Mulching tree leaves: an alternative to disposal

Few things are more beautiful than the changing colors of tree foliage in the fall. Unfortunately, enthusiasm for this autumn splendor is tempered by the knowledge that homeowners, professional grounds managers and golf course superintendents will soon face the inevitable task of leaf disposal.

There's no question that you have to do something with leaves. Tree leaves can shade the turf, robbing it of precious photosynthetic activity in the late fall. Even thin layers of tree leaves trap humidity at the turf surface and increase the chance of snow mold during winter. Thicker layers of leaves can smother and completely kill the turf. Removing the interference from fallen tree leaves also allows your late season nitrogen applications to reach the turf more effectively, and improves the efficacy of late-season broadleaf herbicide applications. Therefore, for optimum turf health, it is critical to remove the tree leaves, or at least break them up.

Dwindling options Years ago, we raked leaves into piles and either burned them, moved them to the curb for garbage pick-up or hauled them to the local landfill. Today, burning tree leaves is banned or at least restricted in most municipalities. Yard-waste bans prevent us from hauling tree leaves to landfills and garbage haulers often no longer take tree leaves left on the curb. If you are lucky, you have your own compost pile or a composting center where you can still dispose of leaves, but this still requires a lot of labor and expense. Land-applying leaves-spreading them on agricultural land and tilling them in-is good method of disposal, but it is expensive.

The easiest and cheapest way to dispose of leaves is to mulch them into the turf. This is not a new idea, but universities have only recently compiled enough data to determine that tree-leaf mulching has no long-term negative effects on the turf. Studies at Michigan State, Cornell, Rutgers and Purdue have concluded that mulching tree leaves is an excellent disposal method that does not harm healthy turf.

The Purdue study * Methods. At Purdue, we are just finishing up a 5-year study looking at the effects of mulching maple leaves into turf. Every October since 1994, we collected maple leaves, ran them through a garden shredder to facilitate handling and applied them to a low-maintenance perennial ryegrass stand.

We applied the leaves at 2,000 and 4,000 pounds per acre in a single application and set aside check plots where we applied no leaves. We selected these rates because a typical forested area will drop about 3,000 pounds of tree litter per acre each year, including twigs and leaves. Our 4,000-pound rate was the equivalent of about 6 to 8 inches of piled leaves. After application to the plots, we immediately mowed the leaves with a mulching mower.

Because we felt that the high carbon-to-nitrogen ratio (C:N) in the tree leaves would eventually limit nitrogen in the turf stand, we annually applied nitrogen using three different rates. Beginning in the spring of 1995, we applied 1.3 or 2.6 pounds of nitrogen per 1,000 square feet per year, and left some plots unfertilized. However, by summer 1996, the ryegrass receiving no nitrogen had declined dramatically. Therefore, we adjusted the rates to 1.3, 2.6 and 3.9 pounds per 1,000 square feet per year.

The turf plots received regular irrigation to prevent drought stress, and herbicide applications to limitbroadleaf and annual weeds. We collected data monthly for clipping weights, visual quality and color ratings, and annually evaluated the turf mat, soil pH and soil nutrient concentrations. We also checked soil microbial biomass and activity, and water infiltration.

* Results. We found that even the high rate of tree leaves had no effect on turf visual quality, color or growth. Although we expected tree leaves to tie up nitrogen in the soil, we saw no long-term effects of tree leaves on turf growth regardless of the nitrogen rate we applied (other than the original plots that received no nitrogen at all). Other studies have shown short-term nitrogen depletion as the tree leaves break down. However, we feel this would not be a problem in most areas because fall fertilization of the turf will almost certainly ensure adequate nitrogen for the grass plants.

We also suspected long-term nitrogen depletion would increase diseases such as red thread, pink patch and dollar spot, but we saw no increase in these diseases regardless of leaf or nitrogen treatment. No differences in weed infestation occurred, regardless of treatments.

No substantial buildup of leaf mat occurred in the soil, though you might expect this to occur after many years of leaf mulching. However, we did see an increase in microbial activity caused by mulching the tree leaves, which apparently prevented the leaf mat from accumulating.

The increased microbial activity is perhaps the most interesting finding of this study. Increased soil microbial activity indicates improved soil quality. Therefore, we expect that the heavy clay soils on which many new subdivisions are now built should improve as the trees mature and their leaves are mulched into the turf.

Our data suggested an increase in water-infiltration rates. This data was not conclusive but, combined with the increased soil microbial activity, suggests mulching tree leaves improves soil properties.

We observed no effect on nutrient availability or pH in the soil. However, we were applying maple leaves with a basic pH to a fertile silt-loam soil with significant buffering capacity. Theory and practical experience tell us that if low-pH leaves, such as oak, are mulched into a poorly buffered soil, such as sand, the pH may decrease significantly after many years of leaf mulching. Of course, corrective measures such as lime applications can counteract this.

But is it practical? Professional turf managers tell us that mulching tree leaves saves tremendous amounts of time for their crews, allowing them to take on more important or profitable jobs. One person can mulch tree leaves on an entire property where it might have taken a crew of four or five to pick up the leaves. Mulching tree leaves minimizes expenses for equipment such as large vacuums and specialized trailers or trucks for hauling. It also eliminates disposal fees at compost sites or landfills.

For some in-house operations, this might be a "no-brainer." However, revenue is an issue that inevitably comes up with contractors whenever a new labor-saving strategy is devised. Why should maintenance contractors eliminate leaf pick-up and disposal when it can be a good source of revenue? This is an important question, and one that may have different answers, depending on the property and the maintenance operation. Isn't mulching tree leaves with mulching mowers still a leaf-disposal service? Your company could still offer a leaf management service, but it could take the form of late-season mulching rather than leaf pick-up and disposal.

Leaf mulching can even be an effective bidding tool, because an overall maintenance program that includes fall leaf disposal could be performed for less money if you employ leaf mulching. Further, maintenance practices perceived as environmentally responsible hold great weight with some clients, so this could be a way to portray yourself as an "eco-friendly" operator.

Is tree leaf mulching for everyone? Probably not--every situation is different. A heavily wooded lawn, already thinned from shade, might not be able to withstand the extra abuse of mulching the tree leaves. Homeowners set in their ways may not be comfortable with the idea of mulching tree leaves when they've always been raked in the past. It is not realistic to expect every property to adopt a leaf-mulching program. However, it is reasonable to evaluate each account and recommend mulching where appropriate. Saving time and money without harming turf makes leaf mulching a winning strategy.

Dr. Zac Reicher is a turfgrass extension specialist and Glenn Hardebeck is a research technician, both at Purdue University (West Lafayette, Ind.).

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