Fallen tree leaves are abundant. Leaves can be an important source of organic matter if managed as a resource. If allowed to flow into surface water, such as through storm drains, leaves can contribute to nonpoint source pollution of surface water.
How can tree leaves, which have always fallen into rivers, lakes and streams, cause water pollution? When any source of plant debris ends up in water and decomposes, it releases nutrients like phosphorous and nitrogen.
As these nutrients combine with other nutrient sources like sediment and fertilizer, nutrient loading becomes more than a water ecosystem can handle and it becomes impaired. Excess algal growth is one sign of nutrient loading.
To manage tree leaves as a resource and help keep them out of surface water, mow them into the lawn to decompose; use as winter mulch in perennial beds and shrub borders; or chop and mix into compost piles.
Mulch mowing is less time consuming than bagging. It returns organic matter and nutrients to soil and ground tree leaves will not cause thatch. Some research suggests mulch mowing leaves reduces weeds. While weed control is sporadic, mulch mowing will eventually improve soil and turf.
Leaving grass clippings while mowing and mulch mowing leaves can decrease the number of fertilization a lawn needs. Using fewer fertilizations can also lead to less nutrient loading of surface waters.
For light leaf layers, mow as usual but without a catch bag. For thicker layers, raise the mowing height as high as it will go and mow twice. According the Michigan State University, up to 6 inches of leaves can be mulched at a time, depending on the type of mower. Push mowers will handle smaller amounts.
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After mulch mowing, there will be an obvious residue of leaves on the lawn surface for a few days. Small leaf pieces eventually sift down into the turf. Once spring arrives, you will notice little to no leaf debris.
If you are lucky enough to have numerous shade trees shedding leaves onto your lawn, grass and leaves could be caught every other mowing. Add these to compost piles, work them into the soil of vegetable or annual flower gardens, or use as winter mulch.
“Leave the leaves” is a new motto from the Xerces Society where pollinator conservation is a key goal. For leaves that blow into perennials beds, shrub borders or tree areas, don’t bother to rake. Leave the leaves to provide winter cover for not only plants but for beneficial insects too.
Another way to use leaves as winter mulch is to place a chicken wire cage around tender plants, i.e. a rose or mum; then fill the cage with leaves once temperatures consistently drop into the 20s overnight. Tree leaves can be used to mulch strawberry beds which need protection to consistently produce in Nebraska.
For a compost pile, leaves are an important source of high carbon material. They can be mixed into compost piles as they are. To speed decomposition, shred tree leaves with a mower before incorporating. Smaller pieces decompose faster since decomposing microorganisms have more surface area to work on.
If you don’t want to mulch mow or use tree leaves as mulch or for composting, it is still important to rake and recycle leaves at yard waste recycling. Do not rake or blow them into streets where they can clog storm drains to cause localized flooding; or be carried to surface water to decompose and contribute to nonpoint.
Kelly Feehan is a community environment educator for Nebraska Extension-Platte County.