Eurasian watermilfoil (Myriophyllum spicatum) is the plant found most frequently in our lakes. An invasive aquatic plant, it was found at just one spot – at a boat launch on Lake Oscaleta – during a plant survey in 1970. Unfortunately, nothing was done about this discovery. At that point we just didn’t understand the threat of invasive species. Today Eurasian watermilfoil (EWM) rings the littoral zone (shallow waters) of all of our lakes.
Eurasian watermilfoil is a submersed perennial that is generally easy to tell apart from other kinds of submersed plants. The leaves are fine and feather-like, arranged in whorls of 4 around a reddish stem. Each leaf generally has between 12 and 20 leaflet pairs. This increased density of leaves is a distinguishing factor from other milfoil species. Growing tips are often red-green and tasseled. After the plant reaches the surface, it may send up flower spikes with tiny pink flowers that rise above the surface and then submerge after pollination. Despite the flowers, milfoil reproduces primarily by fragmentation, and each fragment has the potential to develop into a new plant.
EWM overwinters as a root crown in the substrate where nutrients are stored, although shoots have also been observed to persist all winter and under the ice.
EWM is very adaptable to a range of temperatures, light levels, nutrient levels, and water chemistry. It can grow in deep water or shallow. All of these characteristics typically provide a competitive advantage. This may be why it is the most common plant in our lakes. However, in portions of our lakes, well established native plant beds of bassweed and elodea are able to resist intrusion by milfoil, and in other spots, milfoil and bassweed grow in side-by-side patches.
Many lakes have some natural predators such as weevils and moths that can reduce the growth rate of milfoil, but often they are not present in sufficient numbers to act as a control. Other lakes have tried to supplement the number of weevils in order to control milfoil, but they have met with limited success. Some lakes use mechanical harvesting, which is like mowing the lawn. It can provide some temporary reduction in surface amounts of milfoil, but the fragments can spread to new sites. Herbicides are a common control mechanism but most lakes have to apply these herbicide annually. In our lakes, hand pulling and benthic barriers are feasible local control options.