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An Introduction to Eurasian Watermilfoil

Updated: Oct 8, 2019

Since its first documented occurrence in U.S. waterways in 1942, Eurasian watermilfoil (Myriophyllum spicatum) has proven to be devastating. It is now considered to be among the most threatening of all aquatic invasive species, and for good reason. Aside from being a major nuisance to boaters and anglers, it can significantly reduce the efficiency of drainage and irrigation canals. Because it is such a conspicuous bane to agricultural and recreational land users, public support for Eurasian watermilfoil eradication is often very strong [6].

eurasian watermilfoil

In Colorado, both the Department of Agriculture and the Division of Wildlife direct aquatic nuisance species management. Eurasian watermilfoil (hereafter referred to as EWM) is on the Colorado Division of Wildlife’s Aquatic Nuisance Species Watch List. EWM is designated as a List B species on the Colorado Department of Agriculture’s Noxious Weed List (USDA Code: MYSP2). Its List B designation signifies a potential need for control measures as advised under the state noxious weed management plan [9,17].

The Boulder Valley Conservation District has begun to work with local ditch companies to prevent new infestations [10]. The St. Vrain and Left Hand Water Conservancy District has proposed the use of screens to resist the spread of invasive species, including EWM, into St. Vrain State Park reservoirs [11]. EWM contamination containment programs and watercraft inspection have been implemented at a few state parks [13]. Boulder County wildlife restoration volunteer groups are now including EWM eradication efforts in their programs [19]. Yet, even with well-coordinated management strategies and heightened public awareness, this destructive weed is poised to spread into new areas.


A clear and growing threat

EWM is dispersed primarily through fragmentation. It can become established in waters ranging from <one foot to >20 feet depth. Though it causes the greatest disturbance at 6-15 feet depth, it can blanket shore bottoms at 3-20 feet depth [2]. Once introduced into a new environment, a small colony can competitively displace all other submerged vegetation within two to three years [15]. Even a moderate infestation can dramatically alter the physical, chemical and thermal conditions of riparian habitats, often to the detriment of native flora and fauna [1,2,7]. 

Many aquatic macrophyte species are known to secrete allelopathic substances into the surrounding waters. However, those of EWM (primarily its hydrolyzable polyphenols) are especially potent. Highly algicidal, these compounds strongly inhibit the growth of naturally occurring phytoplankton [21]. This results in an overall reduction of zooplankton, which ultimately impacts native fisheries.

Growing at a remarkable two inches/day during the warm, sunny months, it continues to expand once reaching the water surface to form a shady canopy [3]. These surface mats not only starve nearby aquatic plants for light, but they also promote strong thermal stratification of inshore waters (i.e. an uncharacteristically warm surface layer with an uncharacteristically cool layer below it). EWM canopies therefore pose a significant health threat to humans, pets and wildlife, as they create an optimal nursery for disease-bearing mosquitos [5,7,15,17].

Well-established populations of EWM can reach densities (wet weight) that exceed one kilogram/square foot [4]. These thick, monospecific stands reduce overall habitat quality for numerous aquatic and non-aquatic species. For example, though it displaces plant species favored by herbivorous waterfowl, it provides little nutritive value in return. It can easily choke out the natural spawning beds of numerous native fish species. Furthermore, by providing additional refuge for small fishes, it severely limits the size of larger, predatory fishes [4,6,7,15].

Aggressive proliferation during its growing season can alter water pH as well as disrupt natural phosphorus cycles [7,8,17]. Tangled mats of EWM can cause floods when they block outflows from reservoirs. Decomposing masses foul beaches along the strandline. Dissolved oxygen concentration may plummet to critically low levels when submerged masses die off and decay in the autumn months [3,6,9,17].

Once established, EWM is very persistent. Therefore, prevention is far less costly and complicated than control, much less eradication. Unfortunately, owing to the plant’s weed-like ability to regenerate, total eradication typically cannot be accomplished [4]. Many different means of control have been tested and evaluated. These include physical, chemical and biological methods.


Conventional methods of control

Physical methods of EWM control involve either (1) the direct, mechanical removal of living plant biomass or (2) killing plants in situ by way of habitat manipulation. The simplest form of mechanical removal involves little more a hand rake. More intensive means of mechanical

removal involve the use of heavy equipment such as barge-mounted rototillers and driver-operated dredgers. Habitat manipulation may be carried out through shading (e.g. nontoxic dyes or light-blocking fabrics) or through desiccation (e.g. water level drawdown) [6,7,17].

Chemical control methods have involved the use of “selective” herbicides such as liquid triclopyr (e.g. Renovate) and 2,4-D (2,4-dichlorophenoxy) acetic acid (e.g. Aqua Kleen) as well as the now-banned potassium salt of 2-(2,4,5- trichlorophenoxy) propionic acid (e.g. Silvex) [6,15,16].

Several biological control methods have been proposed. Potential biocontrol agents range wildly from fungi (Mycoleptodiscus terrestris) to weevils (Euhrychiopsis lecotie) to grass carp (Ctenopharyngodon idella). The supposed advantage of biocontrol is that it targets the nuisance species while having minimal impact on native species. Then again, the use of these fungi and weevils, however, could possibly bear unforeseen negative consequences, including secondary invasions; the long-term advantages of using grass carp are even more

questionable, as the fish may only turn its attention to EWM after it has consumed all of the native vegetation [16].

Even the most efficacious and “eco-friendly” control methods have their own drawbacks. Intensive forms of mechanical removal can physically disturb habitat by (1) carving up shore sediments, (2) damaging native flora and (3) substantially increasing water turbidity. Not only can habitat manipulation techniques be similarly disruptive, but they also result in foul waters given that the dead plant matter is left in place to rot [16]. Chemical controls similarly cause water fouling [6]. Due to their inherent risks, biocontrols should be employed only as a last resort, if ever at all.

Simple, manual means of mechanical control (e.g. hand rakes) present a very attractive option as they (1) physically remove plant biomass, (2) are highly selective, and (3) do not involve the use of potentially harmful chemicals or biocontrol agents. On account of the comparatively high costs of labor and dumping, however, manual removal can be fiscally prohibitive [6].

Regardless of the particular method applied, the cost of controlling EWM can be staggering. Millions of dollars have been spent in direct field expenditures to control EWM in Tennessee Valley Authority reservoirs [15]. In Washington, Minnesota, Wisconsin, Vermont and New York, private and government sources spend approximately $1,000,000 per annum on control efforts [7]. On the other hand, there is a heavy price for not controlling EWM; in Michigan, where heavy infestations have negatively impacted valuable recreational areas, EWM is estimated to cost millions of dollars each year in lost tourism revenue alone [16]. Though difficult to assess precisely, indirect costs (e.g. mosquito control) could be even greater.


Known local infestations

At present, EWM is known to occur in as many as 48 states, including Colorado [2]. Its range within Colorado includes the South Platte, Rio Grande, Upper Arkansas and White-Yampa drainages [18]. In Boulder County, where infestation is now widespread, this  invasion has been especially swift. Indeed, some maintain that most or all of the EWM in Colorado originated from a single site within Boulder County; this population has already spread to ten or more areas, primarily along the Front Range [1].

EWM continues to expand into waterways in and around the cities of Boulder and Longmont where it has been dispersed liberally by both human and animal activities. It is now present in Longmont eastward from Golden Ponds to St. Vrain State Park and southward to Boulder Creek and Chatfield State Park [5,11,12,14]. To date, some of these areas are very nearly beyond control; the Colorado Parks and Wildlife’s (CPW) Aquatic Nuisance Species (ANS) Program has already resorted to herbicidal treatments to control EWM at St. Vrain State Park, Chatfield State Park and Lathrop State Park [13].


What we do about it                         

It just so happens that EWM, when properly processed, makes an excellent mulch and soil amendment. It is especially desirable as a top dressing medium for pots and other small contained soil beds, as its fine, fibrous structure allows for excellent moisture retention even when applied in relatively thin layers. It is highly resistant to rot. It is rich in essential minerals such as calcium and phosphorus. Farmers and horticulturists have eagerly received dumpings of EWM (sometimes by the ton) for this use. The Minnesota Landscape Arboretum has used EWM from the Lake Minnetonka Water Conservation District for a decade [20].

Where proper containment and sterilization procedures are exercised, the utilization of EWM biomass in this manner is far preferable to landfill disposal. To Peter Moe, Director of Operations and Research at the arboretum, EWM is "such a valuable material." Not only does it serve to loosen soil and improve its water-holding capacity, but it also releases growth-boosting nutrients and minerals. And, as Moe attests, "it doesn’t have any weed seeds in it."

myriophyllum spicatum

With the consent and support of the Colorado Department of Agriculture and several municipal natural resource managers, we harvest EWM for the purpose of producing high-quality, all-natural microbial inoculants such as PNS ProBio™. Harvest methods are highly selective and minimally disruptive to the natural environment. Strictly manual methods of removal are employed.

We firmly believe that our work, carried out under the auspice of state and local natural resource agencies, significantly improves our area’s natural aquatic resources; rather than being merely sustainable, these activities have a net-positive effect on the environment. Additionally, they serve to protect our local waterways to the benefit of recreational and agricultural land users. Even while it promotes the interests of so many groups, this patently safe and effectual method of EWM extraction is carried out at no cost to any public or private entities.

We take tremendous pride in what we do and are truly grateful to our customers who make it all possible!


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