LEA began working to remove milfoil from the Songo River in 2003. In 2007, we launched a diver assisted suction harvester, the S.S. Libra, modeled after Little Sebago Lake Associations DASH. In the years since, we have continued to refine our techniques and expand our management goals through the use of the following methods.
Benthic barriers are weighted tarps that cover milfoil on the bottom of a river or lake to crush invasive plants and starve them of sunlight. The barriers are used in areas that are heavily infested and have few or no native plants. After several months under the barrier, the plants die and decompose and the benthic barrier can be removed and redeployed. Left in its place is a vegetation free area ready to be re-colonized by native plants.
LEA’s current design of benthic barriers is to use a 20’ x 30’ piece of shrink-wrap that is weighed down by coated rebar woven through slits in the material. Holes are punched in the plastic to allow CO2 from the decomposing milfoil to escape. The barriers are built on land, rolled up, and cached near our work sites on shore. After a final survey, the team decides where to place the barrier. Divers walk into the river, set the barrier down and unroll it over the milfoil. During deployment, a swimmer or kayaker collect any fragments that may be created.
When our program began LEA used 60’ x 40’ barriers (as pictured) made of lawn tarp and galvanized chain with sandbags as weight. While the larger tarps were effective in killing milfoil patches, removal proved to be a difficult task and we scaled down the barrier size.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 of invasive plants is the most intuitive of our control methods. It is labor intensive and involves removing and bagging the entire plant by hand. Once the plants have been removed, they are placed into a mesh dive bag and taken off site to be composted. The greatest challenges during hand removal is to remove as much root as possible and to keep the plant from breaking apart, or fragmenting. The LEA crew and volunteers work very deliberately to avoid making fragment.
We use hand removal in places that would not be practical for our divers to work with the suction harvester such as waist deep water and in and around fallen trees.If hand pulling is supplemented by the use of a suction harvester the efficiency improves greatly.
Diver Assisted Suction Harvesting (DASH)
Suction harvesting is the removal of the whole plant and root by hand with the aid of a specially outfitted boat to increase effieciency. A pump on the boat creates suction in a hose carried by two divers. While carefully avoiding native plants, the divers pull the entire milfoil plant by the root and send it through the hose.
On the boat, the hose discharges plants into a long sluiceway that allows water and milfoil to drops through one of four chutes into mesh produce bags. When a bag is full of milfoil, a crew member closes the chute door and replaces the bag with an empty one. The full bags are trucked away to a safe composting site. While in practice the DASHboat uses the same techniques as hand removal the true difference is volume. It is much faster for a diver to send the milfoil up the hose than to bag it underwater.
Our surveys are carried out by snorkel and our team carries mesh bags to hand pull any clusters of milfoil they may find. If the surveyors find large populations of plants, the site is noted and the crew will return with the suction harvester. Less formal but more frequent surveying is done from our boat as we travel from our boat storage site to our work sites.