Eurasian Water-milfoil - Myriophyllum spicatum
PLANTS: Aquatic perennials with rhizomes and finely dissected, whorled submersed leaves (DiTomaso and Healy 2003). Stems are branched and tawny colored when dry. Wintering buds (turions) are absent (Parkinson et al. 2011).
LEAVES: Leaves are submersed, but become emersed during flowering (or when water levels recede) (DiTomaso and Healy 2003). Emersed leaves are actually bracts that occur below the flowers and are oppositely arranged (DiTomaso and Healy 2003). Submersed leaves occur in whorls of 4 to 5, to 25 mm long. Each leaf is pinnately divided into 24 to 50 linear segments (Lesica 2012).
INFLORESCENCE: Terminal spike of 4-8 cm long that grows erect above the water (emersed). The spike consists of separate male and female flowers growing in the axils of oppositely arranged leaf-like bracts (DiTomaso and Healy 2003).
The relationship of Myriophyllum spicatum (exotic) and Myriophyllum sibiricum (native) has been unclear, but recent treatments considered them as unique species. Where both species are present, the populations can intergrade producing individuals with intermediate characteristics (DiTomaso and Healy 2003).
Flowering in Montana has been observed from July through September.
Montana has at least four species of water-milfoil, and users of this field guide are encouraged to identify specimens using the Manual of Montana Vascular Plants (Lesica 2012). Our native water-milfoil all overwinter as tight balls of unexpanded leaves (turions).
The combination of flowering spikes with bracts less than 4mm, whorled leaves with 14 to 24 pairs of segments, and segments that ascend distinguishes Eurasian Water-milfoil (Parkinson et al. 2011). In addition, leaves have linear segments that are mostly equal in length, and stems growing near the surface branch densely. When removed from the water, plants readily collapse. Turions (wintering buds) are absent in the fall and winter.
Shortspike Water-milfoil (Myriophyllum sibiricum) is native to Montana. It also has flowering spikes with bracts less than 4mm, but its whorled leaves have 4 to 14 pairs of segments that mostly spread. In addition, the lower segments are longest and gradually shorten towards the leaf tip. Turions are present in the fall and winter.
Whorl-leaf Water-milfoil (Myriophyllum verticillatum) is native to Montana. Its flowering spikes have pinnately divided bracts that are longer than the fruits. Its whorled leaves are up to 45 mm long and have 18 to 34 pairs of segments (Lesica 2012). In addition, the lower segments are longest and more abruptly shorten towards the leaf tip. Turions are present in the fall and winter.
Coon’s-tail or Hornwort (Ceratophyllum demersum) is a native plant that initially may appear similar. However, its leaves are linear-forked segments whorled around the stem; they are not pinnately divided (no central mid-rib) (Parkinson et al. 2011; Lesica 2012). Plants are usually sterile and reproduction is mostly by overwintering turions (Lesica 2012).
Introduced throughout North America; native to Eurasia (Lesica 2012).
Observations in Montana Natural Heritage Program Database
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Open water of reservoirs; valleys (Lesica 2012). It tolerates moving water and wave action facilitates fragmentation (Parkinson et al. 2011).
Dispersal occurs primarily by stem fragments and root crown buds (Parkinson et al. 2011). Plants are easily broken by wave action and recreational equipment (Parkinson et al. 2011). In addition, axillary buds easily break off to form new plants. The native water-milfoil species lack buds that easily detach (DiTomaso and Healy 2003; Parkinson et al. 2011).
Eurasian Water-milfoil is an indicator of eutropic (low dissolved oxygen levels coupled with high organic matter) conditions (Parkinson et al. 2011). It grows well in hard, alkaline waters. It also grows well in salinity levels up to 10 parts per thousand (ppt) and survives at 20 ppt salinity (Parkinson et al. 2011).
Plant fragments can root in a wide range of substrates and depths (Parkinson et al. 2011). It roots best in late summer in shallow water (1.6 feet deep) and in rich organic sediment. It roots poorly in coarse sand and gravel substrates. Established plants grow well from 1-4 meters (2-13 feet), and in deeper waters if light penetration is sufficient. Growth is hindered by low light penetration and high dissolved particulate matter.
As an exotic plant, Eurasian Water-milfoil lacks controls that can limit its population. Its prolific growth can outcompete native aquatic vegetation, limiting the plant diversity that provides food for an array of animal life. Dense populations can improve the survival of young fish, but reduces foraging space for predator fish, changing the natural dynamics (Parkinson et al. 2011). The sloughing of lower leaves accumulates on the floor and adds significant amounts of phosphorus and nitrogen to the water column, which increases the internal nutrient loading to a lake (Parkinson et al. 2011). Dense populations decrease dissolved oxygen, which may also change water temperatures (Parkinson et al. 2011).
The following information is adapted from Parkinson et al. 2011.
Eurasian Water-milfoil spreads primarily through plant fragments on boat trailers, recreational equipment, and waterfowl. It can also disperse between water bodies by wind and water flow. Following introduction, populations expand rapidly and may be undergo cycles of dominance and dieback.
* Thoroughly rinse any mud and debris from all equipment and wading gear, and drain the water from the boat before leaving access areas. Pump the bilge before entering another water body as Eurasian Water-milfoil can remain alive in bilge water for several days. Use boat-washing stations when available.
* Remove all plant fragments from the boat, propeller, and boat trailer. Fragments as little as 1-inch long with two nodes are able to root and colonize.
* Dry boats and equipment for 5 days before transporting them to a new water body.
* Do not dispose aquarium water or plants into water bodies.
* Desiccate plant material and/or dispose by securely sealing in plastic bags and placing in the trash for disposal.
* Learn to identify Eurasian Water-milfoil and report findings to the Montana Department of Agriculture; Montana Fish, Wildlife and Parks; County Extension agent; or Weed Coordinator.
Diquat, Endothall, 2,4-D, Triclopy, and Fluridone have been used to herbicide Eurasian Water-milfoil plants in water bodies. Native water-milfoil are also susceptible to some of these herbicides. The herbicide concentration, exposure time before dissipating, timing of chemical control, and other factors are critical to effectively hinder Eurasian Water-milfoil and reduce impacts to native vegetation – see Parkinson et al. 2011, and always follow chemical label instructions and use restrictions. These herbicides must be applied by applicators with an Aquatic Pest Control license. Consult your County Extension Agent and/or Weed District for more information on herbicidal control.
Raking and hand-harvesting can be effective for controlling small populations or early infestations. However, the risk of spread by fragmenting the plants is very high. Fragment barriers around harvest operations have been developed. All plant material should be bagged and desiccated before placing in the trash for disposal. Single harvests should be done when biomass is at its peak. It is recommended to harvest several times during the growing season, and for consecutive years. Areas harvested once can re-generate.
Benthic barriers are mats laid down on the floor of the water body around docks and other high-use areas. They prevent light from penetrating and prevent plants from rooting. They are usually effective, but kill all vegetation. They are removable once the infestation has been destroyed. Barriers must be monitored because sediment will accumulate and provide a substrate for Eurasian Water-milfoil to colonize. Plants can root in 4 cm (1.5 inches) of soil.
Drawdowns lower the water levels to expose plants. This method has been effective at killing plants and reducing infestations when timed with freezing temperatures for 96 hours. This control may require extensive planning and permitting, and may hurt non-targeted vegetation and animal life.
Two insects are being studied for their ability to control Eurasian Water-milfoil: Watermilfoil Moth (Acentria ephemerella) [native to Europe] and milfoil weevil (Euhrychiopsis lecontie) [native to North America].
Non-native water-garden plants should never be dumped near to or within wetlands, streams, rivers, lakes, or ponds. Before purchasing plants, verify that the plant is not invasive.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).
- Additional ReferencesLegend: View Online Publication
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- DiTomaso, J.M. and E.A. Healy. 2003. Aquatic and riparian weeds of the West. Regents of University of California, Division of Agriculture and Natural Resources, Publication 3421.
- Lesica, P., M. T. Lavin, and P. F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
- Montana Department of Agriculture (MDA). 2017. Montana Noxious Weed List. February. Helena, Montana.
- Newton, J., A. Sepulveda, K. Sylvester, and R. Thum. 2016. Potential utility of environmental DNA for early detection of Eurasian Watermilfoil (Myriophyllum spicatum). Journal of Aquatic Plant Management. 54:46-49.
- Parkinson, H., J. Mangold, J. Jacobs, J. Madsen, and J. Halpop. 2011. Biology, ecology and management of Eurasian Watermilfoil (Myriophyllum spicatum L.). EB0193. April reprint. Montana State University Extension, Bozeman, Montana.