Water Smartweed

Persicaria amphibia

Species distinguishing characteristics: 

  • Grows in muddy soils or shallow water
  • Bright pink flowers in dense clusters on a thick, jointed stalk
  • 8 stamens protrude out of flower
  • Alternate, oval, leathery green leaves
  • Prostrate or floating stems when aquatic and erect stems when terrestrial
  • Paper-like sheaths encircle the stem at leaf bases

Family Characteristics: 

  • Often has swollen joints or nodes on stem (Polygonum = many knees)
  • Simple, alternate, toothless leaves
  • Clusters of small flowers with floral parts in multiple of 3 and 4-9 stamens
  • Seeds usually triangular (3-sided)
  • Papery stipules sheath the stem at the base of leaf stalks
  • Family members include common food plants rhubarb (Rheum rhabarbarum), buckwheat (Fagopyrum esculentum), and sorrels (Rumex acetosa)

Growth habitat: 

Perennial, 30–100 cm long/tall, that grows in patches from rhizomes. Habitats, growth habits (erect or prostrate) and the shape of flower clusters distinguish two similar varieties of this highly variable plant.  The terrestrial variety has erect stems and blooms on moist soils.  The aquatic variety has prostrate stems or stems that float and blooms in water (or sometimes in soils previously covered in water).

Leaves and stems: 

Stems have thickened joints and are single or branched near the top.  Stems are erect when terrestrial and prostrate and floating with erect tips when aquatic.  Leaf stalks with prominent paper-like sheaths (stipules), 1–2 cm long, encircle the stem at leaf bases.  Alternate, leathery leaves, 5–15 cm long, with somewhat heart-shaped bases and a white midrib.  Leaves have smooth to wavy margins and often turn rust-colored with age.  Basal leaves are lacking.  In plants growing on land, the stems and leaves are densely haired, and have sharp-pointed tips on the leaf blades.  Plants growing in water have smooth stems and leaves and floating, egg-shaped leaf blades.


Numerous bright reddish-pink flowers in 1-2 dense, elongated, cylindrical to egg-shaped flower clusters, 1­–10 cm long, at the top of a thick flower stalk.  Cup-shaped flowers have 5 sepals, 4–5 mm long, that connect at the base and 8 protruding stamens (pollen organs).  The terrestrialvariety has cylindrical flower clusters that are at least 4 cm long.  The aquatic variety has egg-shaped flower clusters that are rarely 4 cm long.


Rhizomes and creeping runners (stolons) above ground. 


Clusters of shiny, dark brown, lentil-shaped seeds, 2–3 mm long, enclosed by sepals.   

Habitat preferences: 

Moist, muddy soils or shallow water in meadows, ditches, marshes, and edges of streams and ponds at low to subalpine elevations.

Interesting facts: 

The genus Polygonum has recently been split into smaller genera and water smartweed was reclassified as Persicaria amphibia.

Waterfowl and raccoons (Procyon lotor) eat water smartweed seeds.  Some Native American tribes ate the young shoots in the springtime and used a tea of the roots to treat mouth sores and stomachaches.  The name smartweed may be derived from the word “arsmart” due to its use in medieval times for relieving itching and swelling of the backside.

Water smartweeds found in old gold mine tailings in England accumulated significant amounts of gold into their tissues.  Research is currently underway to determine whether the species could be used as a bioassay of minerals in the soil or to clean up badly polluted sites.

Biological Classification: