Persicaria amphibia (Swamp Smartweed)

Plant Info
Also known as: Water Smartweed, Longroot Smartweed, Water Knotweed
Genus:Persicaria
Family:Polygonaceae (Buckwheat)
Life cycle:perennial
Origin:native
Habitat:part shade, sun; moist soil, ditches, along shores, shallow slow-moving or quiet waters
Bloom season:June - September
Plant height:1 to 3 feet
Wetland Indicator Status:GP: OBL MW: OBL NCNE: OBL
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

Pick an image for a larger view. See the glossary for icon descriptions.

Detailed Information

Flower: Flower shape: 5-petals Cluster type: spike

[photo of flowers] 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: Leaf attachment: alternate Leaf type: simple

[photo of terrestrial form leaves] 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.

[photo of ocrea with flaring leafy tip] 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: Fruit type: seed without plume

Fruit is a dark brown seed, shiny or dull, and mostly smooth.

Notes:

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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More photos

Photos by K. Chayka taken Lake and Ramsey counties. Photos courtesy Peter M. Dziuk taken in Anoka and Otter Tail counties.

Comments

Have you seen this plant in Minnesota, or have any other comments about it?

Posted by: Kathy - Big Stone County
on: 2011-09-06 15:42:02

It's been an exceptionally wet year (2011) and this plant is growing on nearby paths where I've never seen it before. Makes sense now that I read that this is a water loving plant. Generally, Big Stone County (on the western border of MN) is classified as semi-arid. There are a whole range of new wildflowers blooming this year.

Posted by: Joan - Ottertail County, outside of Dent
on: 2012-08-24 11:46:14

Canoeing the Ottertail Aut. 22, below Phelps Mill, to West Lost Lake. We saw this plant growing on the shore among cattails and in the water. Have never seen it before, took a photo to help identify it. I was glad to be able to find it on your website. I thought it looked a little like the Smartweed we see growing in fields in Minnesota, and elsewhere.

Posted by: Kara - Little Hanging Horn Lake, Carleton County
on: 2012-09-02 10:40:05

Found growing on the lakeshore in August. Have never seen this plant before. it is very pretty.

Posted by: Marcia - Snake River, Pine City
on: 2016-08-29 15:31:16

First year seeing this pretty flower since moving here 4 years ago.

Posted by: Leah B - Big Willow Park, Minnetonka
on: 2017-07-21 09:04:05

I just noticed this beauty growing on the waters edge, near the path that runs parallel with the road.

Posted by: karl d - BWCA
on: 2017-07-25 16:07:06

beaver pond between Spoon & Sema Lakes - near Knife Lake

Posted by: Liz - Central Lakes Trail, Alexandria
on: 2017-08-25 17:34:03

Found growing along the trail. Lovely!

Posted by: JoAnne K - Ely
on: 2017-09-05 13:20:45

West end of Knife Lake near the portage to Carp Lake (BWCAW). Very few plants in the shallows. the ground there completely covered with rock underwater, somehow these plants found a way to connect with the soil.

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