Description of Method

Rotovators use underwater rototiller-like blades to uproot Eurasian watermilfoil plants. The rotating blades churn seven to nine inches deep into the lake or river bottom to dislodge plant root crowns that are generally buoyant. The plants and roots may then be removed from the water using a weed rake attachment to the rototiller head or by harvester or manual collection. 

In some waterbodies rotovation can be used year-round to control milfoil growth. However, it is most effective in the winter and spring when plants have died back. Summer and fall rotovation usually requires the plants to be cut first since the longer plants wrap around the rototiller head, slowing the rotovation process.

Depending on plant density and sediment type, two to three acres per day can be rotovated. Because of the size of the equipment and high costs, rotovation is most suitable for use in larger lakes or in rivers.

Rotovation is effective for Eurasian watermilfoil removal. Experimental plots have shown that rotovation can produce a high level of milfoil control for up to two seasons. However, milfoil will gradually re-invade the cleared area from adjacent uncleared areas. In milfoil removal test plots in the Pend Oreille River, the growth of native aquatic plants appeared to be stimulated by rotovation. Perhaps removing the milfoil canopy allowed light to penetrate so that native plant propagules could germinate without competition from milfoil. The action of the blades may also stimulate germination. Because of this, rotovation probably would not be a good management method for the control of native aquatic plant species. However, rotovation has also been used successfully in Washington to remove the rhizomes of the fragrant water lily (a non-native, invasive species in Washington).

Because rotovation disrupts the sediment, it can create harmful environmental effects:

• Rotovation churns up the sediment causing water to become temporarily turbid with suspended sediments.
• Plant nutrients in the sediments, such as nitrogen and phosphorus, may be released into the water.
• Long-buried toxic materials in the sediment which may be present from land use activities such as boat building, storm water drainage, or combined sewage outfalls may be released into the water.
• Rotovation may interfere with fish spawning or migration.
• Where salmon runs are present, there is only a limited time window where rotovation is allowed to take place in Washington.

For these reasons, the Washington Department of Ecology, the Washington Department of Fish and Wildlife, and other agencies require permits for rotovation. Although rotovation is used in British Columbia, Canada and on the Pend Oreille River in Washington, rotovation has not become a popular method of plant control in other areas.

Advantages

• Rotovation potentially removes the entire plant rather than just "mowing" off its top like harvesting and cutting.
• Plant density is generally decreased by successive treatments.
• Control generally lasts two growing seasons.
• In some waterbodies, rotovation can be used year-round to control aquatic plants, depending on permit requirements.
• Rotovation may stimulate growth of some desirable native aquatic plants.

Disadvantages

• Rotovation is expensive and large machinery requires regular maintenance.
• Rotovation disturbs bottom dwelling animals.
• Some rotovators are difficult to maneuver around docks and in shallow water.
• Rotovation causes fragmentation which may increase the spread of invasive weeds like milfoil. For that reason, rotovation should only be used in systems where milfoil is already widespread.
• Rotovation is labor intensive. It may require cutting the plants and removing bottom obstacles like logs and rocks (check first with Fish and Wildlife before removing anything from a waterbody).
• Sunken logs can impede rotovation; however some logs may be required to be left for fish and wildlife habitat.
• Underwater utilities, such as gas, water, sewer, telephone or water intake pipes, need to be located before rotovation begins.

Permits

• Rotovation requires hydraulic approval from the Washington Department of Fish and Wildlife.
• A shoreline permit from the appropriate local jurisdiction (city or county) may also be needed and may take up to six months to obtain.
• A Section 404 permit obtained from the Army Corps of Engineers may be required.

Costs

Costs for a private contractor to harvest plants, remove obstacles, rototill, and collect and dispose of plants range from $1,500 to $2,000 per acre. As plant density decreases and obstacles are removed, costs and time needed to rotovate each acre will decrease.

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Questions about this page?
Contact Kathy Hamel by e-mail at kham461@ecy.wa.gov