By Dominic Christensen

Mowing can have a significant impact on vegetation. The types of turfgrass and weed species and the total amount of coverage can be influenced by mowing. The primary objectives for mowing roadsides and parks are to maintain visibility for pedestrians and drivers, reduce turfgrass and weed seed heads, and to improve aesthetics for a community. In this article, I will discuss the height of cut, frequency, and other factors to encourage more effective, efficient, and sustainable mowing practices.

Mowing height

A common myth that was present in a survey in 1969 and is still a myth today is, “cut it short so we won’t have to mow it so often” (White and Bailey, 1969). This may be a good practice if you desire to severely injure or kill most vegetation that is growing along roadsides, but the consequences may be greater weed abundance, and most weeds usually grow taller and so would need more mowing than turfgrass species. Additionally, in that same survey by White and Bailey (1969) most operators did not know the height of cut they were using on roadside vegetation. Recognizing the mowing height is the first step to improving mowing practices. It is also useful to note that roadsides and parks are not golf course greens. The quality of roadside soils are more often poorer and they are not regularly fertilized. Parks and roadsides additionally are usually not irrigated. Therefore, mowing lower than 3 inches will likely both reduce grass quality along boulevards and increase the frequency of equipment maintenance.

In general, only one-third of vegetation should be removed at the time of mowing. This is known as the one-third rule. This prevents turfgrass species from being excessively stressed from a mowing event. For example, if the target mowing height is 2 inches, then it would be ideal to not allow the turf to get taller than 3 inches to only cut off 1 inch during a mowing (1 inch would be ⅓ of 3 inches). If the mowing height is 4 inches, then the length of grass at the time of cut can be up to 6 inches. In the second scenario, this allows the grass to grow two inches. This relationship is a factorial one, meaning for every factored increase in the prescribed mowing height then the amount of growth for the grass can double before the next mow.

The optimal height of cut for common cool-season turfgrasses in a low-input setting has shown to be within the range of 3-4 inches. A low-input setting is defined as little to no supplemental irrigation, fertilizer, and weed control. Mowing lower than 3 inches in low-input environments should be avoided because less leaf tissue will result in lower belowground roots and rhizomes (Juska et al., 1955). Additionally, lower heights of cut for some turfgrasses can promote disease (Hunt and Dunn, 1993). Crabgrass (Digitaria spp.) has been found to be more abundant with a lower mowing height (Davis, 1958; Dernoeden, 1998). Mowing at 2 inches instead of 3 inches in a low-input environment has also been shown to result in less turfgrass coverage, density, and quality (Robins and Bushman, 2020). In a park setting, with grasses and flowering forbs growing together, a higher height of cut has shown to result in more flower blooms for at least one species (Lane et al., 2019).

Mowing frequency

Above, I showed that a higher height of cut will allow the turf to grow more before it needs to be mowed. This will generally take longer and therefore effectively reduce the number of mowings that are needed (Dernoeden et al. 1998). Figure 1 shows the potential impact of mowing at a low height of cut and infrequently, which should be avoided. Deciding on the frequency of mowing should be made based on simply mowing when necessary. Different roadside boulevards, parks, and other turf areas will have different growth rates and some will need to be mowed more often. Less frequent mowing and higher heights of cuts also have the potential to favor abundance of forage grasses in turfgrass; those species could be timothy grass, brome grasses (White and Smithberg, 1969), orchard grass, and reed canary grass in Minnesota and these are usually less desirable in turfgrass settings.

Other factors

Additional factors that need to be considered are the sharpness of mower blades, the timing of mowing, and knowing the grass species that are being mown. It is important to mow with sharp blades, because this will reduce tearing or shredding of leaf blades and therefore can reduce disease. If possible, avoid stressful times of the day or seasons (such as the middle of a dry hot summer for cool-season grasses). Fine fescue species are more susceptible to damage when mowing is poorly timed, so especially avoid mowing on the hottest and driest days of the year. If grass has recently been seeded, early low mowing of seedlings with a mixture of perennial ryegrass and Kentucky bluegrass can favor Kentucky bluegrass abundance (Brede and Duich, 1984). The mower type such as a flail compared to a rotary can have an effect on weed abundance, but is usually less of a factor compared to mowing frequency (White and Smithberg, 1972). In prairie settings, mowing early especially during the year after seeding has been shown to favor forb abundance and result in more flowers (Williams et al. 2007).

Conclusion

Lower heights of cut will require more frequent mowing. More mowing means you are using more resources. The grass will then require more fertilizer, herbicides, and will likely favor less pollinator species. Mowing at a height of cut optimal for the present turfgrass species will keep roadside vegetation healthier, so next time think before, where, and how often vegetation in your cities are being mowed (Table 1).

References

Brede, A. D., & Duich, J. M. (1984). Initial Mowing of Kentucky Bluegrass‐Perennial Ryegrass Seedling Turf Mixtures. Agronomy Journal, 76(5), 711–714. https://doi.org/10.2134/agronj1984.00021962007600050001x

Davis, R. R. (1958). The Effect of Other Species and Mowing Height on Persistence of Lawn Grasses. Agronomy Journal, 50(11), 671–673. https://doi.org/10.2134/agronj1958.00021962005000110009x

Dernoeden, P. H., Fidanza, M. A., & Krouse, J. M. (1998). Low Maintenance Performance of Five Festuca Species in Monostands and Mixtures. Crop Science, 38(2), 434–439. https://doi.org/10.2135/cropsci1998.0011183X003800020027x

Hunt, K. L., & Dunn, J. H. (1993). Compatibility of Kentucky Bluegrass and Perennial Ryegrass with Tall Fescue in Transition Zone Turfgrass Mixtures. Agronomy Journal, 85(2), 211–215. https://doi.org/10.2134/agronj1993.00021962008500020009x

Juska, F. V., Tyson, J., & Harrison, C. M. (1955). The Competitive Relationship of Merion Bluegrass as Influenced by Various Mixtures, Cutting Heights, and Levels of Nitrogen. Agronomy Journal, 47(11), 513–518. https://doi.org/10.2134/agronj1955.00021962004700110006x

Lane, I. G., Wolfin, J., Watkins, E., & Spivak, M. (2019). Testing the Establishment of Eight Forbs in Mowed Lawns of Hard Fescue (Festuca brevipila) for Use in Pollinator Conservation. HortScience, 54(12), 2150–2155. https://doi.org/10.21273/HORTSCI14336-19

Robins, J. G., & Bushman, B. S. (2020). Turfgrass performance of perennial wheatgrass species when grown as monocultures and mixtures. Agronomy Journal, 112(5), 3567–3578. https://doi.org/10.1002/agj2.20346

White, D. B., & Bailey, T. B. (1969). Vegetation Maintenance Practices,Programs, and Equipment on Minnesota Highways (No. 619; p. 69). U.S. Department of Transportation Bureau of Public Roads, Minnesota Highway Department, and Minnesota Local Road Research Board.

White, D. B., & Smithberg, M. H. (1972). Turf Methods and Materials for Minnesota Highways. Final Report 1972 (No. 619). Department of Horticultural Science and the Agricultural Experiment Station, University of Minnesota. http://conservancy.umn.edu/handle/11299/211656

Williams, D. W., Jackson, L. L., & Smith, D. D. (2007). Effects of Frequent Mowing on Survival and Persistence of Forbs Seeded into a Species-Poor Grassland. Restoration Ecology, 15(1), 24–33. https://doi.org/10.1111/j.1526-100X.2006.00186.x