Mechanical Removal

Mechanical removal is the use of machines, in direct contact with the plant, to reduce or remove the population of an invasive plant infestation. There are two main types of plant harvesters; typically harvesters will have a cutting blade that reaches less than two meters below the surface of the water. Alternatively, close-cut mechanical harvesters have been employed. With the cutting blade on the end of an adjustable arm and bottom depth monitored they will cut the plants a foot or less from the lake or river bottom, thus reducing the growth of the plants, which typically regenerate quickly.

Often mechanized harvesters will be equipped with a conveyer to remove the cut plant material from the water body. Doing so reduces the amount of plant material left in the system and therefore decreases plant fragmentation’s impact on the spread of the infestation. Removal of the cut plants immediately opens the water to recreation and improves the aesthetics of the water body. Ecologically the cut and remove practice of mechanical harvesters has its upsides as well. Being that the material is removed and decomposes out of the water rather than in it. Bacteria and invertebrates that would metabolize the dead plant tissue do not deplete the oxygen of the system.

A mechanized harvester is used to clear invasive plants in Wisconsin

Mechanical removal is not without its ecological and economic detractors, however. While most plant fragments are removed some are invariably left in the system. Many invasive aquatic plants are able to spread by fragmentation, notably eurasian milfoil (Myriophyllum spicatum), and variable-leaf milfoil (Myriophyllum heterophyllum). Because of this, some invasive populations benefit on the long term from mechanical harvesting. The fate of the plant material that is removed is also a concern. It has little use as feed animal feed and often it is composted and used as a fertilizer or mulch. Selectivity of harvest is also an issue with machine-assisted removal; native plants growing in and around infestations of exotic plants are equally subject to harvest. Of course, depending on the severity of the infestation, this may not be of concern.

Cost and time are major constraints for this type of removal; a mechanized harvester is usually contracted out for between five hundred and eight hundred dollars a day, and many models cost up to two hundred and fifty thousand dollars to purchase. A harvester is capable of clearing between one and three acres of plants each day, but does not kill the plants. They will grow back resulting in the need for repeat treatments. Multiple treatments tactfully carried out will deplete the plants carbohydrate reserves and eventually may reduce the infesting population.

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