grass
Works Best On:
Forbs, grasses
radar
Selectivity:
Low
attach_money
Cost (acres/year):
Low-moderate ($100–$500 gross)
local_fire_department
Fire:
Moderate–high risk
health_and_safety
Safety:
Moderate risk
houseboat
Cultural Resources:
Low–moderate risk
Environmental Hazards:
(erosion, wildlife, habitat)

Overview

Mowing is one of the most common and useful non-chemical weed control techniques for large areas. Mowers come in a variety of shapes and sizes for many types of situations. There are two categories of mowers: tractor-operated mowers and push behind mowers. The most commonly used tractor-operated mowers brush hogs and flail mowers. Brush hogs have thick, dull blades that spin horizontally under a wide deck, typically dragged behind a tractor. Flail mowers use a set of vertically spinning flat blades to cut vegetation. Like rotary mowers, many flail mowers are often dragged behind a tractor. Specialty mowers, both rotary and flail types, can be operated on an extended arm to mow side slopes and roadsides. Typical push behind mowers include rotary mowers either with a blade positioned under a deck (e.g., lawn mowers) or a brush mower blades, or flail mowers. Push behind mowers are also available with a plastic string as the cutting implement mounted under a deck.

Mowing typically cuts vegetation a few inches above the ground, though some flail mowers can cut vegetation down to the soil surface. Mowing is most often used in flat, even terrain, or in areas with gentle slopes. However, some specialized mowers that can be operated by remote control are now available for uneven surfaces and steeper terrain. These pieces of equipment have tracks instead of wheels and have specialized flail mower heads or rotary decks.

Mowing is commonly used throughout California to non-selectively suppress vegetation in a wide variety of environments such as roadsides, pastures, and parks. Well-timed, intensive, multiple-pass mowing throughout the season can control invasive species on a large scale. Mowing can also be used as a site preparation technique for restoration in wildland settings. In general, mowing is a good tool to suppress all vegetation quickly across a large area. However, “quick” vegetation suppression can only be achieved in specific types of terrain and ground cover. Mowing can also be used to delay seed set of an invasive plant, in preparation for tarping to ensure seed set does not occur. As with all forms of heavy equipment use, cleaning off debris between sites is necessary to minimize the spread of invasive plant propagules.

How to Use

Mowing can be a good way to suppress vegetation that is not large and woody. In areas with woody vegetation, specialized equipment such as masticators can be used.

When mowing invasive plant species, proper timing and frequency of mowing are essential to prevent seed set from occurring. Annuals should be targeted at the flowering stage, but before seed set. Many species of annuals, such as foxtails and yellow starthistle (Centaurea solstitialis), can regrow and produce seed after the initial cutting. A single cut per year will not control any species that sends up secondary growth and flowers after cutting, whether annual, biennial or perennial. If only one pass is made, the lowest mowing height will offer the best weed suppression. If a second cut is planned, mowing at a high initial cutting height will allow space to come back and make a second cut below the height of the original cut. Mowing two or even three times in a growing season with decreasing cutting height may be necessary to achieve seed head control. Often, resources only allow for a single mowing pass over the site.

Biennial species can be targeted in a similar fashion as annual species by mowing right before or at flowering. As with annual species, secondary regrowth may occur and multiple passes may be needed to suppress seed production.

Vegetative perennial species will often resprout after an initial cut and multiple cuttings per year may be needed for seed suppression. Mowing will not eradicate resprouting perennial species but suppression can be achieved.

Smaller woody brush species can effectively be mowed and suppressed with a large rotary mower operated at a high cutting height. Ideally, sites should be mowed multiple times a year for multiple years in order to prevent seed set from occurring in the targeted weedy vegetation.

Wetter sites will often support a much higher level of vegetative growth and will need to be mowed more frequently, often on a set schedule to suppress vegetation and limit seed production. A single cut in higher precipitation zones will not offer good suppression of most species. Dry sites will have less regrowth potential of all species after the initial cut.

Mowing feasibility is limited by terrain and groundcover, both for the equipment powering the mower and the mower itself. Slope, rocks, and large woody vegetation limit where the equipment can be effectively used because uneven terrain, rocks, and woody debris on the ground can contact mower blades and damage the machinery.

Both rotary and flail mowers can be damaged by striking solid materials during the mowing process. Rotary mowers have a cutting height fixed above the ground and are not as likely to come in contact with the ground and solid materials. Flail mower blades can be dropped closer to the ground and are more likely to contact soil and rocks at low cutting height operations. If the mechanism is not damaged, individual flail blades can be replaced if damaged. Generally, cutting height needs to be kept higher for larger equipment compared to small mowers.

Mowing uses machinery with gas engines, and spinning blades can generate sparks, especially where rocks are present. Mower operators should always have the proper fire suppression equipment ready at hand. Whenever there is a risk of starting a fire, spotters and water trucks should be on site.

Special Tips

Different mowing heights can influence the effectiveness of invasive plant treatment. For example, in areas with native perennial grasses invaded by annual broadleaf plants, a high mowing height can suppress the annuals while releasing perennial grass species to compete with weeds.

Timing of mowing is critical, not just for invasive species seed suppression, but for selecting which vegetation is favored by the management action. Timing mowing to maximize impact on the target invasive plants must be weighed against the timing when desirable species may be in growth stages which are sensitive to being cut.

Mowing repeatedly has the ability to select for short species such as non-native basal-rosetted annuals and biennials released from competition.

In instances with thick thatch layers, such as with invasive species like medusahead and perennial pepperweed, mowing can help break up and manipulate the thatch layer which favors the invasive plant. Manipulation of these thatch layers through mowing or other techniques can release native species present in the understory.

On flat ground without rocks, flail mowers can be used to scarify soil in preparation for restoration. This technique, referred to as scalping, is at the interface of mowing and cultivation. However, contact of the flail mower with the soil surface also greatly increases the likelihood of damaging the equipment.

Mastication is a form of mowing that turns woody species into mulch by grinding stumps and small brush down to the soil surface. This can be a very effective technique for woody species that do not regrow after being cut. For species that send up stump suckers or auxiliary root suckers, mastication will only provide temporary suppression and will need to be followed by additional control methods such as pruning, brush cutting, or herbicides. The amount of woody vegetation to be masticated should be considered before treatment. While some woody chip mulch may be desirable, thick layers of mulch can suppress both desirable and non-desirable vegetation and pose a fire hazard. Mastication is especially useful for removing live and dead canes from stands of Himalayan blackberry (Rubus armeniacus), but follow-up treatment is necessary to control this species in the long term. Mastication is also successful at reducing standing biomass from woody species such as brooms, which may be a short-term goal in fuel reduction scenarios, but brooms ultimately need follow up treatment on resprouting stems.

Excessive slope can limit the ability to safely mow, but slope mowers are available to use on roadsides.

Optimal Conditions for Use

Mowing with large equipment is most effective on large areas with smooth, even terrain and easy road access. Soils should not be wet when using large equipment mowers as compaction can occur. Ideally, vegetation to be mowed will not be covered in dew or rain but also not dried up. Smaller push behind mowers can often be used in tighter spaces on steeper slopes, but generally require even terrain for use. For annual species, mowing at the proper time can help limit regrowth potential.

Caveats

Mowing can only be conducted when the terrain and moisture conditions allow safe operation of the equipment. Special heavy equipment operator licenses and training certificates may be required in some places for large mowers.

Spinning blades can strike rocks and other materials, which may create sparks and start wildfires. Chances of starting a fire are lower early in the season, and at higher humidity levels that typically occur earlier in the morning. Anytime there is wind, mowing operations should be cautious because fire risk increases. For roadside mowing applications under high fire risk conditions a water truck should follow mowing equipment.

Mowing may be limited in areas of steep slope. While there are specialized pieces of equipment which can handle steep slopes, not all mowers can be safely operated. Consult the operations manual and manufacturer of the equipment.

Weeds can unintentionally be spread by poor timing, as mowing weeds after they set seed can shatter their seeds leading to an increase in the density of the target weed species. Furthermore, mowing can cause disturbance within the mowed area opening up the canopy to light and which stimulates sprouting.

Likewise, equipment may disperse weeds to the next mowing site if viable seeds are not cleaned from the equipment. Cleaning equipment between sites before transportation is essential to limit seed spread. Machinery should be cleaned on site if possible, to ensure invasive seeds stay on site.

Mowing with large equipment should be avoided when sites are very wet following precipitation. The weight of the equipment can increase the possibility of getting stuck. Additionally, using large equipment on moist soil can cause soil compaction. For mastication of woody vegetation in areas where soils freeze, operation of large equipment during the winter can reduce the likelihood of soil compaction.

As with all cutting techniques, proper timing is vital to the success of the operation. Timing may vary from year to year with fluctuations in the weather. Proper planning is important to ensure the equipment and operator are ready at the optimal time for weed suppression. This is especially important when dealing with contractors. Time and resources should be budgeted to fix and maintain equipment throughout the season of operation.

Potential Hazards to Humans, Environment, and Cultural Resources

Human Safety: High risk. Safe use of equipment requires training and is essential to the operation of large mowing equipment. Proper personal protective equipment (dust mask, ear and eye protection) should be worn by the operator and any individuals within the area. Flying debris and vegetation thrown by the mower pose an injury risk in the area. When using large equipment, there is a high risk of physical injury directly from the equipment itself during loading, hookup, and operation. Difficult and rocky terrain may pose more potential for user injury than flat level ground.

Cultural resources: Low-moderate risk. When abrading the soil surface, identify and avoid surface cultural resources. Cultural resources may be exposed by mowing or masticating. If they are at soil surface level, they may be crushed by heavy equipment.

Habitat: Moderate to high risk. Mowing will remove vegetation, impacting habitat for a variety of species depending on that cover. Impacts will range from moderate to high, depending on the size and frequency of the area being mowed.

Sensitive Species: Moderate risk. Rodents, reptiles and amphibians may be harmed by mowing. Consider sweeping an area prior to mowing to move animals from the site temporarily. Ground and shrub nesting birds maybe be negatively affected if sites are mown during the breading season. Areas with sensitive plants are not appropriate for mowing with large equipment. For non-sensitive desirable species, mowing can either be beneficial or harmful depending on the timing and specific species’ response to cutting.

Erosion: Low to moderate risk. Mowing will leave belowground plant structures in place, but there is increased risk of erosion whenever aboveground vegetation is removed, especially on slopes. Additionally, soil disturbance from the machinery itself may make the site more susceptible to erosion. Frequent mowing with large equipment, or mowing on wet ground, can increase compaction. In areas where mowing height is extremely close to the ground or at ground level, the risk for erosion may be increased.

Fire: Moderate-high risk. Rotating blades can cause ignitions by sparks from blades hitting rocks. Always have assistance and equipment on hand to address ignitions. Do not mow during high fire risk conditions.

Consider Combining with the Following Non-Chemical Methods

Mowing can be combined with several other techniques for successful invasive plant control. Mowing provides an opportunity for quick vegetation suppression, which can be followed by tarping, hand pulling, grubbing, or cutting techniques. Mowing can also be an effective way to prepare a site for competitive planting.

Don’t Use This Technique When/For

Many annual species will be suppressed temporarily by mowing but may then regrow and set seed in a shorter prostrate form. Continuous mowing will favor species with prostrate growth and may shift the plant community to lower growing plants. Do not mow where there are populations of undesirable low growing prostrate species, as these plants will have a competitive advantage when mowed. Mowing vegetation after seed set may lead to an increase in the weed population by stimulating seeds during the mowing process.

Rhizomatous perennial plants’ aboveground growth may be suppressed by mowing. While seed set may be limited, only suppression will be achieved. Multiple cuttings are needed for suppression of the aboveground growth, but rootstock may or may not continue to reproduce and spread. Mowing rhizomatous species with a single cut during the growing season is not recommended. Multiple mowing passes per year for multiple years are needed to suppress populations of rhizomatous perennial plants. Some rhizomatous perennial grass species have been shown to decrease when mowed multiple times per year at the flowering stage. However, many years of mowing are necessary to achieve control of perennial grasses.

References

DiTomaso, J.M., G.B. Kyser, et. al. 2013. Weed Control in Natural Areas of the Western United States. Weed Research and Information Center. University of California. 544 pp.

Ferguson, L., C. Duncan, and K. Snodgrass. 2003. Backcountry Road Maintenance and Weed Management. USDA Forest Service Technology and Development Program. 7100 Engineering 0371-2811-MTDC.

Sheley, R., K. Goodwin, and M. Rinella. 2017. Mowing to Manage Noxious Weeds. Montana State University Extension MontGuide. MT2001104AG.

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
Dan Lubin, Environmental Scientist, California State Parks
JP Marie, Putah Creek Reserve Manager, University of California

Additional Contributors:

Isaac Ostmann, Program Coordinator, Irvine Ranch Conservancy
Shawn Thorin, Program Coordinator, Irvine Ranch Conservancy

Efficacies

Ratings: Excellent (>95% control); Good (81–95% control); Fair (50–80% control); Poor or ineffective (<50% control)

Results are based on an estimation of maximum possible single-season reduction in weed cover and propagule production (=control). Control efficacy was scored for each plant and site characteristic for each management practice individually using best available information, assuming other conditions were optimal. Results for management practices are organized by efficacy rating based on the lowest rating they received for the combination of plant and site characteristics chosen. Rating results provided by the WeedCUT tool are generalized and may not be suitable for all plants or site conditions with the characteristics chosen. Ratings assume that a multi-year strategy will be employed to achieve management goals.
      Rating
Plant Flowering Period
    Winter Good
    Spring Good
    Summer Good
    Fall Good
    Multiple Seasons Poor
    None Poor
Plant Germination
    Winter Good
    Winter / Spring Good
    Spring / Summer Good
    Opportunistic Good
Plant Palatability
    Yes No Information
    No No Information
    Partial No Information
Plant Plant Growth Form
    Grass Good
    Forb Good
    Shrub Poor
    Tree Ineffective
    Vine Poor
Plant Plant Type
    Annual Good
    Biennial Fair
    Perennial Poor
Plant Propagule Production
    Low (<1000/square meter) Good
    Moderate (1000–10,000/square meter) Fair
    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
    Low Good
    Moderate Fair
    High Poor
    None Excellent
Plant Seed Life
    Short (≤3 years) Good
    Moderate (4–10 years) Fair
    Long (>10 years) Fair
Plant Type of Reproduction
    Seed Good
    Vegetative Poor
    Seed & Vegetative Poor
Plant Type of Vegetative Reproduction
    Rhizome / Stolon / Stem Poor
    Bulb / Corm / Tuber Poor
    Root sprout / Sucker / Crown sprout Poor
Site Existing Desirable Plant Cover
    <10% Good
    10–25% Good
    26–50% Good
    51–75% Fair
    >75% Fair
Site Ground Condition
    Muddy Poor
    Smooth Excellent
    Cobbly Poor
    Rocky Poor
Site Habitat
    Marsh / Wetland Fair
    Riparian Fair
    Grassland Excellent
    Shrubland Good
    Woodland / Forest Good
Site Level of Tolerable Disturbance
    Low Fair
    Medium Good
    High Good
Site Slope
    Flat Excellent
    Moderate (10–40%) Fair
    Steep (>40%) Poor
Site Target Area
    <40 square feet N/A
    0.001–0.01 acre N/A
    0.02–0.1 acre Good
    0.2–1 acre Good
    2–10 acres Good
    11–50 acres Good
    51–100 acres Good
    >100 acres Fair
Site Targeted Invasive Plant Cover
    <1% Poor
    1–10% Poor
    11–25% Fair
    26–50% Good
    51–75% Good
    >75% Excellent
Site Vehicle Accessibility
    Roadside Good
    <100 feet from road Good
    100–1000 feet from road Good
    >1000 feet from road Good