Mechanical Physical Methods

Aquatic plant management: mechanical and physical control methods

Aquatic plants are an important part of a healthy lake. They provide habitat and food sources for fish and wildlife, help stabilize bottom sediments, and help prevent shoreline erosion. Some aquatic plants are considered invasive and may cause problems. At other times, native vegetation may become a nuisance. If a lake management service provider suggests that treatment of aquatic plants is warranted (see the “Management Plans” fact sheet), there are several options for control strategies. These include physical/mechanical strategies, herbicides and biological methods (the latter two have separate fact sheets). 

What are mechanical/physical control methods?

Mechanical and physical control methods are often presented together, though some organizations differentiate between mechanical (using a machine to cut/harvest or dredge/excavate) and physical (for example, placing bottom barriers or using water drawdowns). As a whole, these methods include cutting/harvesting, hand-pulling/diver-assisted suction harvesting, weed rollers, benthic barriers/shading, dredging/excavating and drawdown/flooding. There may be other less-common methods not presented here. Work with your lake management service provider to determine which methods are appropriate for your lake. The state’s natural resource agency is also a good resource for technical advice on the management of nuisance and invasive aquatic plants.  In addition, some mechanical control methods described may require a permit. The benefits and drawbacks of each method are presented within each strategy’s description. A general advantage of these methods is not adding toxic and potentially persistent herbicides to the lake, which is generally better for native plants and wildlife and may fit better with community preferences. 

Cutting/harvesting

Manual and mechanical cutting involve slicing off a portion of the target plants and (if harvesting) removing the cut portion from the water body. In addition to actively removing parts of the target plants, destruction of vegetative material may help prevent further plant growth by decreasing photosynthetic uptake and preventing the formation of rhizomes, tubers and other growth types. These approaches can offer quick results but because the plant is still established and will continue to grow from where it was cut, they only provide short-term relief. Cutting depth may be limited to a maximum water depth of approximately 2m; however, some cutters/harvesters can cut to 5m depth. Cutting generally produces a large amount of plant fragments, which can easily disperse and form new stands of vegetation. This may cause an even larger issue if the invasive plants are currently limited in their distribution within a water body.

Hand-pulling and diver-assisted suction harvesting (DASH)

Hand-pulling and DASH (hand-pulling using SCUBA divers) involve removing rooted plants from the bottom sediment of the water body. The entire plant is removed and disposed of elsewhere. Advantages to hand-pulling include: low technology needs and selective. Disadvantages include: labor intensive and limited to shallow, localized areas. Advantages ofDASH include: suitable for deeper waters and moderately selective. Disadvantages include: more expensive and high technology needs. For small populations, the likelihood of eradication from a site can be limited by the attributes of the plant species (some plants have stronger new growth), and the skill or technique of the SCUBA divers.

Weed roller

For the weed roller method, a roller compresses soil and vegetation, leading to short-term control. Advantages of this method include: low operational effort and the ability to leave equipment in place during the season. Disadvantages include: disruption of other bottom-dwelling plants and fish and potential hazards to people using the area. 

Benthic barriers/shading

Benthic barriers can be used to kill existing plants or prevent their growth by shading. Benthic barriers (also known as bottom screens, bottom covers, pond liners or benthic blankets) sit over the substrate at the bottom of a water body, both compressing aquatic plants and blocking out sunlight. Bottom blankets need to be durable and light blocking. Materials such as burlap, PVC plastics, felt-like polyesters and woven synthetics have been used as bottom barriers. Benthic barriers are usually limited to small-scale management in static and slow flowing waters with limited wind/wave exposure. Benthic barriers may be best used for dense, confined infestations, or along shore, or for providing boat lanes or recreational areas around docks, boat launches and swimming areas. Advantages include immediate relief. Disadvantages include: expensive and logistically difficult to maintain, limited to small patches and disruptive of other bottom-dwelling plants and fish. In addition, sedimentation on the benthic barrier may, over time, provide new habitat that could be subject to new invasive aquatic plant populations. Shading can be done with floating blankets. This method has several disadvantages, however. Shading will alter the physical and chemical environment by reducing dissolved oxygen levels, increasing carbon dioxide levels and reducing pH. Because of the potential for gas buildup under plastic barriers, leading to navigational hazards, those types of barriers are prohibited in some states. 

Burlap has been the most common material for shading in recent efforts as 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. Preliminary results indicate some potential for invasive aquatic plant control using burlap barriers. However, continued monitoring and 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, but opportunities for increased efficacy through adaptive management and improved design are also possible, including: 1) selection of burlap material that optimizes invasive aquatic plant control and native vegetation regrowth for a full range of target plants; 2) development of 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 or approaches (e.g., herbicide treatment or native plant propagation).

Riparian (shoreline) vegetation may also provide some degree of shade and avoids many of the disadvantages of benthic/floating blankets. Water dyes have also been used to restrict light penetration, but with less success. The dye is added in liquid form, where it acts to block light transmission through the water column. The use of dyes to manage invasive aquatic plants has not been sufficiently studied to make recommendations. 

Dredging/excavating

The use of dredgers/excavators for plant control, while not species specific, can be effective for controlling some invasive plants. Excavator machinery does have a limited working reach, confining the utility of this method 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 up rooted floating-leaved, submerged and emerged plants. Due to the expense of this method, invasive aquatic plant control is often an auxiliary result of dredging performed for other purposes. However, this method may result in multi-season reductions in plant biomass and density. 

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, so that the water level can be reduced to such a low level that the infested sites are drained enough to expose the target vegetation to either drying in summer or frost in the winter. The success of this technique is based on the mortality of all plant parts, and the timing and length of water level drawdown are related to the target species (e.g. propensity to desiccate, extent of the weed beds). 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 herbicides to dried areas has become more common because it allows the herbicide to directly target the plant populations and greatly reduces movement of the herbicide into the water even when canals are re-filled. Advantages include: can be very inexpensive and effective; disadvantages 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 and small benthic barriers), most will require hiring a professional due to the equipment, scale and safety issues. Most of these strategies require a permit while some do not; consult your local state water agency to confirm. The cost of these efforts will vary widely based on control strategy, area to be treated and other conditions. See “Management Plans” fact sheet for potential funding assistance. As with any control effort, stakeholders should first consult a lake management service provider.