Hydrilla (Hydrilla verticillata)

DESCRIPTION

The Hydrilla (Hydrilla verticillata) is thought to have been introduced to North America in Florida sometime during the 1950's. It is an aggressive, invasive species and has spread throughout Florida and most southern states, as well as California, Delaware, and the District of Columbia. Hydrilla has been categorized as one of the world's worst weeds, and it is certainly among the most notorious of submerged aquatic plant species. Infestations of Hydrilla are extremely severe and can completely choke entire lakes and public water supplies. 

Submersed perennial herb. Rooted, with long stems that branch at the surface where growth becomes horizontal and dense mats form. Small, pointed leaves are arranged in whorls of 4 to 8. Leaves have serrated margins and one or more sharp teeth under the midrib (Godfrey and Wooten 1979). Development of these features may vary with location, age, and water quality (Kay 1992).

(Sources: 1. Sea Grant Nonindigenous Species Site (SGNIS), http://www.sgnis.org/; 2. U.S. Geographic Survey, http://nas.er.usgs.gov/plants/docs/hy_verti.html; 3. King County, WA Noxious Weed Control Program, http://dnr.metrokc.gov/wlr/waterres/smlakes/hydrilla.htm). 

IMPACTS

When Hydrilla established, it results in an array of ecosystem disruptions. Changes often begin with its invasion of deep, dark waters where most plants can not grow. Hydrilla grows aggressively and competatively, spreading through shallower areas and forming thick mats in surface waters that block sunlight penetration to native plants below (van Dijk 1985). 
It has been shown to alter the physical and chemical characteristics of lakes. Colle and Shireman (1980) found sportfish reduced in weight and size when hydrilla occupied the majority of the water column, suggesting that foraging efficiency was reduced as open water space and natural vegetation gradients were lost. Stratification of the water column (Schmitz et al. 1993; Rizzo et al. 1996), decreased oxygen levels (Pesacreta 1988), and fish kills (Rizzo et al. 1996) have been documented. Changes in water chemistry may also be implicated in zooplankton and phytoplankton declines (Schmitz and Osborne 1984; Schmitz et al. 1993).
Hydrilla seriously affects water flow and water use. Recent infestations in the Mobile Delta are reducing flow in small tidal streams and creating a backwater habitat (J. Zolcynski pers. comm. 1998). Its heavy growth commonly obstructs boating, swimming and fishing in lakes and rivers and blocks the withdrawl of water used for power generation and agricultural irrigation.

Effects of Hydrilla Infestation

• waters may become unnavigable
• overheated boat motors are common
• fishing in shallow waters becomes impossible
• fish populations may become stunted
• alteration in fish predator-prey kills
• massive fish kills may occur due to depression of dissolved oxygen concentration
• changes in water quality
• loss of wildlife habitats
• clogging of rivers, streams, canals and ditches occurs, impacting agricultural, domestic and industrial uses
• prevents the use of water systems for recreation
• drownings occur due to entanglement
• impacts the economy of surrounding community
• depresses water-site property values

(Sources: 1. U.S. Geographic Survey, http://nas.er.usgs.gov/plants/docs/hy_verti.html; 2. California  Department of Boating and Waterways, http://www.dbw.ca.gov/Hydrilla.htm)

(Source: U.S. Geographic Survey, http://nas.er.usgs.gov/plants/docs/hy_verti.html)

Different methods or combined methods can be used to control Hydrilla depending on the management goal. In recreational waters the challenge is to control Hydrilla selectively amid native vegetation. Management methods include herbicide, grass carp, and mechanical removal.

Experiments have been done on the Hydrilla fly which eats only Hydrilla. This Asian fly was released in Alabama in 1998. The fly did destroy much of the plant, but populations varied from year to year. Another species being implemented is the grass carp, although possession of the fish is illegal in many states because of the risk of overpopulation. They have been used in small ponds and sterile fish have also shown to be effective. Herbicides are a common control method including copper, diquat, endothall, and fluridone. These chemicals have been effective and have little to no effect on native aquatic plants.

Sources: 1. Langeland, K. A. 1996. Hydrilla vertcillata (L.F.) Royale (Hydrocharitaceae), "The Perfect Aquatic Weed". Castanea 61:293-304, http://plants.ifas.ufl.edu/hydcirc.html; 2. King County, WA Noxious Weed Control Program, http://dnr.metrokc.gov/wlr/waterres/smlakes/hydrilla.htm)

MECHANICAL CONTROL

Hand Cutters
Hand cutting tools are often used against submerged aquatic weeds, including Hydrilla, to combat the plants in small sections of lake or river systems. Hand cutting tools are very effective at providing short-term local control of Hydrilla. As this weed is aggressive, the operator should expect some regrowth in the treatment area during the course of the growing season and understand that repeat treatments could be necessary. If the operator finds that the area infested with this weed is too large to effectively clear, aquatic plant harvesting should be considered as a mechanical alternative.

Harvesting
Although there are a number of manufacturers of aquatic plant harvesting equipment, the design concepts are similar. Most aquatic plant harvesting systems will cut and remove Hydrilla to a depth of 5 to 7 ft. As this biomass is removed from the lake, the water is ready to use at once and there are no restrictions on the use of the area that might be experienced with an herbicide or some biological control treatments.
Harvesting system performance is in the 1- to 3-acre per day range, depending on the equipment mix and shoreline access available. As such, large areas cannot be cleared rapidly. A manager has to factor the amount of acreage to be cleared and the amount of time available before the area needs to be weed free. In some cases other tools may be appropriate if the water needs to be cleared more rapidly.

Rotovation
Rotovators are basically large underwater rototillers. The knives penetrate the soil, till it, and release the root crowns and other plant tissue. The tiller is mounted on an arm similar to a backhoe and can be lowered into the water and held against the sediments. Rotovators can reach the bottom sediments to depths of 20 ft. The rotovator operator tills the vegetation in target areas using a cross-hatch pattern. A number of lines are tilled in one direction; the equipment repositions itself and cuts a number of tillage lines across the original lines at a 90-deg angle. Intensive tilling results in removal of the root crowns of the plant and/ or the below-sediment structures. It will also cut and disturb any crowns that are not removed. This provides long-term mechanical control of this weed. 
Rotovation provides dramatically longer periods of control of hydrilla than does harvesting. This exotic weed poses a major threat to the infested water body and all surrounding waters. mechanical control methods should be considered when the plant is well-distributed as a maintenance tool or where there are limited options.

Bottom Barrier
Bottom barriers have played a critical role in the management of hydrilla, and they have two basic applications when targeting this plant. Bottom barriers have effectively been used to cover pioneering infestations of this weed and prevent the spread of the plant. They have also been used in a maintenance role, opening water around docks or swimming areas for use. 
Bottom barriers are materials that are laid across sections of lake or river bottoms infested with this noxious weed. These barriers are attached to the bottom by pins or sand bags. Common bottom barrier materials are geotextile ground cover cloth or erosion control materials. 
Bottom barriers provide 100 percent control of this weed in areas where they are installed. They also provide long-term control. An ongoing maintenance operation is required to inspect the bottom barrier and clear the mats of sediment buildup.

Driver Dredge
Diver dredge technology is a mechanical control technology that was pioneered by the British Columbia Ministry of Environment. Diver dredging is especially effective against pioneering infestations of Hydrilla. In diver dredging operations, divers use venturi pump systems to collect plant and root biomass. The pumps are mounted on barges or pontoon boats and, normally, each craft will support two dredge systems. The dredge hoses are from 3 to 5 in in diameter and are handled underwater by one diver. Diver dredging can provide excellent results when targeting Hydrilla. When properly applied, this technology will remove the plant and tubers from the lake system and stem the spread of the weed. Pioneering infestations can be eradicated through careful application of this technology. Larger infestations can also be controlled through repeated treatments.

Other CONTROL

The spread of Hydrilla from one body of water to another can be greatly reduced if boaters remove all aquatic weeds from boats, trailers and fishing gear before leaving lakes, rivers, ponds and streams. Fragments can get caught underneath boats and spread the weed from one location to another. Also, aquarium contents should be dumped on ground, not in a body of water or down a drain. A plant suspected of being Hydrilla should be taken to the local County Agricultural Commisioner.

(Source: 1. The U.S. Army Engineer Waterways Experiment Station (WES),     http://www.wes.army.mil/el/aqua/apis/mechanical/hydrilla.html; 2. California Department of Boating and Waterways, http://www.dbw.ca.gov/Hydrilla.htm).

Nonindigenous Aquatic Species-Hydrilla: U.S. Geographic Survey (USGS) 
    http://nas.er.usgs.gov/plants/docs/hy_verti.html
    This site provides very useful and comprehensive information about hydrilla, including origin, distribution, control methods, maps and references. 

King County, WA Noxious Weed Control Program
    http://dnr.metrokc.gov/wlr/waterres/smlakes/hydrilla.htm
    This page includes brief history, method of spread, control, and identification.

The Perfect Aquatic Weed-Hydrilla verticillata: University of Florida
    http://plants.ifas.ufl.edu/hydcirc.html
    This page includes introduction, detailed identification with pictures, description, biology and physiology, and management.

Boater Alert: Hydrilla: California  Department of Boating and Waterways
    http://www.dbw.ca.gov/Hydrilla.htm
    This site contains description and control methods of hydrilla

Prohibited Aquatic Plants- Florida
    http://aquat1.ifas.ufl.edu/prohib.html 
    This page contains a list of all of the prohibited plants in the state of Florida. It also contains links to other sites with further information on these prohibited plants.

Biocontrol of Hydrilla: University of Florida
    http://aquat1.ifas.ufl.edu/hydrill.html 
    This page contains general background information on hydrilla.  It also contains many images of various types and species of hydrilla.

TEKTRAN Invasive Species Research Agricultural Research Service: Hydrilla Research 
    http://www.nal.usda.gov/ttic/tektran/news/hydrilla.htm 
    This site contains 28 research articles.

The U.S. Army Engineer Waterways Experiment Station (WES)    
    http://www.wes.army.mil/el/aqua/apis/mechanical/hydrilla.html
    This site provides a list of mechanical control methods, such as hand cutters, harvesting, rotovation, bottom barrier, and diver dredge.

Hydrilla: Aquatic Plant Management Society Plant Fact Sheet
    http://www.apms.org/plants/hydrilla.htm
    This fact sheet includes general and brief information about hydrilla.

Lake County California  Water Resources Division Hydriall Monitoring
    http://watershed.lake-coe.k12.ca.us/topics/hydrilla/