Manual Removal or Cutting

Hand pulling requires laborers to dig out the entire plant and its roots. Plant material is then deposited away from the shoreline. Pulling is a practical control method if small amounts of aquatic plants are easily accessible and in water less than a few feet deep. Otherwise the process may require the assistance of trained scuba divers.

The effectiveness of pulling depends on sediment type, water visibility and thoroughness of removal. A high degree of control lasting more than one season is possible depending upon the species of plant and if complete removal can be achieved.

A variation of hand-pulling is "diver-assisted dredging." Divers working in deep water hand-pull plants and feed them into a 4" flexible hose. The hose is connected to a vacuum pump, situated in more shallow water, which draws up both water and plant material. Diver-assisted dredging has been used in Florida only in the fast moving waters of the Wakulla River to dredge hydrilla from small, high use areas. In such areas machine harvesting would not be possible and herbicide applications would not be effective.

Advantages to pulling are: control can be selective; control is immediate, and control can be accomplished in sensitive and in hard-to-reach areas. Hand-pulling is usually employed where other methods have not been successful, such as in fast-flowing water, shallow waters.

However, pulling is labor intensive, must be conducted on a routine basis, is limited to low density and small areas. Hand-pulling may also result in a temporary increase in water turbidity. Special consideration must be taken with plants such hydrilla and other plants that reproduce by fragments in order to avoid loose fragments that could colonize other areas within an infested water body, or other water bodies.

Hand cutting requires the use of tools such as rakes, chains, logs, railroad ties, or even old bedsprings, which are pulled through the weed beds in order to sever the plant from the sediment.

Hand-cutting is less labor-intensive than hand-pulling because roots are not removed. Advantages to cutting are immediate removal; minimal cost in small areas; site specificity; and species specificity. Hand-cutting minimizes environmental disruption if done with care. Still, the results are usually short term. Rapid regrowth is possible and the effects of cutting generally last less than one growing season. Cutting is time consuming and labor intensive. It also increases water turbidity. Fragments are numerous and messy; hand-cutting is not practical for large areas.

Water Level Manipulation and Drawdowns

Aquatic vegetation may be controlled artificially by manipulating water levels. Raising water levels can either drown emersed plants or strand floating plants in upland areas. Alternatively lowering water levels, a technique known "drawdown," can be used to expose emersed, submersed, and floating plants to freezing and drying. Water level manipulation techniques are limited to closed water bodies such as lakes, ponds, canals, and reservoirs.

Drawdowns, or dewatering, have been used for many centuries as a means to oxidize and consolidate sediments, alter fish populations, and to control aquatic weeds. The technique requires a dam or other mechanism to lower water levels. The process may be restricted by water use patterns, water rights, or the lack of a predictable source of water for refilling. Drawdowns generally take place in winter to take advantage of drier weather, freezes, and prescribed fire to further stress target plants. Consecutive drawdowns are more successful than a single drawdown.

Light barriers

Blue dye reduces light reaching plants

Light reduction, or attenuation, limits plant growth by reducing light penetration into the water and inhibiting photosynthesis. Sunlight is obstructed by special non-toxic dyes or shade-making structures that are placed along the shoreline. Light barriers are most useful in small ponds that are greater than 3 feet deep.

Bottom Barriers

Bottom covers, or "benthic barriers", are made of growth inhibiting substances such as sand-gravel, burlap, plastic, rubber, fiberglass screens, nylon, and other materials that prevent rooted aquatic plants from growing. These benthic barriers typically kill plants in one to two months and prevent new rooting colonies from becoming established. The duration of control depends on the type of material, application technique, and sediment composition.

Advantages to benthic barriers include the ability to use the barrier at any depth (greater than 3 feet requires the assistance of trained scuba divers), provides immediate control, high level of control, can be installed by homeowners, are hidden from view and do not interfere with shoreline use, and do not create plant fragments.

Nutrient Alteration

Plant growth can be affected if at least one nutrient which is required for growing is in limited supply. Nitrogen, phosphorus, and carbon are the most common nutrients that limit plant growth in lakes. However, unless a lake is oligotrophic, there are usually enough nutrients in the sediment to sustain rooted plant growth. Vascular plants are limited by nitrogen and derive most of their nutrients from the sediments rather than from the water column.

Results from nutrient alteration are very unpredictable and may actually aggravate a plant population. For example, nutrient limitation may control planktonic algae populations and therefore increase light penetration that will promote nuisance plant growth and expand the population. Conversely, adding nutrients through fertilization will create an algae bloom which will limit light penetration and will control rooted sumbersed vegetation.

Fire

The suppression of natural fires has altered historical plant communities and aided fire-intolerant invasive species such as torpedograss, Panicum repens. Plant managers use prescribed fires to suppress plant growth and kill plants that are not fire tolerant. The effect of a single burn is unpredictable, but the results may last for several years. The success of fire is dependent on the intensity of fire, the time of year, and the targeted plant species. Plant managers must consider the effects on soil loss and water quality and impacts on humans and wildlife.

Aeration

Aeration increases the dissolved oxygen in the water and can reduce plant growth, usually causing blue-green algal populations to be replaced by green algal populations. Some researchers have suggested that iron oxide compounds in aerated waters interfere with the photosynthesis of submersed plants as ochre deposits on plant parts. Others say that aeration promotes the growth of filamentous algae that interferes with photosynthesis in submersed plants. Whatever the proximate cause, aeration can be used for some degree of aquatic plant control.

A SURPRISING VARIETY OF PHYSICAL CONTROLS are employed by aquatic plant managers throughout Florida. Although large scale physical control techniques are generally labor intensive, costly, and can produce unexpected results, simple methods such as hand-pulling and benthic barriers allow Florida homeowners to take a proactive role in invasive plant management within their own backyards.

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, physical controls can be costly and energy-intensive 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.