Taking an environmentally sensitive approach to pest management
When moving from Michigan to Missouri some years ago, Michigan State University Extension began promotion of mowing leaves as an alternative to raking them. My Missouri home had a plentiful lawn relative to the amount of leaves dropped, so after practicing this for 10 years, I wondered- is this practice still suggested and have there been any follow up studies on the effects to the soil or turf quality?
Burning of leaves in cities and towns has declined dramatically since the 1970 passage of the Clean Air Act. In the late 1980’s concern developed over the amount of yard waste ending up in landfills, resulting in many states banning yard waste from landfills (e.g. Missouri 1992, Indiana, 1994; Michigan, 1995). While commercial or municipal compost sites developed in response to the situation, some cites have focused on how commercial sites and homeowners could reduce the amount of yard waste they send off their property.
Maintaining a regular mowing schedule in the fall was an idea that appeared to save time and reduce the volume of yard waste. For mowers with bagging units, collecting the chopped leaves and using them as mulch around trees and shrubs is a great option. But capturing the shredded leaves takes time, and many individuals do not have ‘bagging’ mowers. (Mowing the leaves and grass clippings into a ‘windrow’ and then raking them up is an acceptable compromise, but still takes more time.)
Studies in Michigan were conducted in the 90’s to evaluate the effects of leaf mulching on turf, under various conditions. The first study considered three different leaf application rates (none, 3 inches, and 6 inches) of mixed tree species, mulched in with a rotary mower using two passes in October. It also considered 2 different nitrogen applications (2 or 4 lbs of nitrogen/1,000 sq ft). A second study used leaves from maples or oaks. Both studies were done in full sun with Kentucky bluegrass lawns. Michigan State scientist did note that while the nitrogen application improved the turfgrass quality rating, it did not seem to speed the decomposition of the leaves. It was also pointed out that chopping leaves into small particles was important and allows them to filter into the turfgrass canopy making better soil contact.
What about soil quality? In a 1998 study at Michigan State, soil from these various plots were analyzed. Soil pH did not change, but organic matter did increase in response to the leaf mulching. Composition of the grass clippings was also affected; the percentages of carbon and nitrogen both increased with leaf mulching, but the ratio of carbon to nitrogen stayed constant, which is positive. If nitrogen had been reduced, a decline in turfgrass vigor or color may have occurred.
Purdue University also conducted a study considering the effects different leaf application rates may have on turfgrass growth. They applied shredded maple leaves to perennial ryegrass for 4 years. Application rates were 0, 2000, or 4000 lbs/acre in a single application, with leaves being mowed-in with a mulching mower. For comparison to a typical lawn, consider that a typical woodlot will drop about 3,000 lbs/acre of tree leaves and litter per year. Nitrogen was applied in the spring at 0, 1.3 and 2.6 pounds per 1,000 sq ft. In the second year of the study, nitrogen rates were increased to 1.3, 2.6 and 3.9 pounds per 1,000 sq ft because the turfgrass quality was quite poor when no nitrogen was added. Conclusions from this study were all positive, as long as some nitrogen was added. Leaves had no effect on turfgrass quality or color, thatch build-up, soil pH, nutrient availability, weed infestation, or the diseases evaluated (red thread, dollar spot, and pink patch).
A final Michigan State study considered how much leaf litter a low input lawn could handle without significant damage occurring? This was of interest because areas such as municipal parks, low maintenance ball fields and golf course roughs were appealing locations to apply ‘extra leaves’ during the fall peak period. Leaves were from a mixture of deciduous trees applied to a turfgrass mixture of Kentucky bluegrass, perennial ryegrass, and fine fescue. The rates were approximately 6, 12, and 18 thousand pounds/acre (2, 3 & 4 times the typical woodlot production mentioned above) which correlated to a layer 6, 12 or 18 inches thick. No nitrogen was applied for the 3 years of the experiment. While considerable leaf litter was visible into the spring, a 3 inch mowing height reduced this detraction significantly. Furthermore, grass green-up was faster at the higher mulch rates. Since athletic activity was of interest, the surface hardness was tested after 3 years. Application of leaves, regardless of rate, softened the surface. Finally, the Carbon:Nitrogen ratio of the soil thatch layer was analyzed, and while it did increase significantly, it held below a threshold for which it is not considered a problem to turfgrass quality and vigor - 30:1.
So, has this idea caught on? Searching the internet has revealed extension articles promoting this technique throughout other states such as Kansas, Illinois, Minnesota, Texas, and Virginia. Typical leaf drop patterns, in Missouri, has a couple of advantages over states such as Michigan, where the leaves shed abruptly, probably 80% in the month of October. Here, leaves typically fall over several months, with leaves of trees such as ash and maple dropping in October/November and those of oaks falling in November/December. Also, at the end of hot dry summers, a number of trees (e.g. cottonwood) lose quite a few leaves in late August and September to conserve moisture. Rainfall usually returns in late summer and fall, which provides a reason to mow the grass. So if you haven’t tried this before, go for it, you may never go back to leaf removal or raking.
Benefits: Less time, returning nutrients and organic matter to the turf, and the environmental benefits of reduced smoke, less landfill waste, less bag usage, and less transportation costs are just a few.
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REVISED: September 29, 2015