By Josh Friell, former graduate student, currently with The Toro Company
1) For many cool-season grasses under typical lawn management, total dry clippings production may be in the range of 1-3 g per square meter per day or about 4700 kg per hectare per year.
2) Regardless of growth rate, it is most important to remember that proper mowing practices include not removing more than 1/3 of the above-ground tissue in any one mowing. Removing more than the recommended amount can cause undue stress to the turf plant and stop root growth for as much as six days to two weeks.
3) Returning clippings to turf has been shown to reduce nitrogen fertilizer requirements by approximately 98-294 kg nitrogen per hectare per year – that is, by as much as 75% or more.
4) Disposal of lawn waste accounts for approximately 3% of the total energy expenditures associated with lawn maintenance.
5) Recycling clippings has been shown to increase water infiltration rates by 12% over turf where clippings have been removed.
6) Recycling clippings increased net carbon sequestration by the turfgrass by 11 – 59% over removing clippings.
Recycling turfgrass clippings to a lawn after mowing has continually been show to provide several advantages to the turfgrass and the people maintaining it. It is generally suggested that clippings always be recycled, except where: 1) Returning clippings will interfere with the intended use of the turf; 2) The clippings are unusually heavy or excessive clumping of the clippings will occur; or 3) There is significant chance of disease development due the increased tissue present. Often, there are statements found in a variety of literature that claim recycling clippings will lead to excessive thatch accumulation or unsightly brown grass clippings on the surface of the lawn which shade the plants below. Such statements are largely unsubstantiated and the scientific research on the topic has demonstrated significant time, energy, cost, and environmental benefits from clippings recycling, when done properly.
Turfgrass growth rates vary widely among species and are highly dependent on environmental characteristics, management practices, and time of year. However, for many cool-season grasses under typical lawn management, total dry clippings production may be in the range of 1-3 g per square meter per day or about 4700 kg per hectare per year (Hull, 1992; Qian et al. 2003). Regardless of growth rate, it is most important to remember that proper mowing practices include not removing more than 1/3 of the above-ground tissue in any one mowing (Beard, 1973). Removing more than the recommended amount can cause undue stress to the turf plant and stop root growth for as much as six days to two weeks (Crider, 1955).
Allowing the removed tissue to return to the turf, rather than collecting it, has several advantages. For example, returning clippings to turf has been shown to reduce nitrogen fertilizer requirements by approximately 98-294 kg nitrogen per hectare per year – that is, by as much as 75% or more. (Beard and Yoder, 1976; Kopp and Guillard, 2002). This is a result of the fact that on well-fertilized Kentucky bluegrass, for example, between 68-274 kg nitrogen per hectare per year that is contained in the turf tissue may be removed from the turf system if clippings are not returned (Welton and Carroll, 1940; Rieke and Beard, 1974). When clippings are returned to the turf, that nitrogen is slowly released from the senescing plant tissue back into the soil environment. The result is an overall reduction in the total cost of fertilizer as well as the time and energy to apply it.
Calculating the energy expenditures associated with any landscape maintenance activity is a challenging endeavor. There are many inherently subjective decisions involved regarding the number of steps up the supply chain that should be included in the assessment. As such, there is a scarcity of data regarding energy requirements for turfgrass maintenance. One study, however, estimates that disposal of lawn waste accounts for approximately 3% of the total energy expenditures associated with lawn maintenance (Busey and Parker, 1992). While this number is highly dependent on the choices of the turf manager, it demonstrates that significant energy savings may be achieved by simply returning clippings to the turf instead of bagging them for removal.
In today’s world of increasing population and urbanization water use efficiency is of the utmost importance, and practices that help to conserve water are prudent. Ensuring maximum water infiltration into the soil is one important part of proper water management. Doing so minimizes the amount of water that would otherwise be wasted as runoff, and helps keep soil and nutrients from being lost from the lawn into surface waters. Recycling clippings has been shown to increase water infiltration rates by 12% over turf where clippings have been removed (Musser, 1950). This is likely due to improved soil structure resulting from increased soil organic matter (Beard, 1973).
Increasing soil organic matter has other benefits as well. Strong concern over global climate change, coupled with an increased awareness of the need for sustainable soil management, has driven the desire to sequester and retain substantial quantities of carbon in soils under managed turfgrass. It is generally accepted that the average carbon content of grass species is 42.7%, and it follows that returning clippings to the turf will eliminate the excessive removal of carbon from the turfgrass system (Jo and McPherson, 1995). One study found that, not accounting for carbon expenditures associated with management practices, properly managed cool-season turfgrass sequestered between 0.32 – 0.78 Mg carbon per hectare per year when clippings were returned to the turf (Qian et al., 2010). In a subsequent study, it was estimated that recycling clippings increased net carbon sequestration by the turfgrass by 11 – 59% over removing clippings (Qian et al., 2003).
Taken together, the studies above indicate a strong environmental and economic benefit to returning clippings to the turf during mowing operations. Realization of those benefits is dependent on the turf manager adhering to proper mowing, irrigation, and fertilization practices. A turf manager must determine if the required practices fit within his or her maintenance schedule and abilities, and compare the potential for increased time and maintenance against the significant benefits gained by recycling clippings.
Beard, J.B. 1973. Turfgrass: science and culture. Prentice-Hall, Englewood Cliffs, N.J.
Beard, J.B., and R.L. Yoder. 1976. Clipping disposal investigations with rotary lawn mowers. p. 62–67. In Proc. 46th Annu. Mich. Turf Conf.
Busey, P., and J.H. Parker. 1992. Energy conservation and efficient turfgrass maintenance. p. 473–500. In Waddington, D.V., Carrow, R.N., Shearman, R.C. (eds.), Turfgrass. ASA, Madison, WI.
Crider, F.J. 1955. Root-growth stoppage resulting from defoliation of grass. United States Department of Agriculture. Technical Bulletin 1102.
Hull, R.J. 1992. Energy relations and carbohydrate partitioning in turfgrasses. p. 175–206. In Waddington, D.V., Carrow, R.N., Shearman, R.C. (eds.), Turfgrass. ASA, Madison, WI.
Jo, H.-K., and G.E. McPherson. 1995. Carbon storage and flux in urban residential greenspace. Journal of Environmental Management 45(2): 109–133.
Kopp, K.L., and K. Guillard. 2002. Clipping management and nitrogen fertilization of turfgrass. Crop Science 42(4): 1225–1231.
Musser, H.B. 1950. The use and misuse of water. The Greenskeepers’ Reporter 18(2): 5–9.
Qian, Y., R.F. Follett, and J.M. Kimble. 2010. Soil Organic Carbon Input from Urban Turfgrasses. Soil Science Society of America Journal 74(2): 366.
Qian, Y.L., W. Bandaranayake, W.J. Parton, B. Mecham, M.A. Harivandi, and A.R. Mosier. 2003. Long-term effects of clipping and nitrogen management in turfgrass on soil organic carbon and nitrogen dynamics. Journal of Environmental Quality 32(5): 1694–1700.
Rieke, P. E. and J. B. Beard. 1974. Nutrient removal in the clippings of Poa pratensis L. ‘Common’, Festuca rubra L. ‘Pennlawn’, and Agrostis palustris Huds., ‘Toronto’. Agronomy Abstracts.
Welton, F.A., and J.C. Carroll. 1940. Lawn Experiments. Ohio Agricultural Experiment Station