Hydrilla (Hydrilla verticillata) is a highly invasive submersed aquatic weed that poses a significant threat to lakes across the United States. Introduced from Asia via the aquarium trade, it rapidly establishes itself in freshwater systems. Its aggressive growth chokes waterways, forming dense surface mats that interfere with boating, swimming, and fishing. Hydrilla reduces natural biodiversity by outcompeting native plants for sunlight and nutrients. Furthermore, the decomposition of large masses can severely deplete dissolved oxygen levels, leading to fish kills and impacting the health of the aquatic ecosystem. Successfully managing an infestation requires a long-term, multi-pronged strategy targeting both visible growth and persistent reproductive structures.
Understanding Hydrilla and Its Aggressive Spread
Hydrilla is difficult to eradicate due to its remarkable and varied reproductive capacity, primarily through vegetative means. Simple physical removal is often ineffective because the plant easily spreads by fragmentation. Tiny pieces of stem or leaves break off, drift, and quickly root to establish new colonies. Boat traffic commonly causes this fragmentation, inadvertently spreading the plant throughout a lake and to new water bodies.
Two specialized storage organs allow the plant to survive control efforts. Subterranean tubers, modified underground stems, are produced in the lake bottom sediment and can remain dormant and viable for over four years. These tubers are highly resistant to many chemical treatments and often sprout after the main plant mass is controlled, causing re-infestation. Similarly, axillary turions (winter buds) are condensed shoots that detach and overwinter, germinating when conditions become favorable. Effective control must target both the growing plant and these dormant storage structures.
Mechanical and Physical Removal Methods
Mechanical harvesting uses specialized machinery to cut and collect hydrilla biomass from the water. This method offers immediate, temporary relief for navigation and recreational areas, but it is typically expensive and requires frequent repetition. A significant drawback is that cutting produces numerous plant fragments, which can escape collection and accelerate the plant’s spread to uninfested areas.
Dredging is a more drastic physical method involving the removal of lake bottom sediment where hydrilla is rooted and tubers are buried. While highly effective for long-term control in smaller, contained areas, it is extremely costly and causes substantial, temporary ecosystem disturbance. For localized infestations or smaller areas like boat lanes, benthic barriers can be used. These opaque mats are laid directly on the lake bottom to block sunlight, effectively starving the plants underneath.
Before undertaking any large-scale physical alteration like mechanical harvesting or dredging, it is mandatory to seek and obtain the necessary local or state permits. Regulatory agencies oversee these activities to ensure that the methods used do not cause undue harm to the aquatic environment or affect protected species. Failure to secure these permits can result in significant legal and financial penalties.
Chemical Treatment Options
The most common large-scale solution for widespread hydrilla infestation involves the careful use of aquatic herbicides. These treatments are broadly categorized into contact and systemic herbicides, each with a distinct mechanism of action.
Contact Herbicides
Contact herbicides, such as endothall, act quickly by killing plant tissues upon direct contact, disrupting photosynthesis. Endothall is relatively selective and degrades naturally in the water column within five to ten days.
Systemic Herbicides
Systemic herbicides, like fluridone, are absorbed through the plant’s shoots and roots and transported throughout the system. Fluridone works slowly by disrupting the plant’s ability to produce protective pigments. Visual damage appears after seven to ten days, and complete control often requires 30 to 90 days of sustained exposure. This slow, long-term exposure is necessary to successfully control persistent tubers and turions in the sediment. Efficacy depends heavily on maintaining the correct concentration for a sufficient period, which is challenging in lakes with significant water flow or exchange.
Application timing is crucial; fluridone is often most effective when applied in the fall, winter, or early spring when hydrilla growth is minimal. Due to environmental risks, the necessity of precise application rates, and complex permitting requirements, chemical treatment should only be conducted by licensed applicators. Water chemistry, including pH and temperature, must be considered, as these factors affect herbicide stability and effectiveness.
Biological Control and Preventative Measures
Biological control offers a sustainable, long-term approach for maintaining hydrilla levels after initial treatment. The most widely used biological agent is the Triploid Grass Carp (Ctenopharyngodon idella). This sterile, herbivorous fish feeds on hydrilla and is favored because its inability to reproduce allows for careful control of its population and feeding pressure within a closed water body.
Stocking rates are typically calculated based on the acreage of vegetation, often ranging from 7 to 15 fish per vegetated acre for eradication, though a lower rate may be used for maintenance. Careful calculation is necessary to avoid overgrazing, which could eliminate desirable native vegetation and lead to other water quality issues, such as increased algae blooms from released nutrients. Grass carp provide cost-effective, long-term control, often lasting five or more years before re-stocking is needed.
Preventative measures and long-term ecosystem management are fundamental to preventing hydrilla’s return. Reducing nutrient runoff, particularly phosphorus and nitrogen, from surrounding land is important because these nutrients fuel the plant’s rapid growth. This includes managing shoreline erosion and controlling fertilizer use near the lake. Regular monitoring and early detection are also crucial, allowing small, new infestations to be treated quickly before they establish a widespread problem.