Submersed Plants

 
Species: Myriophyllum hippuroides Torrey & Gray, western milfoil
Family: Haloragaceae

Western milfoil is a submersed native plant with a distinctive stout leafy flower spike that rises above the water. Like other aquatic milfoils, western milfoil has underwater leaves arranged in whorls around the stem. Each underwater leaf is feather-like and consists of a series of paired, thread-like leaflets. The above water leaves are not feather-like, are located on the flower stalks, and occur below each flower. The leaves on the flower stalks are smaller than the underwater leaves, although they are larger than the flowers. Western milfoil has bushy underwater growth and large above water leaves.

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Leaf: Two types. Submersed leaves; feather-like, 2 to 5 cm long, often occur in irregular whorls around the stems with 4-5 leaves per whorl. There are 6-20 paired thread-like leaflet pairs per leaf. Emergent leaves: located on the flower stalk, occur in whorls around the stalk, and are longer than the flowers. These leaves measure 5-10 mm long and 1-2 mm wide, have smooth to toothed edges, but are not deeply divided. 

Stem: The yellowish green, highly branched stem grows to 1m or more.

Flower: Tiny flowers with 4 petals occur in the leaf bases above each leaf on emergent stalks. The robust emergent flower stalks (spikes) are 3-12 cm long.

Fruit: The small fruit splits into 4 chambers with each chamber containing one seed.

Root: Rhizomes give rise to numerous smaller thinner roots.

Propagation: Western milfoil spreads via plant fragments, rhizomes, and seeds.

Importance of plant: Western milfoil provides habitat for aquatic invertebrates, which in turn provide food for fish and wildlife.

Distribution: North and Central America. 

Habitat: Lakes, ponds, ditches, and small streams.

May be confused with: When not flowering, most other water-milfoil species. Similar to variable leaf milfoil (M. heterophyllum), an eastern North American species; and eastern water-milfoil (M. pinnatum) both of which have been introduced to the lower Fraser Valley in British Columbia. When flowering, the long, toothed emergent leaves and bushy underwater growth of western milfoil distinguish it from other milfoils native to the northwest. The emergent flower spike, however, could initially be mistaken for parrotfeather milfoil (Myriophyllum aquaticum) or mare's tail (Hippuris vulgaris), but the shallowly toothed edges of the emergent leaves will distinguish western milfoil from these two species.

Photographs: Myriophyllum hippuriodes showing an emergent leaf stalk, a cross section of an emergent leaf, underwater leaves, and a cross section of an underwater leaflet.

Line Drawings: None available 


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