Fallen Leaves? An argument for not raking

Posted: October 25, 2016

Autumn beauty can sure leave a mess behind! Change your mindset about the value of those fallen leaves.

Autumn beauty can sure leave a mess behind! For many people the love of the seasonal aesthetic is lessened by the work involved to clean leaves up -- raking, bagging, and removing. But what if there were a reason to change your mind set about fallen leaves?

Fallen leaves are essential to building healthy soils and to the important roles soil plays in nature. No matter where you live, helping the soil retain or re-create some of these characteristics will increase its ability to function well.

What is soil? According to the Soil Science Society of America, soil is the “unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants." Soil productivity is affected by several characteristics: topsoil depth; texture – ratio of sand, silt, and clay in the soil; structure – how the layers are organized; fertility – level of organic material and nutrient cycling potential; and subsoil – the composition of the parent material or bedrock.

Undisturbed soil will, over time, structure itself into more or less parallel layers above the bedrock. These layers are called soil horizons and are distinguished by sizes of particles, contributions of organic material, and the leaching and depositing of minerals from rainwater. Forest soils are some of the healthiest soils in the world, in large part because the forest ecosystem recycles organic matter. Forest soil is unique in that it has an O-horizon – an entirely organic horizon above the first mineral layer. That first layer of soil is also protected by the duff layer, or layer of undecomposed leaf litter, that will, over time, decompose and enter the soil for plants to use.

The O-horizon has an upper part that is recognizable leaves and twigs (the duff) with decomposition increasing with depth. There is tremendous biological activity happening in the O-horizon with insects and worms, bacteria and fungi, plant roots and their associated fungi fostering decomposition. This activity creates holes (or macropores) that allow for high water infiltration into the soil. The O-horizon moderates soil moisture and temperature and protects the mineral layers below from erosion. This important upper layer acts as a sponge, absorbing and directing tremendous amounts of water to the lower layers. As a sponge, it also absorbs the energy from a falling raindrop and protects the soils below from erosion, and it protects the soil from compaction. In many places, efforts to control stormwater are using principles of green infrastructure – creating areas that mimic the function of forest soil – rain gardens, swales, retention basins. These structures slow the water down and allow it to infiltrate to the soil, improving the water quality, and reducing the quantity flowing overland and causing flooding. Important to stormwater management success will be a consideration of how to maintain the characteristics of the soil that allow for infiltration – mimicking that O-horizon.

Leaves are also a natural fertilizer. As leaves start to senesce (prepare to fall in autumn) nutrients from the tree return to the leaves – opposite of what you would expect. You might think it would be wasteful for the tree to send its resources into a part that is just going to go away, but the nutrients in the leaves become available in the soil when the fallen leaves break down. Trees and plants then can take up nutrients from the soil through their roots. There must be inputs to the nutrient cycle for the plants to get what they need from the soil.

Fallen leaves provide benefits to wildlife – those macroinvertebrates (insects, worms, etc.) that assist with leaf decomposition are important food sources for birds and amphibians. Not to mention amphibians love the cool and moist environment created by fallen leaves. Piling leaves in strategic places – think about making “beds” of leaves – contributes nutrients and provides habitat.

One of the primary methods of leaf disposal is raking and removing them in bags or by burning. Look on those bagged-up leaves as bagged-up nutrients that will be lost from your property. What can you do to leave them in place to contribute to the soil structure and nutrient cycling on your land?

Composting and strategic relocation are two ways to keep them on site. Another is to save your last mowing until after the leaves have fallen. The lawnmower will break (mulch) the leaves into smaller pieces that will decompose more quickly (this assumes you don’t bag your lawn clippings). It’s natural fertilizer for your lawn, garden, meadows, and woods. Keep it on site to build and re-create good soil.

Reconsider your views on those fallen leaves. It might be time to leave them be.

Contact Information

Allyson Brownlee Muth, Ed.D.
  • Forest Stewardship Program Associate
Phone: 814-865-3208