Water lilies (Nymphaea) are attractive aquatic plants that can quickly become a nuisance in a pond. While limited coverage provides shade and habitat, the plants spread rapidly using thick, horizontal underground stems called rhizomes, which anchor them firmly in the sediment. Severe overgrowth creates a dense mat on the water surface, blocking sunlight needed by submerged plants. This surface coverage also compromises the free exchange of oxygen, significantly reducing dissolved oxygen levels and potentially leading to fish kills. Furthermore, excessive plant material decomposition adds nutrients to the water, fueling further unwanted growth.
Manual and Mechanical Removal Techniques
Physical removal is often the first approach for managing water lilies, especially in smaller ponds or for spot treatment. This method involves directly removing the plant material; it is most effective when the entire rhizome is successfully pulled from the substrate. Water lily rhizomes are tough and thick, making complete removal challenging and labor-intensive.
Specialized tools like aquatic weed cutters or rakes can cut the stems (petioles) below the water surface. Repeated cutting will eventually deplete the energy reserves stored in the rhizome, though this requires multiple efforts over time. For extensive infestations, heavy machinery such as hydro-rakes or dredges can excavate the rhizomes and accumulated sediment.
After any removal, it is important to remove all harvested plant material from the water body. This prevents re-rooting or decomposition, which would otherwise release nutrients back into the water and encourage new growth.
Using Aquatic Herbicides Safely
Chemical control is considered when the infestation is too large for manual methods or when rhizomes are difficult to access. Only herbicides specifically approved for aquatic use, such as formulations of glyphosate or diquat, should be applied in a pond. Using non-aquatic chemicals is illegal and dangerous to the ecosystem due to harmful additives.
Systemic herbicides like glyphosate are absorbed by the foliage and travel down to kill the underground rhizome. Contact herbicides like diquat kill only the parts they touch and may require repeated application.
A primary risk of chemical treatment is the rapid decomposition of a large amount of plant matter killed simultaneously. This die-off consumes dissolved oxygen in the water, which can lead to fish suffocating. To mitigate this, treat no more than one-third to one-half of the pond’s surface area at a time, allowing the ecosystem to recover before the next application.
Before applying any aquatic herbicide, check local regulations and state laws, as permits are often required. Consulting a licensed aquatic professional for large-scale treatments is also highly recommended.
Long-Term Prevention Strategies
Long-term management requires an integrated approach that addresses the underlying conditions promoting lily growth. The primary factor fueling overgrowth is an abundance of nutrients, particularly phosphorus and nitrogen. These often enter the pond through surface runoff from surrounding lawns, septic systems, or agricultural fields.
Managing nutrient input is a foundational step in prevention. This can be done by creating buffer zones of vegetation around the pond edge or diverting nutrient-rich runoff. Reducing the nutrient load limits the lilies’ ability to spread rapidly.
Physical barriers also provide effective long-term control. Benthic barriers are heavy, non-toxic mats placed directly on the pond floor. They physically prevent lily rhizomes from rooting in the sediment and block leaves from reaching the surface.
For biological control, sterile triploid grass carp can be introduced. However, they are less effective against the tough rhizomes of mature water lilies compared to softer submerged weeds. Stocking grass carp requires permits in many states and should be used as a long-term management tool after initial reduction, not as a quick fix. Lasting control combines effective initial removal with ongoing environmental management to maintain a balanced, low-nutrient environment.