Milfoil (Myriophyllum spicatum)

Milfoil is a perennial, meaning that it lives for more than two years. The stems and leaves die at the end of the growing season (fall), but the rhizome (underground stem) stays alive and puts up new stems in spring. This is what makes watermilfoil so invasive. The stems always remain submerged, except for the flowers (spikes), which rise out of the water.A single plant may have many stems. The leaves look like tiny feathers, and those near the surface may be reddish in colour. The roots are relatively small and are not deeply anchored in the soil, often not reaching a depth of 15 cm.


Watermilfoil reproduces in two ways: by sexual reproduction (through the formation of flowers and seeds) and asexual reproduction (through stem fragmentation or budding). Stem fragmentation may occur naturally or result from the action of waves, propellors, etc. The stem fragments take root and from each grows a new plant. Watermilfoil growing in high-traffic areas such as beaches and marinas will thus tend to spread rapidly. Watermilfoil invades its habitat very quickly: a fragment only 5 cm long can produce a 1.2-meter stem in a single growing season! It is so invasive that a single plant can produce up to 250 million more by fragmentation! Even more so, watermilfoil is particularly tenacious. Watermilfoil also has a second mode of vegetative propagation: budding. The plant develops buds that survive the winter and put up shoots in spring. A great number of these shoots grow from the base of old stems.


Watermilfoil can anchor itself basically anywhere: in cracks in boulders and on fine soils, such as sand and muck, containing organic matter. It also has a high tolerance to a variety of factors. For instance, it grows equally well on acid peat and in very alkaline waters. It is also found in both sweet and brackish water, and in waters whose nutrient content varies from rich to quite poor. It grows nearly at any depth, but maximum growth occurs between 1.5 m and 4.0 m, depending on water clarity. Finally, watermilfoil can tolerate water turbulence and its soft, flexible stems provide good resistance to mechanical wave action. All these characteristics make watermilfoil a highly invasive plant that is very hard to control.