Milfoil Infested Lake Control Strategies

Eradication – Hand Pulling and Bottom Barrier Installation

Hand Pulling

During hand pulling, operators manually remove milfoil plants from the lake bottom, with care taken to remove the entire root crown and not to create fragments. It generally takes divers to reach and remove plants in deep water.

Bottom Barrier Installation

Bottom barriers are semi-permanent materials laid over the top of milfoil beds and are analogous to using landscape fabric to suppress the growth of weeds in yards.

Click here to learn more about bottom barriers and their environmental impacts

Click here to learn more about installing bottom barriers

Waterbodies suitable for hand pulling and installation of bottom barriers

Due to expense and the time intensive nature of manual methods, sites suitable for hand pulling and bottom screening are limited to lakes or ponds only lightly infested with Eurasian watermilfoil. This method is suitable for very early infestations of milfoil and for follow-up removal after herbicide treatments (whole lake or spot treatments) or diver dredging. To be cost-effective, generally the total amount of milfoil in the waterbody should be three-acres or less in area, if all the milfoil plants were grouped together in one location. If the infestation has advanced beyond this point, it is more effective to consider other eradication techniques such as aquatic herbicides. This method may also be applicable in waterbodies where residents can tolerate no herbicide use such as in a lake used as a municipal drinking water supply. Theoretically, people could use these methods in any waterbody to eradicate milfoil; however, the costs for large-scale projects would become astronomical.

Factors that affect the success of hand pulling include water clarity, sediment type, suppression of milfoil fragments, density of native aquatic plants, and effort expended. It is especially important to have good visibility for the divers to locate milfoil plants. Sometimes diving is only effective in the spring or fall, or during periods between algal blooms. If water clarity is very poor, manual eradication methods may not be suitable for the waterbody. Hydraulic Project Approval is required for all hand pulling and bottom barrier projects. This permit is given in a pamphlet called Aquatic Plants and Fish and is available from the Washington Department of Fish and Wildlife.

Description of a milfoil eradication project in Washington using hand pulling and bottom barriers

Lakes where residents use manual methods for milfoil eradication typically have milfoil lightly scattered singly or in small patches within the littoral zone. To determine the extent of the infestation, survey the littoral zone of the lake immediately prior to starting control work, map milfoil locations, and establish Global Positioning System (GPS).

Hand pulling can begin as soon as milfoil can be easily seen and identified - generally in the spring or as soon as it is discovered in the lake. Despite milfoil’s tendency to fragment more readily during the fall, remove milfoil as soon as possible after its discovery, no matter how late in the season.

Survey Techniques

Lake residents should conduct both surface and underwater surveys several times during the growing season. During the surface survey, a surveyor moves slowly through the littoral zone in a boat, looking into the water (often using a viewing tube), and marking the locations of milfoil plants with buoys. Some lake groups have purchased glass-bottom boats just for this purpose. Surveyors advise wearing wide-brimmed hats, polarized sunglasses, and looking straight down into the water. Wind, rain, or surface disturbance, such as boat wakes, interferes with the ability to see. Morning to noon is often the most suitable time for survey work.

Follow the surface survey immediately with an underwater diver survey. Because known milfoil locations have been marked during the surface surveys, the divers can concentrate their efforts at these locations. Since diver time is expensive, it can be cost-effective to conduct surface surveys before underwater surveys. Arline Fullerton, an experienced scientific diver, describes the survey techniques used by Thurston County to locate pioneering colonies of Eurasian watermilfoil in Long Lake.

In Western Washington, the weather is usually so mild throughout the winter months that the hearty milfoil plants don’t completely die back and are still recognizable. For this reason, the diving schedule on Long Lake begins in March, when divers have the opportunity to survey water lily areas for milfoil before spring growth begins.

Using two certified divers each day, the goal is to examine the entire littoral zone of the lake in a systematic manner, a section at a time. The diver’s support system is a specially designed dive barge operated by another certified diver who stays close to the working divers at all times. The two working divers cover the entire shoreline depth in two sections: zero to seven feet deep and six feet to the edge of the plant line which is usually between ten and fourteen feet in Long Lake. Working substantially in tandem, they swim tight transects, continuously looking side to side for plants. They notify the barge operator of any milfoil finds so that the operator can record all milfoil locations. Divers pull the plants and put them into diver “goodie” bags.

In large shallow areas, the barge operator might drop buoys marking appropriate width lanes for each diver to cover so as not to miss any area accidentally. It is advantageous for the divers to use a compass to keep a straight course. Following the silt trail from the previous transect can also guide the diver. The barge operator uses a map of the lake with all the hot spots from the previous year to aid in alerting divers.

Hand Pulling Techniques

During hand pulling, the divers dig around and beneath the plant roots with their hands or with a tool and gently lift the entire plant out of the sediment. The ease of removal is dependent on sediment type. Diver can readily remove milfoil plants from loose or flocculent sediments. In hard sediments or rocky substrate, divers must use hand tools to loosen the root crown before they can dislodge the plant. Sometimes divers leave fine roots behind; these will not regrow, but it is important to remove the root crown (the fleshy, fibrous roots at the base of the stem). Divers place removed plants into bags for transportation to the surface. Sometimes divers may use a suction device to deliver the plant to the surface. Retain the plants in a sieve, and discharge the water back into the lake.

In locations with denser milfoil colonies, divers should make several passes through the area to ensure that all plants have been located and removed. As the divers work, the people in the support boat mark the locations of milfoil plants. An accurate location is important since divers should resurvey area a few weeks later. There have been instances when overlooked small fragments or plants have grown into large plants upon resurvey. Use removed plants for compost rather than discarding as solid waste.

If colonies are too large for efficient hand pulling, or if repeated visits to the same site indicate that divers miss too many fragments or plants, consider installing bottom barriers. Place burlap bottom barrier (or other biodegradable material) over the plants and anchor it to the lake bottom using natural materials such as rocks or sandbags. The material should cover and extend well beyond the growth zone of the plants. Ecology prefers burlap or other natural materials because they will naturally decompose over a two to three year period.

Some lake groups hire contract divers and surveyors to conduct manual plant removal activities. Other lakes have relied on volunteer efforts. It is important to train any volunteers in plant identification and proper removal methods.

General Impacts of hand pulling

Take special care when hand pulling to prevent the release of milfoil fragments. At certain times of the year (generally after flowering), milfoil plants can fracture into hundreds of fragments, each having the potential to form a new plant. To help contain the fragments, cover individual plants with a mesh bag before pulling. The driver of the diver support boat must take care not to create additional fragments by keeping the boat and propeller out of the milfoil plants. People in the support boat should use net skimmers to retrieve any fragments accidentally released by the divers.

Hand pulling may increase turbidity in the area of removal. This can affect the efficacy of removal if the turbidity interferes with the ability of the divers to see the milfoil plants.

In a Diver's Own Words - A Description of Diver Hand Pulling

Arline Fullerton, scientific diver, provides this advice:

When a large plant is located, the diver must approach slowly; taking note of any small fragments that may have rooted nearby. Once the diver enters the area and removes a plant the disturbed silt may make visibility next to impossible. Marking the spot with a buoy or a long stake helps the diver locate the exact spot later when the silt settles. Milfoil fragments can be wind blown into very shallow water and hide behind logs, sticks, rocks or shore grass. In the case of a large pioneer infestation, it is advisable to have someone on the surface in a canoe, kayak or small boat, catching any escaping fragments as the divers dig up plants. Note of the wind direction should be made as wind direction may indicate the next place one will find new plants.

Sometimes plants that have died back may be difficult to identify. They can look like a black stick with roots, but they are not dead. The divers’ motto for milfoil removal projects; is “when in doubt, pull it out.” Over the years, I have become familiar with the look of milfoil roots as opposed to other lake plants and have used the roots as guides.

As a milfoil plant matures, its shorter side stem growth may develop white roots while still attached to the mother plant. A slight current caused by wind, or, even the wake of a passing ducks foot, will dislodge this growth. This rooted fragment is instantly ready for life as a viable new plant. A diver needs to consider this ability of milfoil to fragment when digging up the root system to remove a whole plant.

It is important to discuss the diver’s ability to achieve neutral buoyancy. This is the point at which the diver is neither too heavy nor too light but can maintain position in exactly the right position to work effectively without disturbing the plant. If it is a very large plant, five or six feet high, the ability to hover motionless can be an advantage. In addition, an advantage is the ability to hover in a slight feet up and head down position. Then if the diver needs to change position, the moving fins are less likely to disturb the plant and cause it to scatter. When wearing a dry suit, a diver can accomplish this with some air trapped in his or her boots.

Divers can recognize plants that will fragment easily by the multiple stems and many rooted side branches. Sometimes some of the stems have collapsed and sunk into the surrounding vegetation rendering them practically invisible. If a diver simply pulled up what he or she could see without some investigation, then stems would be broken off and left to root again. Depending on the size of the plant, the diver may elect to pick off some of the rooted fragments until the plant is of a more easily handled size.

Divers developed several different techniques for removing plants. One way is to grab hold of the top of the plant and wind it around the hand as you move down the stem toward the bottom. Then with the other hand dig up the roots and transfer the whole plant to the “goodie” bag. Another method is to carefully locate the bottom of the stem, loosen the roots and then wind the rest of the plant around the hand. Yet another method of capturing a fragmenting plant is to use the “goodie bag” as a butterfly net and cast the open bag carefully over the entire plant, dropping it down to the bottom and then free up the root system. Each method is effective under different conditions and the diver quickly learns when to use each method.

Follow-up is essential to ensure the success of eradication. Even a few milfoil fragments left in the lake can start a new infestation or boaters may reintroduce milfoil into the lake. Diver and surface inspections should continue at least twice a year during the growing season. Survey work should be as frequent as can be afforded since it is easy to overlook small milfoil plants or fragments.

Long-term follow-up is the key

Once milfoil enters a lake, it generally requires continual maintenance to keep it at low levels. Even if you think milfoil was eradicated, boaters may reintroduce it. As long as the lake group continues surveying, they can identify new introductions quickly and target them for removal before milfoil can reestablish in the lake. Although labor intensive, people have used these manual techniques to successfully eradicate milfoil from a drinking water reservoir in Washington.

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