Rotovation - Underwater Rototilling - An Aquatic Plant Management Method

Rotovators

The Western Aquatic Plant Management Society

Description of Method

Rotovators use underwater rototiller-like blades to uproot aquatic plants (mostly used for Eurasian watermilfoil removal). The rotating blades till seven to twelve inches deep into the lake or river bottom to dislodge plant roots. The plant fragments and root crowns float to the water's surface. Plants and roots may then be removed from the water using a weed rake attachment to the rototiller head, by a harvester, or by manual collection. Rotovation was pioneered in British Columbia by milfoil managers looking for a non-chemical management technique that provided longer term control than harvesting.

Rotovator working In some waterbodies rotovation can be used year-round to control aquatic plant 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 very effective for Eurasian watermilfoil control. Tests 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. Rotovation has also been used successfully to remove the rhizomes of the fragrant water lily. Rotovation appears to stimulate the growth of some native plants, therefore it would not be a suitable control method for these aquatic plants.

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

Rotovation churns up the lake bottom 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 lake bottom 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. However, in British Columbia, rotovation has been used to remove milfoil from spawning beds restoring them to their function.

Although rotovation is used in British Columbia 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 typically lasts two growing seasons.
Rotovation can be used year-round to control aquatic plants, depending on permit requirements.
Rotovators can remove plants from a greater water depth than can harvesters.
Rotovation may stimulate growth of some desirable native aquatic plants.

Disadvantages

Rotovation is expensive.
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.
Rotovation is labor intensive. It may require cutting the plants and removing bottom obstacles like logs and rocks.
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.
Rotovators can leak fuel and hydraulic fluid into the water.

Permits

Permits are required for many types of projects in lakes and streams. Check with your state and local jurisdictions before proceeding with any rotovation project.
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.