Mechanical Controls of Aquatic Plants

MECHANICAL CONTROL

CERTAIN MACHINES HAVE BEEN USED TO HELP COMBAT INVASIVE AQUATIC PLANTS in Florida and throughout the U.S. for more than 100 years. Since the beginning of the 20th century, plant managers have developed a variety of machines that are specially designed to shear, shred, crush, press, pull, convey, lacerate, and remove aquatic weeds from waterbodies.

THEN and NOW

"Destruction boat" Okeechobee, 1930s Bulldozer and chains Early harvester Early conveyor

Conveyor # 15, 1939 St. Johns destroyer Okeechobee, 1937 West Palm canal Dredging machine

Largest harvester 1970s hydrilla rake Testing an
iIsland shredder Bridge protection Harvesting system (2000)

Demonstrating floating
island shredder Removing vegetation
with harvester Weed barge Weed barge

A Bit of History

The use of mechanical controls for invasive aquatic plants began in response to the growing threat of water hyacinth throughout the southeast, especially in Florida and Louisiana. In 1899, the United States Rivers and Harbors Act authorized the construction and operation of "crusher boats" to remove water hyacinth from navigable waterways. Only three years later, in 1901, the Rivers and Habors Act Amendment allowed for the extermination of water hyacinth by mechanical, chemical, or any other means. However, more recent laws passed by the U.S. Congress, and in the 1930s by the Florida legislature prevented the use of chemicals that were harmful to humans, livestock, and wildlife. These laws are still in effect.

In the first half of the 20th century, plant managers were exploring the possibility of using chemical controls, but very few chemical compounds yielded promising results. Most of the chemicals were rejected because of their ineffectiveness or obvious toxicity to non-target species.

"HYACINTH ELEVATOR BOAT" and "HYACINTH DESTRUCTION BOAT"
Original drawings provided by the U.S. Army Corps of Engineers, Jacksonville District


"elevator boat", 1914	"elevator boat", 1914	"destruction boat", 1939	"destruction boat", 1945

Chemical use restrictions imposed by federal and state laws resulted in the almost exclusive use of mechanical means for invasive aquatic plant management, including the use of crusher boats, saw-boats, harvesters, and "elevators". Although by the 1950s researchers had successfully developed an array of herbicidal and physical (non-mechanical) means for controlling invasive plants, eco-managers in Florida and throughout the U.S. continued to employ machines.

ADVANTAGES of MECHANICAL CONTROLS

Under certain circumstances, mechanical controls can be more beneficial than other methods for managing invasive aquatic plants. Advantages include:

Immediate control in small areas by clearing away vegetation.
Oxygen remains in water when decomposing plant material is removed from the waterbody.
Water is immediately available, unlike water-use restrictions associated with some herbicidal controls.

TYPES of MECHANICAL CONTROLS (MACHINES)
USED FOR THE REMOVAL of INVASIVE AQUATIC PLANTS

Machines used in aquatic plant management come in all sizes and shapes and include models designed for specific plant types (floating, submersed, emersed), and specific plant habitats (open water, canals, shorelines, wetlands).

Crusher boats, or Kennys, were used in Florida to control water hyacinth until the late 1940s. The machines hauled plants aboard, crushed them with 40,000 pounds of pressure per square inch, and discharged them into the water or onto the shore. Disposal in the water was the most likely method since shore disposal was inhibited by shallow water, dense tree growth, or the protests of waterfront land-owners.

Saw boats were used in the 1940s to control water hyacinth. They were able to operate in shallow water and closer to shore than were crusher boats. The saw blades cut close to the surface and were able to shred water hyacinth without lifting them out of the water. Still, the inability to access all the plants along the shore-line allowed continued regrowth of water hyacinth.

Hi-ballers were floating machines that collected plants, ground them up, and used a water canon to fire the plant slurry across the water to the shore. Hi-ballers were used mainly for canal maintainence, such as in remote areas of the Everglades, but also have been used to create boat trails along shorelines. Obviously, the use of the hi-baller is possible only where shoreowners permit the disposal of ground up plants and wildlife on their property.

Harvesters remove plants from the water, which alleviates water quality problems associated with the decay of cut plants left in place. These machines cut vegetation, remove it from the water with a conveyor system and dispose of the plant matter onshore. Harvesters operate in depths of about 5-6 feet, and some are large enough to haul up to 13 tons of vegetation. (Densely packed water hyacinth can weigh 200 tons per acre.) Sometimes, separate shuttle barges transport the plant matter to shore while the harvester continues working. Unfortunately, plant harvesters also remove fish, amphibians, reptiles, birds, and other wildlife by including them in the harvested plant material.

Mechanical weed cutters cut aquatic plants several feet below the surface of the water and then permit the cut plants to flow downstream. Unlike harvesting, the cut vegetation is not collected by the weed cutter. Cutting generally takes place during the summer when plants are closest to the surface and more likely to interfere with water users. An assortment of cutting machines is commercially available, ranging from portable boat-mounted devices to large barge-like units. Specialized cutters can work in shallow areas not accessible to harvesters, and can cut a seven-foot swath in weeds in water as shallow as 10 inches or as deep as five feet. These larger machines can cut up to one acre of plants in an hour. Importantly, fish habitats can be retained if vegetation is not cut too short.

Rotovators use rototiller-like blades to churn seven to nine inches deep into the bottom, extracting the entire plant. They are primarily used for plants that have buoyant root crowns such as Eurasian water milfoil (Myriophyllum spicatum). The loose floating plants and roots are then collected and removed by an attachment to the machine, by harvester, or by hand. Rotovators can clear two to three acres per day and are most often used in winter or spring when plants have died back. Control generally lasts for two growing seasons. Rotovation is expensive and labor intensive. The machines are difficult to maneuver, and bottom obstacles are problematic. Because rotovation severely disrupts the sediments, it can produce negative environmental impacts such as increased water turbidity, release of plant nutrients from the sediments, release of toxic residues bound in the sediments, and disruption of bottom dwelling animals, fish spawning and migration. Rotovators are not used in Florida.

Dredges and pumps act like underwater vacuum cleaners to remove aquatic plants, organic detritus, plant roots, and associated sediments from the hydrosoil. It is an expensive process, especially if a nearby disposal site is not available. Dredging results in suspended sediments, has serious habitat impacts, and short-lived effects if not conducted below the photic, or light penetrating, zone. Hand-held, diver-assisted dredges can be effective for clearing underwater (submersed) plants from some areas. Divers pull the plants up by the roots and "feed" them into the suction dredge where they are collected in baskets on shore or on an attending barge. Dredging is generally restricted to use in extreme cases.

Dragline and trackhoes are large shovel machines. Using a long cable, a dragline casts a shovel which is dragged back in, collecting plants along the way. The shovel full of plants is emptied into a truck or piled along the shoreline. Trackhoes have claw shovels which reach out to approximately 25 feet, dig down, and pull plants back to shore. Trackhoes are primarily used for ditch maintenance and near-shore plant clearance.

Weed rollers are a relatively new kind of machine for controlling aquatic plant populations. The heavy roller can be up to 30 feet long and rolls forward and reverse in a 270 degree arc. It compresses the plants and soils in the area, usually around a boat dock. Rollers require minimal supervision and can be left in place. Frequent use impedes weed growth. Rollers also disturb bottom dwelling species and inhibit fish spawning.

A shredder of note still in use is called "Cookie Cutter." Two counter rotating blades shred plants and floating islands (tussocks) of plants, peat and muck. (Floating islands occur when muck and plants rise to the surface during reflooding after prolonged periods of drought; they can be quite large and sturdy, supporting even medium size trees.) Cookie Cutters generally are used to dismantle very dense floating islands, including trees up to four inches in diameter.

Here are links to manufacturers of mechanical controls for aquatic weeds.

DISADVANTAGES of MECHANICAL CONTROLS

While machines may be preferred in certain situations, a variety of disadvantages limits their more general use for controlling invasive plants in Florida's water bodies. Today, plant managers generally restrict the use of mechanical controls to small areas such as bridges and flood control structures, where immediate control is needed, or in marinas, swimming areas, fast flowing water, and boating trails, or where chemical, biological, and physical (non-mechanical) means of control are not practical.

High cost: The machines generally are expensive, costing tens to hundreds of thousands of dollars, and requiring frequent maintenance and repairs.
Labor intensive: Personnel must be trained to be qualified to operate and maintain the machinery and supervise its use.
Slow and inefficient: Machines generally clear only a couple of acres per day. The largest and most efficient machine can clear 8-10 acres per day.
Rapid regrowth: Vegetation can regrow quickly, necessitating frequent repeated treatments throughout a single growing season.
Native plants removed by machines are disadvantaged as invasive plants regrow faster than native species and are able to recolonize an area more quickly.
Floating plant fragments: the spread of some invasive species can be promoted when plant parts drift away to grow and take root elsewhere.
Disruption of soils and native vegetation: Machines are not selective. Native species may be removed or disturbed by the churned bottom.
Destroys wildlife: Wildlife that live and nest in aquatic vegetation can be entrapped and killed as the machine gathers, grinds up or removes the invasive plants.
Not all water bodies are suitable: The use of machinery can be restricted due to a waterbody's size, depth, and current, or the number of obstructions such as trees, docks or bridges in the waterway.
Increases water turbity: Sediments on the bottom are churned and disrupted, muddying the waters.
Disposal of removed vegetation: To remove vegetation from the waterway means plant mangers must locate suitable areas in which to dispose of large amounts of plant material.

Despite certain disadvantages, mechanical control of invasive aquatic plants remains suitable for many of Florida's waterways. Plant managers carefully select the most appropriate mechanical control by evaluating factors such as the plant species in question, the disposal of the targeted plants, management objectives and uses of the water body, funding, and the physical characteristics of the targeted water body. No single type of machine is universally effective.

MECHANICAL REMOVAL of CERTAIN INVASIVE PLANTS

Water hyacinth, water lettuce and other floating plants:
Initially, passive devices such as log booms and barriers were used to prevent the spread of water hyacinth in Florida's waterways. The plant's rapid growth prompted the Rivers and Harbors Act of 1899, a stimulus for the development of mechanical controls for invasive plants. The first machines used to control water hyacinth, such as conveyor belts, grapples, and derricks, were largely ineffective because they could not operate in shallow water along shorelines, the mass and volume of water hyacinth was an impediment (one acre of water hyacinth can weigh nearly 200 tons), and the machines could not keep up with the plant's growth rate. Managers were more successful after the development of crusher-boats and saw-boats.

Hydrilla, Eurasian water milfoil, and other submersed plants:
Mechanical control such as cutting and harvesting should be used only when the submersed plant population has completely infested the water body. If mechanical control is used while hydrilla is spreading, the fragmented parts will root, thereby speeding the invasion. Specially designed aquatic plant harvesters are used to cut and collect submersed plants from waterways to clear boat lanes for navigation.

PLANT DISPOSAL

Wet water hyacinth plants can weigh in excess of 200 tons per acre, each acre filling up to 100 truckloads of uncompressed plants that must be disposed of. The sheer weight and volume of the harvested aquatic plants presents a significant disposal problem. Invasive aquatic plant managers are confronted with the task of finding suitable land-locations on which to pile plants for longterm decay, obtaining permissions and permits from landowners and governing agencies, and transporting the plant mass to the disposal sites.


Piles dry slowly	Birds eat harvested fish, small animals	Harvested fish	Millions of tons of plants to harvest	One truck holds (3) tons

In efforts to help solve harvested plant disposal problems and to offset managment costs, for many years researchers and others have sought economically viable options for the commercial use of harvested aquatic plants. For information about how plants have been used to produce biogas, animal feed, soil additive, fiber board, furniture and other products, go to the Utilization of harvested plants page of this web site.

PREVENTION

Preventing the introduction and spread of non-native plants in Florida's waterways is the most effective and least expensive means of restoring Florida's natural freshwater habitats.

Like all plant management techniques, mechanical controls are costly tools to use in the fight to combat the non-native aquatic plant populations that are infesting Florida's lakes, rivers and wetlands.

Time and money spent on managing invasive species can be saved in the first place by preventing the introduction and spread of invasive species in the state's waterways. Public cooperation is an essential part of restoring Florida's natural habitats.

Please read here about steps you can take to help prevent invasive aquatic plant problems.

For more information about specific mechanical controls, visit:

Mechanical Controls web site,
U.S. Army Corps of Engineers

The Creators

This page was authored by Sarah Cervone, with research assistance by Rebecca Hassell.
Data is from the APIRS Database.
This page was designed by Sara Reinhart.
Photography and graphics are by Ann Murray, Sara Reinhart and Vic Ramey.
Vic Ramey is the editor.

DEP review by Jeff Schardt and Judy Ludlow.

Main Index

This project is a collaboration of
the Center for Aquatic and Invasive Plants, University of Florida,
and the Bureau of Invasive Plant Management, Florida Department of Environmental Protection


"Destruction boat"	Okeechobee, 1930s	Bulldozer and chains	Early harvester	Early conveyor


Conveyor # 15, 1939	St. Johns destroyer	Okeechobee, 1937	West Palm canal	Dredging machine


Largest harvester	1970s hydrilla rake	Testing an iIsland shredder	Bridge protection	Harvesting system (2000)


Demonstrating floating island shredder	Removing vegetation with harvester	Weed barge	Weed barge