Lily pads (Nymphaea species) are rooted aquatic plants that can quickly transform a water body into a dense green carpet. Their rapid, expansive growth stems from robust, horizontally spreading root structures called rhizomes, which anchor the plants firmly. Excessive coverage severely reduces sunlight for submerged life and restricts the critical exchange of oxygen between the water and the air. This can lead to dangerously low dissolved oxygen levels that stress fish.
Physical Removal and Mechanical Control
Physical removal involves manual labor or specialized equipment, best suited for small ponds or select areas. The key to successful physical removal is not simply cutting the floating leaves, but targeting the tough, subterranean rhizomes. Tools such as specialized aquatic weed cutters or heavy-duty rakes are used to slice through or drag out these root structures from the mudline.
All severed plant material, including leaves, stems, and rhizome fragments, must be completely removed from the water. If left to decompose, this material recycles nutrients back into the system, fueling future growth. For larger infestations, mechanical solutions like aquatic harvesting equipment or dredging may be employed to remove the biomass and rhizomes. This method is labor-intensive and often requires repeated efforts since any remaining rhizome tissue can readily sprout new plants.
Biological Control Options
A natural, long-term approach involves introducing sterile triploid grass carp (Ctenopharyngodon idella). These herbivores consume a wide variety of aquatic plants. The triploid designation means the fish are sterile, preventing them from reproducing and establishing uncontrolled wild populations.
Grass carp are sometimes less effective on established lily pads, as they generally prefer softer, submerged vegetation over the tough, waxy leaves of water lilies. However, they will consume lily pads when their preferred food sources become scarce, providing sustained grazing pressure that inhibits regrowth. Determining the correct stocking rate depends on the pond’s size and the density of the existing vegetation. Pond owners must check with local and state regulatory agencies, as the use of triploid grass carp is often regulated and may require a permit to prevent unintended ecological disruption.
Chemical Treatment Strategies
Chemical control is highly effective but requires careful attention to product selection and application protocols. Only herbicides specifically formulated and approved for aquatic use by the Environmental Protection Agency should be used. Using standard terrestrial herbicides can severely harm the aquatic environment and violate environmental laws.
Two primary types of aquatic herbicides are used for lily pad control: contact and systemic. Contact herbicides, such as those containing Diquat, kill only the plant tissue they directly touch, causing the leaves and stems to die back quickly. Since the deep-seated rhizome is generally unaffected, multiple applications are necessary as new growth emerges from the surviving root system.
Systemic herbicides, such as aquatic formulations of Glyphosate or 2,4-D, are absorbed through the leaves and translocated throughout the entire plant, reaching and killing the rhizomes. This action provides a more complete and longer-lasting kill of the plant structure. A non-ionic surfactant, which helps the herbicide penetrate the lily pad’s waxy cuticle, is often mixed with the systemic product to increase its effectiveness.
A major risk with chemical treatment, especially for large, dense infestations, is the rapid decomposition of the dead plant material, which can dangerously deplete the dissolved oxygen in the water. This sudden drop in oxygen can lead to a fish kill. To mitigate this hazard, applicators must treat no more than 25% to 50% of the infested area at one time, allowing two weeks for the initial vegetation to decompose before treating the next section. Due to the inherent toxicity and application complexity, consulting a licensed aquatic applicator is often the safest and most reliable strategy for large-scale treatments.
Long-Term Prevention and Maintenance
Once lily pads are removed, preventing their return relies on managing the environment that allowed them to thrive. The primary driver of excessive aquatic plant growth is an overload of nutrients, particularly phosphorus and nitrogen, which often enter the water through surface runoff. Implementing vegetative buffer strips of native grasses and plants around the pond perimeter helps to filter and absorb these nutrients before they reach the water.
Reducing the internal nutrient load can also be accomplished through aeration, which helps to circulate the water and limit the release of nutrients from the bottom sediments. Another strategy is the careful application of pond dyes, which shade the water column and reduce the sunlight penetration necessary for new plants to establish themselves. Consistent monitoring and early intervention on small, localized patches of new growth are far more manageable than tackling a widespread re-infestation.