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Montana Field Guides

Cutleaf Water-milfoil - Myriophyllum pinnatum
Other Names:  Cutleaf Watermilfoil

Undetermined Origin
Not Documented

Global Rank: G5
State Rank: SNA
(see State Rank Reason below)

Agency Status
MNPS Threat Rank:

External Links

State Rank Reason (see State Rank above)
Myriophyllum pinnatum is not documented with verified specimens in Montana. It is reported to occur in southern British Columbia to Oregon and in eastern North America (Giblin et al. [eds.] 2018). n Montana it was reported from aquatic plant surveys conducted in 2011 at Cliff Lake, Wade Lake, and Red Rock River in Montana during aquatic plant surveys. However, specimens to verify this data have not been found. Proper identification of Myriophyllum species require careful collections to obtain flowering or fruiting structures, use of an appropriate and current taxonomic key, and time spent studying the specimen. Potential specimens should be provided to and verified at one of our State Herbaria (University of Montana, Montana State University, or Montana State University-Billings) and will likely require genetic testing. A conservation status rank is not applicable (SNA) because this plant is not known to occur in Montana.
General Description
PLANTS: Aquatic, perennials with finely dissected, whorled submerged leaves, and emerged leaves within the inflorescence. Stems may branch. Wintering buds (turions) are absent. Source: Giblin et al. [eds.] 2018.

LEAVES: Submerged leaves are well-developed, in whorls of 3-6 with 6-13 segments, but usually with some alternately arranged as well. Emerged leaves are well-developed (larger than the flowers or fruits), whorled, linear, lanceolate, narrowly oblong, elliptic, or ovate in shape, and occur within the inflorescence. Plants may have at the base of branches or shoots pinnate, whorled, and, mostly unreduced leaves. Source: Giblin et al. [eds.] 2018.

INFLORESCENCE: An emergent, terminal spike. The spike consists of separate male and female flowers and emergent leaves. Emergent leaves are greater than the flowers and fruits. Emergent leaves within the upper half of the inflorescence are either a) pinnately divided or lobed more than half-way to midvein or b) entire to toothed. Source: Giblin et al. [eds.] 2018.

Range Comments
It is reported to occur in southern British Columbia to Oregon and in eastern North America (Giblin et al. [eds.] 2018). The NRCS PLANTS database shows this plant occurring in the eastern and mid-western US States and in British Columbia and Saskatchewan, Canada (

Lakes, sloughs, ditches, and slow streams (Giblin et al. [eds.] 2018).

Reproductive Characteristics
On the same plant male and female flowers are separate, generally one per axil, and usually subtended by 2 or more tiny bracteoles. In general male flowers grow above the female flowers in the terminal spike-like inflorescence. Male flowers have 4 stamens with anthers of 0.4-0.9 mm long. Male flowers have 4 sepals of 0.1-0.5 mm long. Female flowers have 4 petals of 1.5-3.0 mm long. Source: Giblin et al. [eds.] 2018.

Fruits are drupe-like or nut-like with 4 carpels. Fruit segments are cylindrical with 2 winged longitudinal ridges that have 6-12 prominent cross-ribs, sharply angled faces and often with small warts or bumps. Faces are smooth. Source: Giblin et al. [eds.] 2018.

Plants do not develop turions.

  • Additional ReferencesLegend:   View Online Publication
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    • Giblin, David E., Ben S. Legler, Peter F. Zika, and Richard G. Olmstead (editors). 2018. Flora of the Pacific Northwest: An Illustrated Manual. Second Edition. University of Washington Press in Association with Burke Museum of Natural History and Culture, Seattle, Washington. 882 pp.
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Citation for data on this website:
Cutleaf Water-milfoil — Myriophyllum pinnatum.  Montana Field Guide.  .  Retrieved on , from