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Montana Field Guides

Variable-leaf Milfoil - Myriophyllum heterophyllum
Other Names:  Twoleaf Water-milfoil, Broadleaf Water-milfoil

Aquatic Invasive Species
Undetermined Origin
Not Documented

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

Agency Status
USFWS:
USFS:
BLM:
MNPS Threat Rank:
C-value:

External Links






State Rank Reason (see State Rank above)
Myriophyllum heterophyllum is an aquatic plant native to North America, but introduced into many portions of the continent. It has not been documented in Montana. A 1958 herbarium specimen (http://www.pnwherbaria.org/) from Flathead Lake is mis-identified, and likely represents a Myriophyllum species that is native to Montana. 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 5-28 leaf segments. Emerged leaves are well-developed (larger than the flowers or fruits), whorled, and occur within the inflorescence. Emerged leaves are widely ovate and gradually transition upwards from pectinate (arranged like the tines of a comb) to entire (no tines or teeth). Emerged leaves dry to become dark green or black. Source: Giblin et al. [eds.] 2018.

INFLORESCENCE: An emergent, terminal spike. The spike consists of separate male and female flowers. Source: Giblin et al. [eds.] 2018. See Reproductive Characteristics for distinguishing characteristics.

Range Comments
Variable-leaf Milfoil is historically known in North America from the southeast and mid-west and considered native to North America (Howard 2019). However, it has been introduced into many portions of the U.S. (Howard 2019). In Canada Two-leaf Water-milfoil occurs in British Columbia where it is apparently native (NRCS PLANTS database; Giblin et al. [eds.] 2018).

Habitat
Lakes and ponds (Giblin et al. [eds.] 2018).

Reproductive Characteristics
FLOWERS
On the same plant male and female flowers are separate, generally one per axil, and usually subtended by 2 or more bracteoles. In general male flowers grow above the female flowers in the terminal spike-like inflorescence. Male flowers have 4 stamens with anthers of 1.3-2.2 mm long. Male flowers have 4 sepals of 0.5-0.9 mm long. Female flowers have 4 petals of 1.5-3.0mm long. Source: Giblin et al. [eds.] 2018.

FRUITS
Fruits are drupe-like or nut-like with 4 carpels that remain fused for a third to a quarter of their upper margin as it matures. Fruit segments are orbicular to broadly elliptical at their middle. Faces are warty or bumpy and bluntly 4-angled.

Plants do not develop turions.

Management
Variable-leaf Milfoil can be introduced when folks release their aquarium plants into water bodies, by attaching to boats or trailers, or by waterfowl (Howard 2019).

In Connecticut hybrids between Myriophyllum heterophyllum and Myriophyllum pinnatum have been found; propagation occurs primarily vegetatively (Moody and Les 2002).

Contact information for Aquatic Invasive Species personnel:
Montana Fish, Wildlife, and Parks Aquatic Invasive Species staff.
Montana Department of Natural Resources and Conservation's Aquatic Invasive Species Grant Program.
Montana Invasive Species Council (MISC).
Upper Columbia Conservation Commission (UC3).

Threats or Limiting Factors
There are indications that plants are capable of developing monotypic stands (Thum and Lennon 2006).

References
  • Literature Cited AboveLegend:   View Online Publication
    • Moody, M.L. and D.H. Les. 2002. Evidence of hybridity in invasive watermilfoil (Myriophyllum) populations. Proceedings of the National Academy of Science, 99(23): 14867-14871.
    • Thum, R.A. and J.T. Lennon. 2006. Is Hybridization Responsible for Invasive Growth of Non-indigenous Water-Milfoils? Biological Invasions 8(5): 1061-1066.
  • Additional ReferencesLegend:   View Online Publication
    Do you know of a citation we're missing?
    • 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.
    • Howard, V. 2019. Myriophyllum heterophyllum Michx. U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, Florida. Accessed on June 14th from
  • Web Search Engines for Articles on "Variable-leaf Milfoil"
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Citation for data on this website:
Variable-leaf Milfoil — Myriophyllum heterophyllum.  Montana Field Guide.  .  Retrieved on , from