Physical Removal

Physical removal is acting directly on plants to remove them without the use of machinery, such as hand pulling, or alteration of the abiotic factors in the environment. Light attenuation is an example of physical control of invasive plants.

Worldwide perhaps the most used form of aquatic plant management is hand pulling. The goal of hand removal is to remove the whole plant including the root; this can sometimes be difficult, though, as many aquatic plants are brittle. Operation costs for hand removal are minimal; that fact coupled with the highly selective nature of hand removal makes it popular.

Hand removal is slow and in deep water may require scuba gear. For these reasons it is best suited for shallow areas and for small operations by shorefront landowners or lake associations.

If hand pulling is supplemented by the use of a suction harvester the efficiency improves greatly. Suction harvesting is the removal of the whole plant and root by hand with the aid of a suction device. A diver pulls the plant and then transports it to the surface via a vacuum hose. The plants then travel down a trough where they are bagged and disposed of. This method of management is, like hand pulling alone, highly selective, but faster. The selectivity of hand pulling and suction harvesting is dependent on the training of whoever is pulling the plants. Suction harvesting removes plant material from the water and therefore does not combat the infestation in a way that would result in oxygen depletion.

A disadvantage of suction harvesting is that it requires scuba gear. And unlike hand pulling you must have at least two people, one to pull the plants underwater and one on the surface, both to monitor safety and to bag the plants. High cost and time commitments are perhaps the largest of disadvantages.

A 2002 drawdown on the Rodman Reservoir in Florida to kill hydrilla

Benthic barriers are also a form of physical control. The barriers are a lining used to cover the plants at the bottom of the lake or river in order to block light and prevent the plants from photosynthesizing. Any number of materials can be used, from geotextile fabrics and lawn tarps, to clay or other sediments. Sediments are however not the preferred material because they have been known to be recolonized, unlike synthetic materials. While black works the best (because it blocks the most light) barriers of all colors have been used with varying degrees of success, including clear. Whatever material is used, it must be permeable to the gasses that escape from the decomposing plant material and respiration of the decomposers. If not the barrier will billow up and both become ineffective and pose an obstacle to recreation. Benthic coverings are very effective on a small scale and are excellent for high traffic areas.

The downside to the use of benthic barriers is the high cost of materials, deployment, maintenance, and recovery, which can total twenty to fifty thousand dollars per acre. Because of this restriction they are mostly only suitable for small-scale deployment. There is no way to selectively target specific plants with a benthic barrier hence any covered native plants will be killed. As mentioned above, though, in most cases there are few native plants to worry about as they have typically been out competed for resources by the invasive by the time that the use of a benthic barrier is a viable option. Benthic barriers kill all plant life under them; because of this there can be oxygen depletion during and after decomposition.

Similar in principal to benthic barriers is the use of on-shore light blocking structures or light attenuating dyes to reduce the rate of photosynthesis by the aquatic plants. Light attenuation is an effective method of pest plant control and is quite inexpensive, but it is nonselective and many find it to be unattractive. Like some other methods of aquatic invasive plant control it may lead to oxygen depletion.

Controlling the abiotic factors of temperature and moisture is achievable through drawdown of the water level. Eighty-five percent of waterways in the United States are controlled. On a controlled waterway it is possible to manage the depth of the water in order to kill invasive species. Like suction harvesting and benthic barriers, this method was also used effectively on the Songo River. With the water level down the plants die in one of two ways either, by desiccation or freezing. It is a cheap method of controlling plants in the littoral zone of a water body.

In the case of drawdown, though it seems that the disadvantages outweigh the advantages. A long-term drop in later level will have negative recreational and ecological impacts. For boaters a change in water depth may expose damaging rocks. Ecologically water level changes in a lake or river environment alters the riparian ecotone, the area where two distinct habitats meet, and may alter food web interactions. Lastly, it can be rather complicated politically depending on the degree of water level change.

Back to previous page