Cutting: String Trimmers / Brush Cutters

Non-Chemical Weed Control: String Trimmers and Brush Cutters, Collin Rath from Cal-IPC on Vimeo.

Special Tips

One way to improve the effectiveness of string trimmers or brush cutters for weed suppression is to roughly mulch weeds during the cutting process. This can be achieved by bringing the cutting head down sideways over the cut stems multiple times as they lay on the ground after initial cutting. Alternatively, mulch cutting can be achieved by making multiple cuts, starting at the top and trimming small portions of the plant as you move down the stalk. Mulching material during treatment may take more time, but in certain instances may be beneficial. Mulched material covers bare soil and helps vegetative material decompose quicker.

Invasive perennial bunch grasses may be effectively suppressed by turning the cutter at an angle and trimming deep into the crown. Additional follow-up will be needed to control secondary growth from the sides.

String trimmers may also be used to “scalp” the soil surface when controlling annual weed species. As the string is flexible, the cutter can be used at an angle bringing the string in contact with the soil surface (not in rocky areas). Seedlings, or established low-growing vegetation, can be brought down to the soil surface creating bare ground with the scalping method. Scalping the soil surface can be a good site preparation before restoration activities.

Circular blades can be used on brush cutters to clear small diameter woody or very fibrous vegetation. Typical blades will allow 2 inches of cut, for a total effective use on vegetation with stems less than 4inches in diameter. When targeting woody vegetation, a slow directed use of the blade is needed, compared to the swinging motion used when targeting other vegetation. Either a string trimmer or brush cutter can be used to cut suckers or sprouts from larger woody vegetation which was felled with a chainsaw. Cutting these sprouts may need to be done multiple times over a few seasons for suppression of the root system. If proper precautions are taken in moister environments where fire danger is low, a metal blade can be used to cut and mulch dry materials for fire clearance in areas where weeds have already dried. Heavy string can sometimes be substituted in place of a metal blade to reduce the risk of fire.

When using solid blades, make sure to ease into cutting thicker vegetation. If the cut is made too quickly, there is possibility for the blade to bounce off target vegetation. Only use blades designed for brush cutting. Do not use circular saw blades, as they are not designed to handle the impacts that occur when brush cutting.

Optimal Conditions for Use

Flat dry sites are ideal conditions for the use of string trimmers or brush cutters. However, they can be effectively on a variety of vegetation in rougher terrain and may be the best option available for vegetation suppression on steeper slopes. Gravel, rocks and cobbles, however, can be problematic, inhibiting effective cutting especially with metal blades.

Sites which are dry typically need to be cut less often for weed suppression and vegetative growth. It is better to cut at the appropriate timing for the target species and before the dry season occurs. Often, especially for annual species, there will be limited regrowth potential if precipitation does not follow a cutting event. Wetter sites will often support a much higher level of vegetative growth and will need to be cut more frequently, often on a set schedule to suppress the vegetation to limit reproduction. Vegetation that is very wet, or excessively dry, can be more difficult to cut. A year-to-year cutting schedule may need to be altered based on precipitation received.

Caveats

One assumption of using a string trimmer is that the target weed species is not closely intermixed with non-target desirable species. If desirable and undesirable vegetation are intermixed, using a string trimmer may have negative impacts to desirable vegetation. While string trimming may still be appropriate, the impact to desirable species should be weighed against the benefit of controlling the invasive plant. Off-target impacts can be especially great if multiple cuts are planned throughout the growing season to suppress the invasive species.

When vegetation becomes dry, and there is any potential for the generation of sparks, metal-bladed blade trimmers should not be operated. While plastic-bladed and string trimmers have less potential to generate sparks, fire danger still exists both from sparks and from a hot motor coming in contact with dry vegetation. Care should be taken with all gas-powered equipment during and after operation, as the hot exhaust from the machinery is a potential ignition source. Electric or battery powered string trimmers may be a viable alternative when fire risk is high. Lastly, as with any equipment, it is important to clean the equipment (especially cutting heads) after each use and to make sure the operator is not moving weed seed through clothing or equipment.

Potential Hazards to Humans, Environment, and Cultural Resources

Human safety: Low to moderate risk. Using a string or blade trimmer has potential to cause harm to humans operating the equipment and standing in the vicinity of operation. String and blade trimmers have the ability to generate flying debris, which could injure the user or someone close by. Direct contact with the cutting head is not common, as it is extended on a shaft, far away from the user. It is of upmost importance to use proper personal protective equipment (PPE) including, but not limited to, eyewear or face shield, long pants or chaps, hearing protection and gloves. If used for extended periods of time, using a harness and padded gloves will reduce fatigue and back strain from holding a vibrating tool. Repeated long-term use of this equipment can cause repetitive stress injuries. Steep terrain can increase the risk and hazard associated with using the equipment. Equipment typically uses a gasoline and oil mix, and extended exposure to fumes can cause nausea and other health risks. Wearing a particulate or organic vapor respirator can minimize the user’s exposure to fine particles from mowing, exhaust particulate, and toxic fumes. Anyone planning to use a respirator for work in the state of California must follow Cal-OSHA guidelines and work for an employer who is compliant with Cal-OSHA employer guidelines. Caution should be taken when string-trimming weeds that have foliage or sap that may cause contact dermatitis. It is always important to wear proper PPE when string trimming, but even more important when targeting species associated with contact dermatitis

Cultural resources: Low risk. As there is minimal soil impact utilizing this equipment, generally the risk for impacting cultural resources is low.

Habitat: Low to moderate risk. This technique will remove vegetation various species depend upon. Small low density weed infestations will have minimal impact, where large monocultures which are cut, will have more negative impacts.

Sensitive species:Moderate risk. Pre-disturbance surveys should be conducted before brush cutters or string trimmers are used in areas where sensitive species occur. In instances where sensitive plant species are intermixed with the target weed, there is potential to injure them through the use of the equipment. Sensitive plant species should not be cut. When ground nesting birds are present, these areas should be avoided to protect nests from noise, direct damage, and flying debris. If the vegetation to be trimmed is dense and there are concerns for frogs, rodents, etc., a site can be cleared with the help of a biological monitor. Alternatively, impact to these species may be reduced if the trimmer can be used in a top-down manner to allow species to flee. Verify species that may be present before beginning work to determine if this is appropriate and review and use accepted avoidance procedures (e.g., 100’ buffers around actively nesting bird species) to minimize impact. Gas powered versions are motorized and can disturb birds and other wildlife both via physical disturbance and through noise (although this is not usually a severe disturbance).

Fire: Low risk (string trimmers), Moderate risk (metal-bladed brush cutters. Metal blades can create sparks when they hit on rocks or other hard objects. Wildfires have been started by brush-cutting. Do not use metal blades under high fire risk conditions and always have assistance and extinguishers on hand to address ignitions from sparking.

Erosion: Low risk. This technique poses little erosion risk as it leaves the below-ground plant structures intact and causes minimal soil disturbance. If large acreages of plant material are cleared on steep slopes, there is some increased risk of erosion occurring. Mulching plant materials that are cut may help cover the soil surface and reduce the risk of erosion.

Consider Combining with the Following Non-Chemical Methods

String trimmers or brush cutters can be used in conjunction with a variety of other techniques to combat specific weeds. Suckers from trees that have been cut with a chainsaw can be effectively targeted with a brush cutter. Brush cutters can also be used to follow up and clean up plants that could not be targeted with a mowing operation.

As string trimmers and brush cutters often do not control the species they cut, but instead suppress the vegetation and seed production, it can be good preparation for follow-up work with manual removal, grub hoeing, and other techniques to kill the target plant. Brush-cutting can also be used as site prep for tarping, solarizing, or mulching when done low to the ground.

Don’t Use This Technique When/For

Perennial vegetation that spreads by rhizomes or other underground reproductive structures will not be controlled by cutting with a string or blade trimmer. Aboveground growth may be suppressed with repeated applications, but the roots may continue to spread, and resprouting along spreading roots may be encouraged after the initial cut is made.

Additionally, any species that reproduces via above-ground stem fragments (e.g., Japanese knotweed, Bermuda grass, Cape ivy) should not be targeted with a brush cutter or string trimmer because pieces of the aboveground vegetation can be spread with the potential to re-root and establish multiple plant populations.

Do not use this technique under red flag wildfire conditions. Do not use a metal blade under fire-prone conditions.

References

Milbrath, L. R., A. DiTommaso, J. Biazzo, and S. Morris 2016. Tolerance of Swallowworts (Vincetoxicum spp.) to multiple years of artificial defoliation and clipping. Invasive Plant Management 9:1-11

Shelton, A.L. 2012. Mowing anytime after midsummer can manage Japanese stiltgrass. Invasive Plant Science and Management 5:209-216

Tarasoff, C.S., K. Streichert, W. Gardner, B. Heise, J. Church, and T. Pypker. 2016. Assessing benthic barriers vs. aggressive cutting as effective yellow flag iris (Iris pseudacorus) control mechanisms. Invasive Plant Science and Management 9:229-234

Ward, J. and T. Mervosh. 2012. Non-chemical and herbicide treatments for management of Japanese stiltgrass (Microstegium vinineum). Journal of Invasive Plant Sciences and Management 5(1):9-19

Authors and Credit

Lead Author: Tom Getts, University of California Extension Weed Specialist, UC ANR

Co-authors:

Jessica Bailey, Landscape Architect, California Native Grassland Association
Pamela Beitz, Integrated Pest Management Specialist, East Bay Regional Parks District
Jutta Burger, Science Program Director, California Invasive Plant Council
Sandy DeSimone, Science Director, Audubon Starr Ranch
Shani Pynn, Senior Plant Restoration Ecologist, Riverside Corona Resource Conservation District

Additional Contributors:

Ling He, Rangeland Conservationist, USDA-NRCS
Isaac Ostmann, Program Coordinator, Irvine Ranch Conservancy
Shawn Thorin,Program Coordinator, Irvine Ranch Conservancy

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