Variable Milfoil: The Invasive Aquatic Threat Few Are Talking About
Variable milfoil is rapidly spreading, threatening lakes, rivers, and native ecosystems while disrupting recreation and property value.
The tranquility of North America’s freshwater lakes and rivers is being threatened by a silent, but formidable, invader: variable milfoil (Myriophyllum heterophyllum). While some aquatic invaders like zebra mussels and Eurasian watermilfoil are frequently discussed, variable milfoil has spread widely with little public awareness, quietly choking ecosystems, hampering recreation, and challenging resource managers.
What Is Variable Milfoil?
Variable milfoil is a submerged, perennial aquatic plant distinguished by its finely divided, feather-like leaves arranged in whorls around a main stem. With both underwater and emergent leaves—the latter forming a recognizable flowering spike in summer—the plant can reach up to 15 feet in length and features stems ranging from green to bright red or reddish-brown.
- Typically, 4-6 leaves per whorl, each bearing 5-14 pairs of tiny leaflets.
- Produces both submersed and emergent leaves, the latter arising above the water as blade-shaped bracts and small white flowers.
- Stems can be heavy and stiff, helping distinguish it from native milfoil species.
The plant grows rooted in a variety of substrates—mud, sand, or gravel—across both still and flowing freshwater systems, tolerating depths from 1 to 5 meters.
Native Range and Expansion
Variable milfoil is native to the Southeastern and Midwestern United States, where natural predators keep it in check. Its range, however, has dramatically expanded:
- First observed in New England (1932, Bridgeport, Connecticut); now established throughout New England, the Adirondacks, and parts of the Pacific Northwest.
- Spread rapidly via human activities—especially releases from aquariums and fragments escaping boats and trailers.
In its native areas, native insects and fish control its spread, but in northern lakes, the lack of natural predators has allowed the plant to explode and dominate in a short time frame.
How Does Variable Milfoil Spread?
The primary mechanism of spread is fragmentation. When boats, paddles, or currents break stems, the fragments can float away, root, and start new colonies:
- Variable milfoil can survive out of water for hours if kept moist—for example, on a boat trailer’s carpeted bunk.
- Reproduces also via rhizomes (lateral roots), turions (dormant buds), and seeds.
- A single fragment may establish a dense bed within one season.
Dense colonies can dominate lakes and slow-moving rivers within just a few years.
Why Is Variable Milfoil a Problem?
Variable milfoil is particularly aggressive and outcompetes native vegetation, resulting in diverse ecological, economic, and recreational impacts:
- Forms thick mats near the water surface that block sunlight, reducing biodiversity and degrading the aquatic habitat.
- Can lower dissolved oxygen levels—sometimes to zero beneath dense mats—harming or excluding fish, insects, and other wildlife.
- Reduces water quality and impedes water flow, potentially increasing mosquito breeding areas.
- Dense growth hinders swimming, fishing, boating, and can entangle or endanger swimmers.
- Negative economic impacts on tourism and waterfront property value; managing infestations is expensive and labor-intensive.
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Comparing Variable Milfoil with Other Aquatic Invasive Milfoils
| Feature | Variable Milfoil (M. heterophyllum) | Eurasian Watermilfoil (M. spicatum) | Northern Milfoil (M. sibiricum) |
|---|---|---|---|
| Leaf Whorls | 4–6 leaves, 5–14 leaflets/leaf | Usually 4 leaves, 12–21 leaflets/leaf | 5 leaves, 5–9 leaflets/leaf |
| Color | Green to red stems | Reddish stems | Green stems |
| Emergent Flower Spike | Present (3–6 inch, summer) | Shorter, less prominent | Short, sometimes absent |
| Spread | Fragmentation, rhizomes, seeds | Fragmentation, seeds | Fragmentation, seeds |
What Does Variable Milfoil Look Like?
Variable milfoil is distinctive but commonly confused with other milfoils. Identification tips include:
- Whorled, feather-like leaves—submerged leaves are limp and divided, emergent leaves are linear, small, and stiff.
- Stems often red or reddish brown.
- Dense clusters of emergent spikes above the water surface in summer with small, white, or pinkish flowers.
However, positive identification often requires an expert or sending samples to a laboratory, as hybridization between milfoil species is possible, complicating management efforts.
How Did Variable Milfoil Become So Widespread?
The major vectors for the spread of variable milfoil beyond its native range are human activities:
- Aquarium plant introductions: Dumping aquarium water or plants into local waterways—frequently cited as the original introduction method in the northeastern U.S.
- Boating and fishing: Plant fragments attach to propellers, anchors, trailers, and gear, hitchhiking from lake to lake.
- Movement of infested water: Even small residual water containing fragments can cause infestations elsewhere.
Without intervention, new infestations multiply and quickly take hold; in many regions, variable milfoil is now the most widespread aquatic invasive.
The Ecological and Economic Consequences
The impacts of unchecked variable milfoil are diverse and severe:
- Biodiversity loss: Displacement of native aquatic plants, altering food webs and habitat for fish, insects, amphibians, and waterfowl.
- Oxygen depletion: Thick mats reduce sunlight below, causing submerged plants to die off and decomposition to deplete dissolved oxygen, occasionally leading to fish kills.
- Altering shorelines: Accumulated biomass changes sedimentation patterns and shoreline morphology.
- Decreased recreational value: Dense growth inhibits swimming, boating, water skiing, and angling, leading to reduced tourism and property value.
- Increased management costs: Control or removal is expensive, requiring ongoing intervention once established.
Variable Milfoil Management and Control
Once established, variable milfoil is extremely difficult to eradicate, but several management techniques are in use:
- Hand Pulling: Effective only for small infestations, requires care to avoid fragmenting the plant, which could worsen spread.
- Benthic Barriers: Physical sheets placed on the lakebed smother plants but are impractical for large areas.
- Mechanical Harvesting: Can clear plants temporarily but often increases fragmentation and spread elsewhere in the waterbody.
- Herbicide Treatments: Certain approved chemicals can reduce infestations, but may harm native plants and require repeated applications.
- Biological Controls: In parts of the U.S., weevils and other insects native to the plant’s home range show promise, but more research is needed for safe widespread use.
The best approach is prevention. Rapid detection and immediate response to new outbreaks are essential. Public education and responsible boating practices help minimize spread:
- Always clean, drain, and dry boats, gear, and trailers after use in infested waters.
- Never release aquarium or water garden plants into natural water bodies.
- Support local monitoring and early detection programs.
Legal Status and Management Efforts
- Variable milfoil is listed as a noxious, prohibited, or regulated species in many states.
- Sale, import, or possession is often illegal where the species is invasive.
- Many regional management plans focus on containment rather than eradication due to the high cost and complexity of full removal.
Some regions have implemented volunteer-based removal teams, citizen science monitoring, and community-driven initiatives to supplement government and conservation authority efforts.
Frequently Asked Questions (FAQs) About Variable Milfoil
Q: How do I recognize variable milfoil in my local lake?
A: Look for feather-like submerged leaves arranged in whorled clusters (4–6 per node). In summer, emergent spikes with small, white flowers may be visible. However, positive identification can be difficult due to hybridization—contact local experts for confirmation.
Q: Can variable milfoil be completely removed once it has invaded a waterbody?
A: Complete eradication is rare once an infestation is well established, as removal methods often cause fragments to spread, starting new colonies. The focus is typically on containment and reduction of impact.
Q: Why is fragmenting the plant so dangerous?
A: Each fragment can root and establish a new colony. Recreational activities, poor removal practices, or boating can easily exacerbate spread.
Q: Who is responsible for managing variable milfoil?
A: In most regions, state and provincial authorities, lake associations, and local conservation organizations share responsibility, with public participation essential for prevention and monitoring.
Q: What can I do to help prevent the spread?
A: Clean all recreational equipment thoroughly, never release aquarium plants, and report sightings to authorities. Community awareness is key to prevention.
Conclusion: A Hidden Threat Demanding Attention
While often overlooked, variable milfoil is one of the most serious aquatic invaders across North America. Its stealthy spread and devastating ecological impact highlight the urgent need for greater awareness, robust prevention strategies, and collaborative management. By understanding the threat and supporting local efforts, individuals and communities can help protect the health and beauty of our waters for future generations.
References
- https://dem.ri.gov/sites/g/files/xkgbur861/files/programs/benviron/water/quality/surfwq/aisfs/variable-milfoil-fs.pdf
- https://www.adkwatershed.org/variable-leaf-milfoil
- https://plant-directory.ifas.ufl.edu/plant-directory/myriophyllum-heterophyllum/
- https://invasivespecies.wa.gov/priorityspecies/variable-leaf-milfoil/
- https://fieldguide.mt.gov/speciesDetail.aspx?elcode=PDHAL04060
- https://adkinvasives.com/Invasive-Species/Detail/41
- https://www.snohomishcountywa.gov/6568/102271/Invasive-Plant-Control-Invasive-Milfoil
- https://nyis.info/species/eurasian-watermilfoil/
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