Milfoil refers to a group of aquatic plants found in freshwater systems across the globe. Correct identification is important because many species are native and beneficial, while others are non-native and highly invasive. Invasive milfoil can cause serious ecological and recreational problems in lakes and ponds. Understanding the plant’s physical characteristics and growth habits is essential for effective management.
Detailed Physical Characteristics
Milfoil is a submersed, rooted perennial plant known for its delicate, feather-like leaves. These leaves are arranged in whorls, or circular patterns, around the main stem, typically with four or five leaves at each node. The soft, bushy appearance is a primary visual cue for identification.
The most telling feature is the number of leaflet pairs extending from the central leaf axis. The stems are generally slender and range from light green to reddish-brown, especially near the water surface. Stems elongate upward, sometimes reaching 30 feet in clear water.
When stems reach the surface, they often branch out extensively, creating dense, tangled mats of vegetation that float horizontally. In mid-summer, small, inconspicuous flowers may emerge on reddish spikes extending a few inches above the water line.
Habitat and Growth Patterns
Milfoil thrives in still or slow-moving freshwater environments, including lakes, ponds, reservoirs, and quiet river sections. The plant is rooted in the sediment and can grow in various substrates, such as rocky, sandy, or mucky bottoms. It prefers nutrient-rich, fine-textured sediments, often found in disturbed aquatic systems.
While milfoil can root in depths up to 30 feet in clear water, dense growth and mat formation typically occur in shallower areas, between 3 and 15 feet deep. The species exhibits rapid growth in early spring, starting photosynthetic activity earlier than native plants due to its tolerance for cooler water. Reproduction occurs through fragmentation, where a small piece of stem or leaf breaks off, drifts, and establishes a new colony by rooting in the sediment.
Native Versus Invasive Varieties
The distinction between native and invasive milfoil relies on leaf structure and behavior when removed from the water. The most common invasive species is Eurasian Watermilfoil (Myriophyllum spicatum), which is generally the source of public concern. A key indicator is the number of leaflet pairs, which typically ranges from 12 to 21 pairs per leaf.
In contrast, native species like Northern Watermilfoil (Myriophyllum sibiricum) have fewer leaflet pairs, often ranging from 5 to 10. The invasive Eurasian Watermilfoil is also identified by the “wilt test”: when lifted from the water, the delicate leaflets collapse and hang limply against the stem, resembling a closed umbrella.
Conversely, the native Northern Watermilfoil retains its structure and remains rigid when lifted. Native species do not pose the same threat of forming dense, light-blocking surface canopies. Invasive varieties are known for their aggressive ability to form thick, monoculture mats that negatively impact the ecosystem.
Distinguishing Milfoil from Other Aquatic Plants
Several aquatic plants are frequently mistaken for milfoil due to a similar submerged, bushy appearance. The two most common look-alikes are Coontail (Ceratophyllum demersum) and various species of Waterweed (like Elodea or Egeria).
Milfoil is separated from Coontail by root structure and texture. Coontail lacks true roots, is often free-floating, and has stiffer, forked, or serrated leaves, giving it a rough texture when pulled through the hand. Milfoil is a rooted plant with a smooth, fine, feathery texture that feels soft to the touch. Coontail leaves also lack the fine leaflet pairs characteristic of milfoil.
Waterweed species, such as Elodea, grow submersed and in whorls around the stem. However, their leaves are simple, broader, and blade-like, unlike milfoil’s intricate, feathery structure. Focusing on the whorled arrangement, feather-like nature, and specific number of leaflet pairs helps determine if the plant is milfoil and if it is the aggressive invasive type.