Montana Field Guide

PLANTS: Aquatic, perennials with finely dissected, whorled submerged leaves, and reduced or absent emerged leaves. Stems may branch. Over-wintering turions (cylinders or balls of small leaves) are present. Source: Giblin et al. [eds.] 2018.

LEAVES: Submerged leaves are well-developed (up to 40 mm long) and mostly in whorls of 4 with (6-)10-24 leaf segments. Emerged leaves are reduced or absent within the inflorescence (shorter than flowers or fruits), whorled. Source: Giblin et al. [eds.] 2018.

INFLORESCENCE: An emergent, terminal spike. The spike consists mostly of separate male and female flowers (some may be bisexual) and emerged leaves are absent or less than the flowers and fruits. Flower bracts are less than 4 mm long. Source: Lesica et al. 2012; Giblin et al. [eds.] 2018.

Montana has 1 exotic and 3 native Water-milfoil species. Their identification requires a close examination and users should consult Biology, Ecology, and Management of Eurasian Watermilfoil [Myriophyllum spicatum] (Parkinson et al. 2011), Manual of Montana Vascular Plants (Lesica et al. 2012) or Flora of the Pacific Northwest-2nd Edition (Giblin et al. [eds] 2018).

Common Water-milfoil - Myriophyllum sibiricum, native, desirable:
* Combination of flowering spikes with emergent leaves less than 4 mm and whorled submerged leaves with 4 to 16 pairs of segments that mostly spread or are perpendicular to the apex.
* Submerged leaves are often in whorls of 4 with 6-16(-24) segments. Segments spread or lay perpendicular to the rachis at base, but may ascend towards the apex.
* Plants remain stiff when removed from water.
* Lower pair of segments are longest and gradually shorten towards the leaf tip.
* Turions present: dark green, broadly cylindrical, composed of reduced and thickened leaves, and may remain persistent on next year’s new growth.

Eurasian Water-milfoil - Myriophyllum spicatum, exotic, noxious, invasive:
* Combination of flowering spikes with emergent leaves less than 4 mm and whorled submerged leaves with 14 to 24 pairs of segments that ascend.
* Submerged leaves have linear segments that are mostly equal in length.
* Vegetative shoot tips are often dense.
* Plants readily collapse when removed from water.
* Turions (cylinders or balls of small leaves) are absent.

Hybrid Eurasian X Common Water-milfoil, exotic, noxious, invasive:
Historically, the relationship of Myriophyllum spicatum and Myriophyllum sibiricum has been unclear, but recent treatments indicate they are unique species. Where both species are present, the populations can intergrade producing hybrids with intermediate characteristics (DiTomaso and Healy 2003). Genetic testing is necessary when morphological characteristics are in doubt (Thum personal communication). In Montana the hybrid has been found in waterbodies where both species occur and will grow invasively. Hybrids have not been found in water bodies that lack one of these species, indicating that hybrids are not self-reproducing (Thum personal communication). Herbicides that traditionally control Eurasian Water-milfoil are not effective on hybrid plants (Thum personal communication).

Whorled Water-milfoil - Myriophyllum verticillatum, native and desirable:
* Emergent leaves are longer than the flowers and fruits and pinnately divided or lobed more than half-way to mid-vein.
* Flowers have 8 stamens.
* Submerged leaves are generally in whorls of 4, often with 12-22 segments.
* Fruit segments are round(-ish) with shallow, longitudinal ridges and no wings or cross-ribs.
* Turions present (balls of small leaves that develop from tips of vigorous vegetative shoots): brown to red-brown and 1-5 cm long.

Andean Water-milfoil - Myriophyllum quitense, native and desirable:
* All emergent leaves are medium to dark green, smooth, longer than the flowers and fruits, and dry to dark grey.
* Emergent leaves on the upper half of inflorescence are ovate and serrate.
* At the base of branches or shoots are strap-shaped, entire to pectinate (toothed like a comb), somewhat opposite, and very reduced leaves.
* Flowers have 8 stamens.
* Submerged leaves are in whorls of 4-5 with 6-12 segments.
* Plants remain stiff when removed from water.
* Turions absent.

Common Hornwort - Ceratophyllum demersum, native, desirable:
* Submerged leaves have linear-forked segments that whorl around the stem. They are not pinnately divided (no central mid-rib).
* Flowers are submerged, but usually plants are sterile and reproduction is mostly by overwintering turions (Lesica et al. 2012).

Shallow to rather deep water of ponds and lakes in the plains, valleys, and montane zones of Montana (Lesica et al. 2012).

FLOWERS
On the same plant male and female flowers are usually 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 pink petals and 8 stamens. Source: Lesica et al. 2012; Giblin et al. [eds.] 2018.

FRUITS
Fruits are drupe-like or nut-like with 4 carpels that remain fused for a quarter to a third of their upper margin as it matures. Fruit segments are 2-3 mm long. Source: Lesica et al. 2012; Giblin et al. [eds.] 2018.

Turions present: dark green, of reduced and thickened leaves, remnants may persist at shoot bases of next year’s new growth (Giblin et al. [eds.] 2018).