Hydrilla, also known as “hydra weed” or “water thyme,” is an aggressive, invasive aquatic plant that poses substantial challenges to freshwater ecosystems. Originating from the Old World, primarily Asia, Africa, and Australia, this submersed perennial herb was introduced to the United States in the 1950s, likely through the aquarium trade. Its widespread nature and rapid growth make it a significant problem in lakes, ponds, rivers, and canals, where it forms dense mats that can disrupt natural processes and human activities.
Identifying Hydrilla
Hydrilla (Hydrilla verticillata) can be identified by several distinct physical characteristics. Its slender stems, which can grow up to 25 feet long, branch out extensively, particularly near the water surface, forming thick, dense mats. The small, pointed, bright green leaves are typically arranged in whorls of three to ten leaves around the stem, with five leaves per whorl being the most common arrangement. A key identifying feature is the presence of noticeable serrations, or saw-toothed edges, along the leaf margins, which can feel rough to the touch. Many hydrilla leaves also have small spines or teeth on the underside of their reddish midrib, further distinguishing them.
Subterranean Structures
Below the water, hydrilla often produces small, potato-like tubers, typically dull-white to yellowish, which grow at the ends of underground stems in the sediment. These tubers, ranging from 4 to 15 mm long, store food and allow the plant to survive adverse conditions and overwinter. Additionally, hydrilla produces compact, spiny, dark green buds called turions along its leafy stems. Both tubers and turions are reproductive structures that can detach from the parent plant and establish new populations, contributing to its rapid spread.
Distinguishing from Similar Plants
Distinguishing hydrilla from similar aquatic plants like native elodea (Elodea canadensis) or Brazilian elodea (Egeria densa) is important for proper management. Native elodea typically has smooth-edged leaves arranged in whorls of three, without tubers. Brazilian elodea usually has whorls of four to six leaves, which are often larger and have smooth edges, lacking the distinct serrations and midrib spines found on hydrilla leaves. The presence of subterranean tubers and turions is a definitive characteristic of hydrilla, as these structures are not produced by elodea or Brazilian elodea.
Why Hydrilla is a Problem
Ecological Disruption
Hydrilla’s aggressive growth habits cause significant disruption to aquatic ecosystems. Its ability to grow rapidly, sometimes up to an inch per day, allows it to form dense, thick mats that block sunlight from reaching native submerged vegetation. This shading leads to the decline and displacement of native plant species, which are crucial for providing food and shelter for local wildlife, thereby reducing biodiversity. The thick mats also interfere with water flow, which can cause issues in canals and irrigation systems.
Water Quality and Wildlife Impacts
As hydrilla mats decompose, they consume large amounts of dissolved oxygen in the water, leading to hypoxic conditions that can harm or kill fish and other aquatic organisms. This oxygen depletion can severely impact fish populations, altering predator-prey relationships and leading to an overall decline in fish numbers and size. Furthermore, hydrilla can host specific types of blue-green algae that produce neurotoxins, which have been linked to avian vacuolar myelinopathy (AVM), a disease affecting waterfowl and raptors.
Recreational and Economic Impacts
Beyond ecological impacts, hydrilla infestations have considerable effects on recreation and local economies. The dense surface mats make recreational activities such as boating, fishing, and swimming difficult, sometimes impossible. This interference can deter tourists, leading to substantial economic losses for communities that rely on water-based recreation and tourism. Property values of waterfront areas can also decrease, and property owners may face considerable costs for ongoing management efforts.
Controlling Hydrilla
Controlling hydrilla requires a comprehensive and often ongoing approach due to its rapid growth and various reproductive methods.
Prevention
Prevention is a primary strategy to avoid new infestations. Individuals can help by thoroughly cleaning boats, trailers, fishing gear, and other equipment to remove all plant fragments and mud before moving between different water bodies. Proper disposal of aquarium plants and avoiding the introduction of non-native species into natural waterways are also important preventative measures.
Mechanical Control
Mechanical removal involves physically cutting, harvesting, or dredging the plant from the water using specialized equipment. While this method offers immediate results, it is often a temporary solution because hydrilla can regrow from even small fragments left behind. Therefore, mechanical removal is usually combined with other control techniques for more lasting effects.
Chemical Control
Chemical control involves applying EPA-approved aquatic herbicides to kill or inhibit hydrilla growth. Herbicides like fluridone and endothall are effective, with fluridone being a slow-acting systemic herbicide that disrupts photosynthesis throughout the plant, and endothall acting as a contact herbicide. Professional application is often necessary to ensure proper dosage and to minimize harm to native plants and aquatic life. Multiple treatments may be required, and these applications can be expensive.
Biological Control
Biological control methods utilize natural enemies to manage hydrilla populations. Triploid grass carp, which are sterile, herbivorous fish, are commonly used because they readily consume hydrilla. Stocking rates vary, but typically range from 2 to 50 fish per acre, with 7 to 15 fish per surface acre often providing effective control in smaller bodies of water. However, grass carp are not a complete solution; they may consume desirable native plants, and their effectiveness can be limited in heavily infested areas or open water systems where they might migrate away. A multi-faceted approach, integrating prevention, mechanical, chemical, and biological methods, is generally the most effective strategy for hydrilla management.