Hydrilla (Hydrilla verticillata) 

Description

Hydrilla, a member of the Hydrocharitaceae family, is a submersed, vascular hydrophyte. It is often referred to as the perfect aquatic plant because of its ability to adapt and aggressively compete in its environment. Depending upon the conditions it grows under, it has highly polymorphic characteristics. Two different biotypes of hydrilla exist. The female dioecious biotype populations only produce female flowers, while the monoecious biotype populations have both male and female flowers upon the same plant. It is generally rooted in sediments, but fragments can break free, survive, and re-establish in a new location. Branching of hydrilla is sparse until it reaches the water's surface, and then bifurcation becomes extremely profuse, forming thick, dense mats in the upper parts of the water column. Hydrilla forms above and below ground stems called stolons and rhizomes, respectively, which gives rises to new vegetative growth.

The leaves of hydrilla are typically 2-4mm wide by 6-20mm long and occur in whorls of 3-8. H. verticillata can be identified by the presence of fine serrations on the margins of the leaves. In addition, the dioecious biotype may have sharp spines along the underside of the leaves' midrib. Hydrilla produces hibernacula in the form of turions (overwintering/dormant buds). The dormant buds are formed either on the leaf axil or terminally on rhizomes and are known as axillary turions/turions and tubers/subterranean turions, respectively.

Native Range and Distribution

The distribution of hydrilla is worldwide, occurring and dominating in aquatic communities in all continents except Antarctica, but is most likely to be native to the warmer regions of Asia. There are two different and distinct introductions that have occurred in the United States. Since the 1950s hydrilla's dioecious biotype has been found in canals near Miami and the Crystal River of Florida. This first introduction into the wild is thought to be a result of releases from aquarium trade. This dioecious biotype is typically found as far north as South Carolina, and there are even some population further north. A monoecious biotype discovered in the Potomac River, near Washington, D.C., in 1982, marks the second introduction in the United States. This second invasion is conspicuously different from the first because it is a different biotype, and therefore its source is from outside of the United States. The monoecious biotype is thought to be hardier to temperate climates, and its distribution usually expands north of South Carolina. Since its two separate introductions, hydrilla has rapidly spread throughout the southern United States' lakes, rivers, reservoirs, ponds, and canals through anthropogenic activities. More recently hydrilla has been expanding its distribution northward, and is currently established in the Cayuga Lake Inlet, NY. It has also been found in the Erie Canal at North Tonawanda, NY, several small ponds in Broome County, nine lakes and ponds in Long Island, and the Croton River in Westchester County.

Threats and Impacts

The biology of hydrilla makes it a perfect aquatic plant with the ability to outcompete and displace other hydrophytes. The growth habit of hydrilla encourages competitiveness by growing up to one inch per day, and then branching copiously near surface of the water. This enables the plant to effectively capture sunlight and shade preexisting aquatic macrophytes. Hydrilla also displaces native aquatic plants by its ability to utilize available nutrients for growth. It is made up of 90% water, and thus more plant material can be produced from fewer available nutrients. 

Since hydrilla is so successful in establishment, dominance, and reproduction, it is important to understand its economic and environmental impacts. Hydrilla is associated with a reduction in flow of drainage canals which can lead to flooding and damage to shorelines and structures. In irrigation canals it also impedes flow and cogs intake pumps. In one case clogged intake pipes due to hydrilla has cost a hydroelectric facility over $4 million in lost electrical production and $525,000 in losses of game fish due to reduced water flow and dissolved oxygen. It is also known to disrupt utility cooling reserviours by interupting flow patterns for adequate cooling. When hydrilla forms dense mats, which is often, it interferes with navigation of both commercial and recreational vessels.

Due to it's rapid growth characteristics, hydrilla can quickly form a monoculture which can negativly impact fish populations. For example, largemouth bass populations are negatively impacted when hydrilla coverage exceeds 30%. Another study has compared a native aquatic plant to hydrilla while examining regrowth and establishment after floods have uprooted and scoured the two plants from the sediments. Sousa, Thomaz & Murphy (2012) have found that both plants start regenation at the same time, but hydrilla has a much higher rate of biomass increase. They suggest that hydrilla can outcompete native, aquatic plants after a major environmental event such as a flood.

Management

Three main management techniques have been researched extensively, developed, and used to curtail hydrilla's negative impacts:
• Mechanical - Mechanical techniques are limited by efficiency and logistical issues. It cost Florida approximately $2400/ha for mechanical harvesting and long term effectiveness is low, probably due to hydrilla's persistent tubers.

• Stocking of triploid grass carp - When properly stocked the use of grass carp can be an extremely effective and inexpensive biological tool for invasive aquatic plant management.

• Systemic and contact herbicides - The most commonly used chemical for hydrilla is fluridone, especially for large scale management. Fluridone is a systemic herbicide that is translocated within the plant's tissues. Unlike contact herbicides, fluridone is effective in controlling regrowth from subterranean turions for up to 1.5 to 2 years after a single application. Additionally hydrilla is especially sensitive to fluridone, and small concentrations of 5-150ppb have proven effective. As a result of its sensitivity, fluridone can be selective which helps prevent a mass die off of all native and non-native aquatic macrophytes. Additionally, the application of fluridone for hydrilla control is economically feasible and costs managers $125-880/ha.