A Feathery Invasive in Every U.S. State
Eurasian watermilfoil (Myriophyllum spicatum) is a submerged aquatic invasive plant native to Europe, Asia, and northern Africa that has become established in all 48 contiguous U.S. states and most Canadian provinces. It is one of the most widespread aquatic invasive plants in North America — exceeded in range only by the near-ubiquitous purple loosestrife among aquatic invasives — and has been actively managed in U.S. water bodies since the 1950s and 1960s.
The plant is named for its distinctive feathery foliage: each submerged leaf is finely divided into 14–24 pairs of thread-like leaflets arranged in a pinnate pattern (resembling a feather), giving the plant an appearance resembling a tiny fern or bottle brush when examined closely. In aggregate, large milfoil beds create a visual impression of underwater meadows, and the plant's appearance led to its initial use as an aquarium plant — which is almost certainly the primary pathway for its early introductions to North American water bodies in the 19th and early 20th centuries.
Introduction and Spread History
The exact date and method of introduction of Eurasian watermilfoil to North America is debated. It was first documented in North American water bodies in the early 20th century, with the earliest confirmed specimens from the Chesapeake Bay region (Maryland, Virginia) in the 1940s. Population explosions and widespread recognition of the management problem began in the late 1950s and 1960s, when milfoil had spread throughout the Chesapeake Bay watershed and was expanding rapidly through recreational boating in lake regions of the North and Midwest.
By 1970, Eurasian watermilfoil was documented in 20+ states. By 1990, it was in all contiguous states. The spread mechanism is the same as for hydrilla and other fragmentation-reproducing aquatic invasives: boat propellers, trailers, and bilge water carry plant fragments between water bodies whenever boats are moved. A single stem segment with one node can establish a new plant in an uninfested lake. Lakes near popular recreational boat launching areas — which receive boats from many different water bodies — are at highest risk of new introduction.
Biology and Competitive Traits
Eurasian watermilfoil is a highly competitive submerged plant that succeeds through several biological mechanisms. Unlike many native milfoil species, Eurasian watermilfoil can grow in a wide range of water quality conditions — from relatively clear oligotrophic lakes to turbid, nutrient-enriched water bodies. It begins growth earlier in spring than most native submerged plants, capitalizing on cool water temperatures (as low as 5–10°C) to establish a canopy before native vegetation begins to grow. This early-season competitive advantage can be decisive in allowing milfoil to dominate the littoral zone before native plants can compete.
Milfoil stems elongate rapidly in spring, growing from overwintering rootstocks at the sediment surface, reaching the water surface to form a lateral canopy. Once at the surface, lateral branching creates a dense horizontal mat that shades all submerged life below. Fragmentation occurs naturally as wind, current, and wave action break off stem tips — and dramatically increases when boats pass through milfoil beds. Each stem tip fragment produces roots at nodes within days to weeks and establishes a new plant. This fragmentation biology is simultaneously the primary spread mechanism and a fundamental challenge for mechanical harvesting programs. See Eurasian watermilfoil growth and fragmentation for detail.
The Hybrid Problem
Eurasian watermilfoil hybridizes naturally with the native northern watermilfoil (Myriophyllum sibiricum) wherever both species co-occur. The resulting hybrid (M. spicatum × sibiricum) is morphologically intermediate between the parents and can be difficult to distinguish in the field — leaflet counts typically fall between those of the parents (9–13 pairs). Importantly, hybrid milfoil may show enhanced growth and competitive ability compared to either parent ("hybrid vigor"), and may also show different — sometimes reduced — responses to herbicide treatment compared to pure Eurasian watermilfoil. The hybrid is documented across the northern U.S. and southern Canada and is a growing management complication in lakes where both parent species are established.
Management Overview
Eurasian watermilfoil has been managed in U.S. water bodies for more than 50 years, generating a large body of experience with what works and what doesn't. Herbicide treatment (particularly with triclopyr and ProcellaCOR), biological control using the native milfoil weevil (Euhrychiopsis lecontei), and winter drawdown in regulated impoundments are the most effective management tools. Mechanical harvesting provides short-term navigational relief but does not provide lasting control and can spread infestations through fragment dispersal. For complete management guidance, see Eurasian watermilfoil control methods.
References
- Smith, C.S. & Barko, J.W. (1990). Ecology of Eurasian watermilfoil. Journal of Aquatic Plant Management 28:55–64.
- Aiken, S.G., et al. (1979). The biology of Myriophyllum spicatum. Aquatic Botany 6:379–390.
- Moody, M.L. & Les, D.H. (2002). Evidence of hybridity in invasive watermilfoil. Proceedings of the National Academy of Sciences 99(23):14867–14871.