Milfoil is a feathery, submerged aquatic plant with finely divided leaves arranged in circles (called whorls) around a long, flexible stem. The name itself is a clue: “milfoil” comes from Latin for “thousand leaves,” describing the way each leaf splits into numerous hair-like segments that fan out underwater like a tiny bottlebrush or a bird’s plume. Several species exist in North American waters, and telling them apart matters because some are native and ecologically beneficial while others, particularly Eurasian watermilfoil, are among the most aggressive aquatic invaders on the continent.
The Basic Look of Watermilfoil
If you pull a milfoil stem from the water, the first thing you notice is the leaves. Each leaf is divided into many paired, thread-like segments called leaflets that radiate outward from a central leaf stalk, giving it a feathery or comb-like appearance. Underwater, these leaflets spread open and look lush and bushy, but once out of the water, the leaves collapse against the stem and look stringy and limp. This dramatic difference between the submerged and above-water appearance trips up a lot of first-time identifiers.
The leaves grow in whorls of three to six around the stem, spaced at fairly regular intervals. Stems are smooth, round, and can be green, reddish-brown, or pinkish depending on the species, time of year, and water conditions. Individual plants can send stems several feet toward the surface, and in shallow water the stems often branch heavily near the top, forming dense mats just below or at the waterline. These canopy-like mats are one of the most visible signs of a milfoil infestation when you are looking at a lake from a boat or dock.
When milfoil flowers, it sends a small spike above the water surface. The flowers are tiny, inconspicuous, and typically pinkish or yellowish, arranged along a short stalk that pokes just a few inches above the waterline. Most people never notice milfoil flowers unless they are specifically looking for them, which is why identification almost always relies on the leaves and stems rather than blooms.
Eurasian Watermilfoil Versus Native Watermilfoil
The species most people are concerned about is Eurasian watermilfoil (Myriophyllum spicatum), an invasive plant that has spread across much of the United States and southern Canada since the mid-twentieth century. It outcompetes native plants, forms surface mats that interfere with boating and swimming, and alters lake ecosystems. But North America also has native watermilfoil species, particularly northern watermilfoil (Myriophyllum sibiricum, sometimes called M. exalbescens), which looks strikingly similar. The resemblance between the two creates real headaches for lake managers and volunteers trying to survey for invasives.
The most reliable visual distinction comes down to counting leaflet pairs on a single leaf. Eurasian watermilfoil typically has 12 to 21 pairs of leaflets per leaf, giving it a denser, more finely divided appearance. Northern watermilfoil usually has fewer, roughly 5 to 12 pairs, and the leaflets tend to be slightly thicker and more widely spaced. When you hold a leaf from each species side by side, the Eurasian leaf looks lacy and crowded, while the native leaf looks a bit more open and sparse.
Other differences help but are less definitive on their own. Eurasian watermilfoil stems are often reddish or pinkish, especially near the growing tips, while northern watermilfoil stems tend to stay green or olive. Eurasian milfoil also tends to branch more aggressively near the water surface, forming those distinctive canopy mats, whereas native species are more likely to stay lower in the water column without creating a thick surface layer. The flower spikes of Eurasian watermilfoil are usually longer and have more flowers than native species, but since flowering is seasonal and brief, this trait is less useful in practice.
Why Leaflet Counts Are Trickier Than They Sound
On paper, counting leaflet pairs seems straightforward. In the field, it is anything but. Milfoil leaves are small, and the leaflets are hair-fine. Counting them accurately often requires pulling a stem from the water, laying a leaf flat on a white surface or clipboard, and using a hand lens. Water movement, tangled growth, and the leaf’s tendency to collapse when removed all complicate matters. Most field guides recommend counting leaflets on several leaves from the middle portion of the stem, since leaves near the tip or base can have atypical numbers.
The bigger problem is overlap. Some Eurasian watermilfoil plants in nutrient-poor or cold water produce leaves with as few as 10 to 12 pairs of leaflets, which falls squarely into the range expected for native milfoil. Environmental conditions like light level, water depth, nutrient concentration, and current speed all influence how many leaflets a plant produces and how densely it grows. A single plant can look noticeably different in a shallow, sunny bay compared to a deeper, shaded spot in the same lake. This plasticity means no single visual trait is foolproof.
Hybrid watermilfoil further muddies the picture. Eurasian and northern watermilfoil readily cross-pollinate, producing hybrids whose leaf characteristics fall somewhere between the two parent species. These hybrids are increasingly common in many lakes and can only be confirmed through genetic testing. They tend to be even more aggressive than pure Eurasian watermilfoil in some settings, so misidentifying a hybrid as native has real management consequences.
Parrotfeather and Other Milfoil Relatives
Not all milfoil species are fully submerged. Parrotfeather (Myriophyllum aquaticum) is a striking exception that looks very different from the watermilfoils most lake users encounter. It produces two distinct types of foliage: emergent stems that rise up to about a foot above the water surface and submerged stems that can extend up to 16 feet below. The emergent leaves are stiff, grayish-green, and arranged in whorls, with each leaf divided into roughly 20 linear segments (about 10 pairs of leaflets). They genuinely look like tiny parrot feathers fanning out from the stem, which is how the plant got its common name. Submerged leaves on the same plant are darker green or even reddish-orange, softer, and more limp.
1Utah State University Extension. Parrotfeather (Myriophyllum aquaticum) Identification and Management in WaterwaysBecause parrotfeather grows partly above the waterline, it is easier to spot than fully submerged species. It forms dense, bushy mats along shorelines, in slow-moving streams, and in ditches. It is also invasive in many regions and can clog waterways. If you see a feathery aquatic plant with stiff growth sticking well above the water surface, parrotfeather is a strong possibility. Standard watermilfoils like Eurasian and northern milfoil rarely produce more than a tiny flower spike above the waterline.
Several other Myriophyllum species occur in North America, including whorled watermilfoil (M. verticillatum) and low watermilfoil (M. humile). Whorled watermilfoil can look very similar to both Eurasian and northern milfoil, though it often has conspicuous leaf-like bracts along its flower spike that the others lack. Low watermilfoil is much smaller and typically grows in shallow, muddy margins rather than open water. These species are far less common in everyday encounters, so most identification questions come down to Eurasian versus native milfoil.
What Milfoil Looks Like Through the Seasons
Milfoil changes appearance dramatically over the course of a year, and knowing what to expect in each season helps with identification. In spring, new growth emerges from fragments or specialized dormant buds. Stems are bright green, relatively short, and have not yet branched heavily. This early-season growth can be easy to miss because it has not yet formed the dense surface mats people associate with milfoil problems.
By midsummer, Eurasian watermilfoil is typically at peak biomass. Stems have grown to the surface, branched into a thick canopy, and may be producing flower spikes. The surface mats are most visible and problematic during this period. Leaves near the tips can appear reddish. Native milfoil also reaches its peak in summer but usually does not form the same dense surface canopy.
In fall, milfoil begins to senesce. Stems become darker, sometimes brownish, and some species produce specialized overwintering buds called turions. Northern watermilfoil, for example, forms turions on its stem tips starting in September or October. These turions gradually elongate and become heavier through the autumn, eventually sinking to the bottom by April, where they assume a distinctive U-shape at the base of the new plant the following season. Turions remain attached to the previous year’s growth until the parent stem decays, so you can sometimes find them dangling on old stems in late fall.
2Aquatic Botany. Turions of Myriophyllum exalbescensBy winter, in temperate climates, much of the visible plant material has died back. What remains are the turions resting on the bottom, any root crowns that survived, and loose stem fragments. Eurasian watermilfoil can overwinter as root crowns and fragments even without producing true turions, which is one reason it spreads so effectively: a fragment carried on a boat trailer to a new lake in autumn can root and establish a new population the following spring.
Plants That Get Confused With Milfoil
Several common aquatic plants share milfoil’s feathery, finely divided leaves and routinely get mistaken for it. The most frequent lookalike is coontail (Ceratophyllum demersum), a rootless submerged plant with forked, fan-shaped leaf segments. Coontail leaves are stiffer and coarser than milfoil, and the segments fork in a Y-pattern rather than branching from a central leaf stalk like milfoil’s comb structure. If you pull up a plant and the leaves look like tiny antlers rather than tiny feathers, it is probably coontail.
Bladderwort (Utricularia spp.) is another common source of confusion. Its finely divided leaves look feathery at first glance, but a close look reveals tiny bladder-like structures along the leaf segments. These bladders are the plant’s insect-trapping organs, and nothing resembling them appears on milfoil. Bladderwort also tends to be lighter green and more tangled, without the neat whorled leaf arrangement of milfoil.
Fanwort (Cabomba caroliniana) produces submerged leaves that fan out in a similar feathery pattern. However, fanwort leaves are opposite (growing in pairs) rather than whorled, and each leaf divides into segments that spread like a hand-held fan. The overall texture of a fanwort bed underwater looks slightly different, with a more layered, delicate quality, but the distinction requires examining how leaves attach to the stem.
The practical test comes down to leaf arrangement and structure. If leaves are whorled (three or more emerging from the same point on the stem) and each leaf has paired segments branching off a central stalk like a comb, you are looking at a milfoil. If the leaves fork, are opposite, or have obvious bladders, you are looking at something else.
Tools and Methods Beyond the Naked Eye
When visual identification is inconclusive, especially when trying to separate Eurasian from native milfoil or detect hybrids, more advanced methods come into play. Genetic testing using DNA markers is the gold standard for distinguishing species and confirming hybrids. Several university extension labs and state agencies offer this service; you mail in a stem sample and get a species identification back. For individual lake surveys, this is the most reliable approach available.
At a landscape scale, researchers have been developing remote sensing methods to detect and map Eurasian watermilfoil from the air. Drone-mounted multispectral cameras that capture light beyond what the human eye can see have shown promise for identifying milfoil beds in shallow, clear water. In a multi-year study across several sites in Lake Huron’s Les Cheneaux Islands, researchers using drone-based multispectral imaging achieved roughly 77 to 79 percent accuracy in correctly identifying Eurasian watermilfoil, which exceeded most previous remote sensing efforts for submerged vegetation. The method relies on the fact that different aquatic plant species reflect and absorb light slightly differently, especially in the near-infrared range, where differences invisible to our eyes become detectable to the right sensor.
3Digital Commons @ Michigan Tech. DETECTION AND CLASSIFICATION OF EURASIAN WATERMILFOIL WITH MULTISPECTRAL DRONE-ENABLED SENSINGFor most property owners and lake volunteers, though, a hand lens, a white tray to lay leaves flat on, and a printed field guide remain the most practical tools. Many state environmental agencies publish free watermilfoil identification guides with side-by-side photographs of Eurasian versus native leaves. Taking clear photos of the whorled leaves, stem color, and any flowering parts and sending them to your state’s aquatic invasive species coordinator is often the fastest path to a confirmed identification.
Signs of Milfoil Damage From Biological Control
If you are surveying milfoil and notice stems that look hollow, brownish, or collapsed at certain points, you may be seeing evidence of biological control at work. The native milfoil weevil (Euhrychiopsis lecontei) is a small aquatic beetle whose larvae mine through milfoil stems, consuming tissue from the inside. A single larva can damage roughly 15 centimeters of stem on average, causing that section to lose buoyancy and eventually break apart.
4Biological Control. Developmental Rates of the Native Milfoil Weevil, Euhrychiopsis lecontei, and Damage to Eurasian Watermilfoil at Constant TemperaturesThese damaged stems do not mean the plant is a different species. They just indicate weevil activity, which some lake management programs deliberately encourage. If you pull up a milfoil stem and find hollowed-out sections or tiny exit holes, the plant’s identification is unchanged, but you are getting a glimpse of one of the few natural checks on Eurasian watermilfoil growth.
Practical Tips for Field Identification
A few habits make field identification more reliable. First, always collect samples from the middle of the stem, not the very tip or the base. Tip leaves are often immature with fewer leaflet pairs, which can make Eurasian milfoil look like native milfoil. Basal leaves are often degraded. Middle-stem leaves give the most representative count.
Second, look at more than one plant. Milfoil beds often contain a mix of species, and a single grab sample from the edge of a bed may not represent what dominates in the center. Pull stems from several spots.
Third, note the growth form from above before pulling anything up. Does the bed form a thick canopy at the surface, or does it stay lower in the water column? A dense surface mat in water deeper than a few feet is a strong indicator of Eurasian watermilfoil, since most native species do not canopy as aggressively. This is not definitive by itself, but it helps narrow the possibilities before you start counting leaflets.
Finally, pay attention to timing. Late summer through early fall is the easiest window for identification because plants are at peak biomass, flower spikes may be present, and the differences between species are most pronounced. Spring surveys are harder because young growth of all species tends to look similar, with shorter stems and fewer distinguishing features.