Mechanical Physical Methods

Aquatic plant management: mechanical and physical control methods

Aquatic plants are an important part of a healthy lake. They provide a home and food for fish and wildlife, help stabilize soil, and help prevent shoreline erosion. Some aquatic plants are considered invasive and may cause problems. At other times, native aquatic vegetation may become a nuisance. If a lake management service provider says treatment of aquatic plants is needed (see “Management plans” page), lake stakeholders have many options for lake weed control. These include physical/mechanical, aquatic herbicides, and biological methods. The latter two have separate fact sheets.

What are mechanical/physical control methods?

Mechanical and physical aquatic weed control methods are similar, though some groups differentiate between them. Mechanical examples are using a machine to cut/harvest or dredge/excavate. Physical examples are placing bottom barriers or using water drawdowns. As a whole, these methods include harvesting, hand-pulling/diver-assisted suction harvesting, weed rollers, benthic barriers/shading, dredging/excavating, drawdown/flooding and even using a lake weed rake. There may be other less-common methods not presented here. Work with your lake management service provider to decide what is best for your lake. The state’s natural resource agency is also a good resource for management advice of nuisance and invasive aquatic plants. Also, mechanical control may require a permit to be done. The pros and cons of each method are within each strategy’s description. An advantage of these methods is not adding toxic or persistent chemicals to the lake. This may be better for native plants and wildlife and for stakeholders’ risk preferences.

Cutting/harvesting

Manual and mechanical cutting involve cutting a portion of the target plants and (if harvesting) removing it from the waterbody. In addition to removing parts of the target plants, destruction of vegetative material may help prevent further plant growth by decreasing its energy. This could prevent the formation of rhizomes, tubers, and other growth types. These short term approaches can be quick to allow recreational use of a waterbody. Although where the plant is still established it will continue to grow from where it was cut. Cutting depth may be limited to a water depth of only 2m, but some lake weed cutter kinds can cut to 5m depth. Cutting produces a large amount of plant fragments. These can disperse and form new stands of vegetation. This may cause a larger issue if the invasive plants are currently limited in the water body area.

Hand-Pulling and Diver-Assisted Suction Harvesting (DASH)

Hand-pulling and DASH (hand-pulling using SCUBA divers) remove rooted plants from the soil of the water body. The entire plant is removed and disposed of elsewhere. Benefits to hand-pulling include: low-technology and selective. Drawbacks include: labor intensive and limited to shallow, localized areas. Benefits to DASH include: can reach deeper waters, moderately selective. Drawbacks include: more expensive and high-technology needs. For small populations, the chance of eradication from a site is limited to the plant species (e.g. shoot strength for marginal aquatic plants), and the skill of the SCUBA divers for submerged aquatic plants.

Weed roller

A weed roller compresses soil and vegetation for short-term control. Benefits include low effort and the ability to leave equipment in place during the season. Drawbacks include: effect on other plants and fish and it presents a hazard to people using the area.

Benthic barriers/shading

Benthic barriers can be used to kill existing plants or prevent their growth. Benthic barriers are also known as bottom screens, bottom covers, pond liners, and benthic blankets. They sit on the bottom of a water body, compressing aquatic plants and blocking out sunlight. Bottom blankets need to be durable and light blocking. Materials like burlap, PVC plastics, felt-like polyesters and woven synthetics have been used as bottom barriers. Benthic barriers are usually limited to small scale management. They work best in still or slow waters with limited wind and waves. Benthic barriers may be best used for dense infestations or along shore. They also work to provide boat lanes or recreational areas around docks, boat launches, and swimming areas. Benefits include: provide immediate relief and can be relatively low-cost. Drawbacks include: difficult to maintain, limited to small patches, and disrupts other plants and fish. In addition, new soil on the benthic barrier may, over time, make habitat that is vulnerable to new invasive aquatic plant populations. Shading can also be done with floating blankets. This method has several disadvantages, however. Shading will alter the physical and chemical environment by reducing O2, increasing CO2, and reducing pH. Because of the potential for gas build up under plastic barriers, leading to navigational hazards, those types of barriers are prohibited in some states.

Burlap has been the most common material in recent efforts. It is biodegradable (does not pollute the aquatic environment) and the mesh size can be selected to allow native plant growth while preventing nuisance plants. Initial results indicate potential for invasive aquatic plant control using burlap barriers. However, continued maintenance is required to confirm long term control and degradation of the barriers and to document native plant (and invertebrate) recolonization in treated areas. Limitations and challenges for the use of burlap barriers exist. However, increased efficacy through adaptive management and improved design are also possible. This includes: 1) selection of burlap material that optimizes invasive aquatic plant control and native vegetation regrowth for a full range of target plants; 2) developing additional strategies to promote re-vegetation by native vegetation – including seeding burlap mats; and 3) integrated pest management that combines benthic barriers with other control tools (e.g. herbicide treatment, or native plant propagation).

Riparian (shoreline) vegetation may also provide some degree of shade and avoids many of the drawbacks of benthic or floating blankets. Pond dye like aquashade also restricts light penetration, but with less success. The pond dye is added in liquid form, blocking light transmission through the water column. The use of pond dyes to manage invasive aquatic plants has not been studied enough to make recommendations.

Dredging/excavating

The use of dredgers/excavators for plant control, while not species specific, can help control some invasive plants. Excavator machinery has a limited working reach to narrow waters, such as ditches, channels, drainage or irrigation systems, ponds or small rivers. This method also leads to a large amount of suspended sediment. Excavators can be used either for harvesting floating plants, or by digging rooted floating-leaved, emerged and submerged aquatic plants. Due to the expense of this method, invasive aquatic weed control is often an auxiliary result of dredging performed for other purposes. However, this method may lower plant biomass and density for longer.

Drawdown/flooding

Water level drawdown is limited to waters with a controlled outflow like ponds or reservoirs and gated irrigation canals and flood control channels. In these, the water level can be reduced to a level that the infested sites are drained to expose the target aquatic vegetation. They then either dry out in the summer or frost in the winter. The success of this technique is based on the mortality of all plant parts. Therefore, the timing and length of water level drawdown is related to the target species (e.g. propensity to desiccate, extent of the weed beds). Consequently, aquatic weed eradication can only be achieved if the water level drawdown is applied for at least several months so that either freezing or dryness affects the sediment to the depth where plant fragments are present. The application of soil-active, systemic aquatic herbicides to dried areas has become more common because it allows the aquatic herbicide to directly target the water plant populations and greatly reduces movement of the herbicide into the water even when canals are re-filled. Benefits include: can be very inexpensive and effective. Drawbacks include: can have severe environmental and recreational impacts, and requires a water control structure.

How to undertake mechanical/physical control strategies

While some mechanical/physical control strategies may be performed by the individual (e.g., hand-pulling, small benthic barriers, pond rake), most lake weed removal will require hiring a professional due to the equipment, scale, and safety issues. Most require a permit to remove lake weeds. Consult your local state water agency to confirm. The cost will vary based on control strategy, area to be treated, and other conditions. See “Management Plans fact sheet” for potential funding assistance. With any control effort, stakeholders should first consult a lake management service provider.