The aquatic plants commonly known as “lily pads” represent a diverse group of species, primarily from the genera Nymphaea and Nuphar. Determining if they are invasive requires a careful, localized answer, as they can be either native or non-native depending on the water body. While some species are beneficial in their native environments, others exhibit aggressive growth that causes environmental and economic harm, classifying them as truly invasive. The plant’s status depends entirely on its origin within a particular ecosystem.
Defining the Term: When is a Lily Pad an Invasive Species?
A species is legally defined as truly invasive when it is non-native to a specific ecosystem and causes or is likely to cause environmental or economic damage, or harm to human health. This definition distinguishes a genuine invasive threat from a mere nuisance species, which is often a native plant that simply becomes overabundant in its natural range.
Common native species often mistaken for invasive plants include the White Water Lily (Nymphaea odorata) and Spatterdock (Nuphar lutea). Nymphaea odorata is native to the eastern United States, where its rapid growth can create a dense nuisance. However, if this same plant is introduced to the western United States, it is classified as invasive because it is outside its native range and aggressively displaces local flora.
A clearer example of an invasive plant often mistaken for a lily pad is Yellow Floating Heart (Nymphoides peltata), which is native to parts of Europe and Asia. This aggressive non-native species was introduced globally, primarily through the water garden trade, and is officially listed as a noxious weed in North America and New Zealand. Nymphoides peltata forms dense, monotypic mats that are highly destructive to aquatic ecosystems, requiring regulatory control.
Ecological Consequences of Excessive Growth
The primary negative impacts of excessive lily pad coverage stem from the physical and biological changes they force upon the water body. When these plants cover more than an estimated 25% of the water’s surface, the aquatic ecosystem suffers significant distress. The dense surface canopy blocks the penetration of sunlight, which is necessary for the growth of submerged aquatic vegetation (SAV) on the lake bottom.
The loss of SAV leads to a reduction in food and shelter for many invertebrates and fish, disrupting the lower levels of the food web. Furthermore, the massive amount of plant material that dies and sinks consumes large quantities of oxygen during decomposition. This process, combined with reduced surface aeration caused by the leaf mat, can lead to dangerously low dissolved oxygen (DO) levels, potentially stressing or causing fish kills, particularly during warm weather.
The physical structure created by thick lily pad growth alters the habitat, making it unsuitable for certain aquatic life and impeding the movement of fish and waterfowl. The stagnant, low-oxygen areas beneath the leaves can also become ideal breeding grounds for mosquitoes. The extensive root systems, known as rhizomes, anchor the plants firmly in the sediment, trapping organic debris and contributing to the gradual shallowing of the water body over time.
Practical Approaches to Control and Removal
Managing excessive lily pad growth requires a multi-faceted strategy that recognizes the plant’s tenacious root structure. The thick, tuber-like rhizomes can be one to six inches thick and extend deep into the sediment, making simple leaf removal ineffective for long-term control.
Non-chemical methods for removal include manual pulling and mechanical harvesting.
- Hand-pulling or using specialized aquatic rakes can be effective for small patches, but they must completely extract the stout rhizome to prevent rapid regrowth.
- Mechanical harvesters are used for larger infestations.
- All cut or pulled plant material must be removed from the water to prevent it from decaying and contributing to nutrient load and oxygen depletion.
- Continual cutting of the leaves just below the surface over multiple seasons can deplete the plant’s stored energy, eventually leading to its demise.
Chemical control involves the careful application of aquatic herbicides like 2,4-D or systemic options that are absorbed by the plant and travel down to kill the root system. Because herbicides can cause a large mass of plants to decompose simultaneously, potentially leading to a severe drop in dissolved oxygen, application must be targeted and often requires permits, especially in public or connected waterways. Ultimately, the most sustainable control strategy depends on correctly identifying the plant as either a native nuisance or a regulated invasive species.