Disk water hyssop
Other Common Names: roundleaf water-hyssop
Weed class: monitor list
Native to: Native to the central United States
Why Is It a Noxious Weed?
This plant is on the monitor list - it is not a listed noxious weed in Washington. Please contact its sponsor Jenifer Parsons at jenp461@ECY.WA.GOV with the Department of Ecology to report locations or for more information.
How would I identify it?
This perennial herbaceous plant is native to parts of the United States and Canada but is not native to Washington State. Plants grow in shallow water or stranded in mud when water recedes.
Flowers grow from leaf axils (where leaves connect to the stem), one to two from each axil, on short stalks (6-23 mm). Flowers somewhat bilaterally symmetrical, white, with 5 petals, the petals 4.5 to 9 mm long.
Leaves are oppositely arranged on the stems, somewhat rounded with smooth margins and 6 or more veins radiating up from the base.
Stems are typically floating, unless plants are stranded on mud. Roots form from lower stem nodes. Stems with various hairiness, moderate to dense on new growth.
Fruit Seed Description
Fruit is a capsule, spherical to broadly ellipsoid, 3.5 to 5.5 mm long with many small seeds.
Where does it grow?
Plants grow in ponds, sloughs, swamps, ditches, rivers, aquatic, stranded in mud. Herbarium collections document plants growing in the Umatilla National Wildlife Refuge.
How Does it Reproduce?
Plants spread by seed and by rooting stem fragments.