water Smartweed (Water Knot Weed)
Polygonum amphibium (P. amphibium stipulaceum)

Preferred Water Depth and Inundation Tolerance

Species has a wide tolerance for inundation (Max: 20”/Min: moist soil).

Wildlife Value

Seeds are eaten by waterfowl. Serves as a major food source for purplish copper butterfly. Also provides cover for waterfowl and fish.


Used in lower shoreline zones. Wave tolerant.

Availability, Establishment, and Maintenance

  • Achenes, cuttings, rootstocks, and rhizomes may be used. Plants and rootstocks have limited availability from commercial vendors. Collect achenes when they are mature (July-Sept.).
  • One source recommends storing achenes in fresh, 32-34o F water for 3 to 6 months to stratify.
  • Other sources indicate that seed germinates best when stored dry at 40o F followed by light at 70o F.
  • Broadcast in the spring on wet site.
  • To use cuttings, take 2-6 inch cuttings from top shoots and plant on site in wet area by burying lower part of cutting. Spring drawdown increases plant production.
  • Achene production is high in areas shallowly flooded during the growing season.

Water Smartweed (Water Knot Weed)
Polygonum amphibium (P. amphibium stipulaceum)

Mature Height
Up to 3 feetWater Smartweed (Water Knot Weed)

Plant Type
Perennial emergent herb

Indicator Status


Nutrient Load Tolerance

Salt Tolerance

Siltation Tolerance
Moderate to high

Flowering Color and Time
Red to pink
June to August

Light Preference
Partial to full sun

Seeding Rate
.5 - 1.5 lbs/acre

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