Water lilies (Nymphaeaceae) are rooted aquatic plants with large, circular leaves that float on the water’s surface. While they add beauty, excessive growth creates significant ecological and recreational problems. Dense stands cover the water, limiting sunlight penetration needed for submerged native plants. This coverage also inhibits the natural exchange of gasses, potentially depleting dissolved oxygen levels detrimental to aquatic life. Decaying plant material contributes to the organic sediment layer, fueling future growth.
Mechanical and Manual Removal
Physical removal directly addresses lily pad overgrowth, offering immediate relief and removing biomass. This method is best suited for smaller, contained areas or for maintaining control after initial reduction. Techniques range from simple hand-pulling to using specialized equipment like amphibious excavators.
The primary challenge is the plant’s robust underground structure, the rhizome. Water lilies quickly regrow from remaining fragments of this thick, tuberous root system anchored deep in the sediment. Physically digging out the entire rhizome is the most effective mechanical control, though it requires substantial effort and specialized tools.
A temporary solution involves cutting the stems (petioles) several feet below the water surface. This detaches the floating pads, forcing the plant to expend energy producing new leaves. All cut material must be removed from the water to prevent decay, which releases nutrients and contributes to oxygen depletion. Repeated cutting throughout the growing season is necessary to suppress regrowth and prevent the plant from replenishing energy reserves.
Chemical Control Strategies
The use of aquatic herbicides is often the most effective method for achieving systemic control of large lily pad infestations. Herbicides approved for aquatic environments, such as those containing glyphosate or 2,4-D, are formulated to be absorbed by the plant and translocated down to the rhizome. Glyphosate is a systemic herbicide, commonly applied as a liquid spray directly to the floating leaves, moving throughout the plant to kill the entire structure, including the roots. Treatment is most successful when the plants are mature and actively drawing energy downward, typically in late summer or early fall.
The application of any aquatic herbicide is governed by strict regulations, and users must follow the manufacturer’s label precisely. Permits are often required from state or local environmental agencies before any chemical treatment can take place in a public water body. It is highly recommended to hire a licensed and certified applicator who understands the nuances of aquatic chemistry and local ordinances.
A primary concern with chemical treatment is the risk of dissolved oxygen depletion, especially when treating dense stands of vegetation. As the treated plants die and decompose, the bacteria responsible consume large amounts of oxygen from the water, which can lead to fish kills. To mitigate this risk, applicators treat only a percentage of the total infestation at one time, allowing the oxygen levels to stabilize before subsequent applications. Furthermore, certain herbicides, like diquat, are fast-acting contact chemicals that are quickly inactivated in muddy water, making water clarity an important factor in product selection.
Biological Control Options
Biological control involves introducing organisms that feed on the target vegetation, providing a long-term, self-sustaining management solution. The triploid grass carp, a sterile version of the white amur (Ctenopharyngodon idella), is the most common biological agent used for aquatic weed control. These fish consume vegetation and can offer effective control for several years, often between eight to ten years.
Water lilies are generally not the preferred food source for grass carp; they often target softer, submerged plants first. If no other vegetation is available, carp will consume lily pads, but control may be incomplete or take significantly longer to achieve. Stocking rates are highly variable, ranging from 10 to 40 fish per surface acre, depending on the plant species, the density of the existing biomass, and the desired level of control.
Regulatory approval is frequently mandated before stocking grass carp, as permits ensure the fish are sterile and stocked at appropriate densities. Stocking too few fish results in ineffective control, while stocking too many can lead to the complete removal of all aquatic vegetation. This removal negatively impacts the lake ecosystem by eliminating fish cover and food sources. For best results, grass carp are often introduced after initial chemical or mechanical methods have reduced the bulk of the lily pad biomass.
Long-Term Prevention Through Nutrient Reduction
Addressing the root cause of excessive plant growth—an abundance of nutrients—is fundamental for long-term prevention. Aquatic plants thrive in water bodies with elevated levels of phosphorus and nitrogen, primarily originating from runoff. Managing the inflow of these nutrients is far more sustainable than perpetually removing the resulting biomass.
One highly effective preventative measure is the establishment of riparian buffer zones, which are vegetated areas along the shoreline. These plantings filter stormwater runoff, absorbing nutrients and trapping sediment before they can enter the lake. Reducing the use of nitrogen and phosphorus fertilizers on lawns and agricultural fields near the water body also directly limits nutrient loading.
In-lake solutions can also help manage the nutrient availability within the water itself. Aeration systems disrupt the stagnant conditions favored by water lilies and improve overall water quality. The application of products containing beneficial bacteria and enzymes can help break down organic matter on the lake bottom, reducing the muck layer and making fewer nutrients available for plant uptake. Specialized compounds, such as aluminum sulfate, can be used to chemically bind phosphorus in the water and sediment, effectively locking it away from the plants.