Persicaria amphibia (Swamp Smartweed)
|Also known as:
|Water Smartweed, Longroot Smartweed, Water Knotweed
|part shade, sun; moist soil, ditches, along shores, shallow slow-moving or quiet waters
|June - September
|1 to 3 feet
|Wetland Indicator Status:
|GP: OBL MW: OBL NCNE: OBL
|MN county distribution (click map to enlarge):
|National distribution (click map to enlarge):
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One or two spike-like racemes at the top of the stem, each 2 to 4 inches long. Flowers are 1/8-inch across, bright pink with 5 tepals (petals and similar sepals) and long pink-tipped stamens. The clusters may be long and slender or short and more thimble-shaped depending on whether the plant is the aquatic or terrestrial form.
Leaves and stem:
Leaves are alternate, up to 8 inches long and 3 inches wide, toothless, hairy to smooth, blunt-tipped or tapering to a sharp point, slightly tapered to rounded at the base, on a stalk typically up to 1¼ long though some leaves may be stalkless and aquatic forms may have much longer stalks.
At the base of the leaf stalk is a sheath (ocrea) that wraps around the stem. In some forms the tip end is green and flares out, like a collar around the stem, otherwise it is brown and papery. Like the leaves, the ocrea may be hairy or not and sometimes has cilia-like hairs less than ¼ inch long along the edge. Stems are slightly ribbed, prostrate to erect, branched or not, hairy or not, and may root at the nodes in the lower plant, creating dense colonies.
Fruit is a dark brown seed, shiny or dull, and mostly smooth.
Swamp Smartweed, formerly Polygonum amphibium, is a quite variable species with both terrestrial and aquatic forms, which are treated as separate varieties in some references though they are not recognized in Minnesota at this time. The terrestrial form (var. emersa) is mostly erect, has hairy leaves with consistently wavy edges and a taper to a sharp point, and slender flower clusters up to 4 inches long. The aquatic form (var. stipulacea) has mostly hairless, more oblong leaves that are flat, often with a blunt tip, and float on the water's surface, as well as shorter, fatter flower clusters and, when stranded on land, ocrea with the green flaring tip. While there are a number of other smartweeds that have pink(ish) flower spikes, Swamp Smartweed has the most shocking pink flowers of the lot and is probably the most easily recognizable species in this genus. Of note is multiple references specifically state this species has no dark splotch on the leaf surface that is typical of some other smartweeds, but it is not unheard of—see additional images below!
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- terrestrial Swamp Smartweed plants in shallow lake water
- aquatic form of Swamp Smartweed
- a colony of aquatic Swamp Smartweed
- a colony of terrestrial Swamp Smartweed
- stranded aquatic form near a pond
- pollinators on Swamp Smartweed
- leaves with dark blotch
- leaf hairs
Photos by K. Chayka taken Lake and Ramsey counties. Photos courtesy Peter M. Dziuk taken in Anoka and Otter Tail counties.
Have you seen this plant in Minnesota, or have any other comments about it?