Forbs, grasses, shrubs (some)
Low–moderate ($100–$500 gross)
Low–moderate (wildlife, habitat)
String trimmers and brush cutters are widely used power tools. A favorite tool of urban landscape maintenance crews, their use is also common by vegetation management crews in natural landscapes and open spaces. These tools are used to sever vegetation at or near the soil surface.
String trimmers or brush cutters consist of a spinning cutting implement on a handheld pole powered by either gas or electricity. They are typically mounted on a shoulder harness. The cutting implements are often interchangeable to varying degrees depending on the manufacturer and model of the particular trimmer or brush cutter. String trimmers use a plastic string, which cuts by whipping against upright plant stems. String thickness should match the robustness of the target species. Most string trimmers can dispense more string by bumping the head on the ground during operation. More robust string can be used with fixed heads (not a bump feed) for thick-stemmed or mixed plant communities. For woody and thicker-stemmed dry weeds, plastic or metal blades can be used. Blades range from single fixed semi-round blades to multiple articulated individual blades. Metal blades can also be used to target smaller diameter woody vegetation. Personal protective equipment, such as long pants, gloves, and eye protection should be worn during operation. Metal-bladed trimmers and bush cutters can create sparks that cause fire. Always use precaution when using powered equipment in dry conditions.
How to Use
Select the appropriate string or cutting head for the target species and install according to the manufacturer’s specifications. Hold the trimmer so that the cutting head is near the ground and start the trimmer. When using, keep the cutting head as close to the ground as possible and move it side to side to sever the stems. If there are desirable plants, stop the blade from spinning by releasing the throttle, or sweep the trimmer over the top of the desirable plants. Avoid trimming desirable plants when possible. Under dry conditions, where vegetation could catch fire, have a fire extinguisher and a response plan to reduce the risk of fire, especially when using metal blades which can strike sparks if they hit rocks. Using plastic blades or string will greatly reduce fire risk.
Annual treatment frequency to eliminate seed production differs depending on the target species and climate. For annual species, this will be as many times per year as new populations germinate or regrow after initial cutting. For perennial species, this will be once or more during the growing season depending on the level of control desired. Certain broad-leaf perennial species and grasses may need to be cut back three to four times a growing season to prevent seed production. Annual or biennial plants should be targeted just before or at flowering. Cutting needs to take place before seeds are mature enough to finish setting. For most grasses this is before “milky” stage of seed development.
Perennials should be cut in the vegetative stage, just before flowering begins, and multiple cuts will be needed to prevent seed production. For all species, secondary and tertiary growth throughout the season may begin to flower at a much shorter height and frequency than the initial spring growth. Experimentation may be needed to determine what combination of timing and treatment interval is necessary to achieve suppression for an individual site or species. Revisiting the sites every two to four weeks may be necessary to control regrowth throughout the growing season.
When working with taller vegetation, multiple cuts or very rapid cutting passes may be needed to avoid the stalks falling on the cutter. Depending on the accumulated amount of vegetation being cut, it may be necessary to remove the vegetation from the site using rakes or other tools in order to release desirable vegetation. Slash piles may also be an effective alternative, when material cannot be removed off-site.
String trimmers are most effective at smaller sites to ensure that target weeds are cut at the optimal stage in their development for maximum suppression. Utilizing a string trimmer or brush cutter may be useful for controlling resprouts following other control techniques such as mowing or felling trees with a chainsaw. Likewise, this technique’s effectiveness can be improved by hand pulling, grubbing, or using herbicide to manage regrowth or plants that were missed in order to ensure seed set does not occur.
String trimmers and brush cutters need to be maintained according to manufacturer recommendations for optimal function. Two-stroke gas powered equipment should be run with the correct octane fuel mixed with the specified ratio of oil for good performance. Carburetors and sparkplugs should be routinely maintained, and gearboxes should be properly lubricated. Adequate string should be loaded into the cutting head at the beginning of the day and metal blades should be kept sharp for good performance. For battery operated equipment, correct charging and discharging specifications should be followed to maximize battery performance and longevity. Read and follow all manufacturer instructions to ensure the equipment lasts and performs out in the field.
Non-Chemical Weed Control: String Trimmers and Brush Cutters, Collin Rath from Cal-IPC on Vimeo.
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.
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.
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
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
Rangeland Conservationist, USDA-NRCS
Isaac Ostmann, Program Coordinator, Irvine Ranch Conservancy
Shawn Thorin,Program Coordinator, Irvine Ranch Conservancy
|Winter / Spring||Good|
|Spring / Summer||Good|
|Plant||Plant Growth Form|
|Low (<1000/square meter)||Good|
|Moderate (1000–10,000/square meter)||Good|
|High (>10,000/square meter)||Fair|
|Plant||Rate of Spread|
|High (doubling in <10 year)||Fair|
|Moderate (50–75% increase in 10 years)||Fair|
|Slow Rate (25% increase in 10 years)||Good|
|Plant||Resprouting / Regenerative Capacity|
|Short (≤3 years)||Excellent|
|Moderate (4–10 years)||Good|
|Long (>10 years)||Fair|
|Plant||Type of Reproduction|
|Seed & Vegetative||Poor|
|Plant||Type of Vegetative Reproduction|
|Rhizome / Stolon / Stem||Fair|
|Bulb / Corm / Tuber||Fair|
|Root sprout / Sucker / Crown sprout||Fair|
|Site||Existing Desirable Plant Cover|
|Marsh / Wetland||Fair|
|Woodland / Forest||Excellent|
|Site||Level of Tolerable Disturbance|
|<40 square feet||Excellent|
|Site||Targeted Invasive Plant Cover|
|<100 feet from road||Excellent|
|100–1000 feet from road||Good|
|>1000 feet from road||Good|