Decodon verticillatus (Swamp Loosestrife)

Plant Info
Also known as: Water Willow
Family:Lythraceae (Loosestrife)
Life cycle:perennial
  • State Special Concern
Habitat:part shade, sun; swamps, lake shores, shallow mucky water
Bloom season:July - September
Plant height:2 to 4 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

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Detailed Information

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

[photo of flowers] Pinkish purple flowers whorled in a dense cluster around the stem at the leaf axils. Flowers have 4 to 7 crinkled petals (usually 5), each about ¼ inch long, elliptical but often fleeting and not always easy to distinguish. 8 to 10 white-tipped pink stamens arch out in a spidery array extending nearly double the length of the petals. Over-all flower structure is nearly 1 inch across.The calyx holding the flower is a thin walled, paper-like cup with gnarly appendages. Multiple flowering stems, rarely branched, shoot from the base of the plant.

Leaves and stem: Leaf attachment: opposite Leaf attachment: whorl Leaf type: simple

[photo of leaves] Leaves are opposite or more often in whorls of 3 or 4, elliptical to lance-like, tapered similarly at both ends, 2 to 8 inches long, ½ to 2 inches wide, toothless, finely hairy on the underside and smooth on the upper surface, the mid-vein reddish near base, on a short stalk. Stems are long and arching, up to nearly 10 feet in length but the overall plant height rarely exceeds 3 to 4 feet. Stems root at the tips as they re-meet the substrata, are sometimes slightly hairy, angled with 4 to 6 sides, often pinkish red on blooming canes and woody at the base. 

Fruit: Fruit type: capsule/pod

[photo of fruit] Fruit is a globular capsule up to 1/3 inch across, containing numerous seeds. Capsules persist through winter. Minnesota plants rarely form set seed but spread as clonal mats via re-rooting stems.


Being at the northwest fringe of its natural range, Swamp Loosestrife is restricted to lakeshores in east central Minnesota. Because this region has seen such heavy development over the past decades it should come as no surprise that another native species has been put at risk. According to the DNR it has been listed as a Special Concern species since 1984.

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More photos

Photos courtesy Peter M. Dziuk taken at several locations in Anoka county.


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

Posted by: Ellen S.
on: 2016-10-29 15:40:39

How can I tell this apart from purple loosestrifge

Posted by: K. Chayka
on: 2016-10-29 16:17:12

Flowers are arranged differently: Decodon verticillatus has flowers whorled in the leaf axils, Lythrum salicaria has a single spike at the tip of a branch.

Posted by: Frances WM - 27170 Lakewood Dr. Isanti MN 55040 (on our property)
on: 2017-09-12 15:38:03

There is a creek that runs through our property called Spirit Creek which runs into the Rum River. The creek is the outlet for Long Lake. This plant is massive, almost to the point of chocking out the creek. Now this year, it has spread onto our shoreline in front of our home on Long Lake. A few years ago I checked with DNR and they said this is the farthest west that they have seen this plant.

Posted by: Joseph Capecchi - Kanabec County
on: 2018-08-18 11:43:13

It is on our shoreline on Knife lake

Posted by: Darrick P Wotachek - Isanti, MN
on: 2020-07-01 10:55:50
MN DNR hydrologist, Conservation Officer, and County Wetland Specialist witnessed almost 1/2-1 acre of water willow adjacent to the outlet of Long Lake. Photos were taken and documented. There is a chance that the entire creek has more populations further down the watershed.
Posted by: Nakomi Rudlong - Pelican Lake, Wright county
on: 2020-07-05 12:21:44

At the northern boat landing in Monticello. New landing put in. Many wildflowers.. I have seen this about four feet from shore.

Posted by: Holly Buss - Burnsville
on: 2023-08-05 12:28:26

Big Blake Lake, WI - in a narrows - it covers both sides. Beautiful!

Posted by: Elizabeth Tiller - Dakota Co, Eureka Twp, Chub Lake inlet
on: 2023-09-23 18:47:58

Lovely large stand at the inlet creek to Chub Lake

Posted by: Cal Harth - St. Croix State Park, Pine County
on: 2023-09-29 14:41:32

A small island in the St. Croix River near the mouth of the Kettle River has a patch of this plant covering about 300 square feet. Over 30 years ago I saw it there, without knowing of it's rarity.

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