Ministry of Agriculture, Food and Rural Affairs

Ontario Weeds: Water smartweed

Table of Contents

1. Name
2. Other Names
3. Family
   • Photos and Pictures
4. Terrestrial or dry habitat forms
5. Aquatic forms
6. Flowers
7. Habitat
8. Similar Species
9. Related Links

Name: Water smartweed, Polygonum amphibium L.,

Other Names: renouée amphibie, renouée aquatique, P. natans (Michx.)

Family: Buckwheat or Smartweed Family (Polygonaceae)

Photos and Pictures

Water smartweed.Top of flowering plant.

Terrestrial or dry habitat forms: With stems erect to nearly prostrate, smooth or more or less hairy; leaves alternate, lanceolate to oblong, up to 20cm (8in.) long; ocrea membranous or slightly leaf-like in texture, its outer margin smooth or torn and either with or without a distinct horizontal, leaf-like flange.

Aquatic forms: Rooting in shallow water with floating, hairless stems, elliptic leaves and ocreae without horizontal, leaf-like flanges.

Flowers: Inflorescence of both forms a compact, ovoid- or conic-cylindric, spike-like raceme, 1-4cm (2/5-1¼in.) long and 1-1.7cm (2/5-2/3in.) wide, usually blunt; flowers light to deep pink, without true petals, the coloured sepals 2-5mm (1/12-1/5in.) long. Both terrestrial and aquatic forms have a thick, lens-lens-shaped, pale brown to nearly black achene, 2.5mm (1/10in.) long and wide. Flowers from June to September.

Habitat: Water smartweed is a native plant in shallow water and along the margins of lakes, ponds, rivers, creeks and roadside ditches, and persists in depressional areas in cultivated fields throughout Ontario.

Similar Species: Both terrestrial and aquatic forms of Water smartweed are distinguished from the closely related Swamp smartweed by their short, thick, more or less cylindric inflorescences, the occasional presence of pubescence and a leaf-like flange on the ocrea of the terrestrial form, and from most other Smartweeds by having thick, tough, forking rhizomes.

Related Links

... on general Weed topics
... on weed identification, order OMAFRA Publication 505: Ontario Weeds
... on weed control, order OMAFRA Publication 75: Guide To Weed Control

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