The Science of Autumn Leaf Management
Take stock in what autumn leaves can do for your garden and the insects and animals that overwinter there.
By Thomas Christopher
I’m old enough to remember when fall was a season redolent with a special rich scent. My sisters and I would help my father rake the dry fallen leaves into piles on the curb, and then he would put a lit match to them. We’d stand around, watching the piles smolder, our eyes tearing when the wind blew the smoke our way. That odor of burning leaves would fill the whole neighborhood. And when the piles burned down, my older sister, the wild one, would ride through the embers on her bicycle, throwing out a trail of sparks.
By the time I was a teenager, leaf burning had been banned as a source of air pollution. Which of course it was. What surprises me in retrospect, however, is that my father, a thrifty Yankee born and bred, ever wasted such a precious resource in this fashion.
Today, the dispute is not so much about how to dispose of the leaves but rather how to put them to work. Oh, plenty of homeowners still rake the leaves out to the curb so that the town can haul them away, but the smart gardeners and the environmentalists know that the autumn leaf harvest is too precious to waste in this manner.
For gardeners, they are a free and abundant source of organic matter. Run over with a mulching mower, the leaves can be left in place to sift down into the lawn and decompose to serve as a nutritious top dressing, one that feeds the grass as it decomposes. Or they can be raked into piles and then run over by the mower or fed through a leaf shredder to reduce them to a sort of brown confetti. This can be applied directly to your beds as a handsome mulch or stacked and allowed to decompose for a season or two before a similar use.
When recycled in this fashion, the leaves not only add organic matter to the soil in your beds, they also help to suppress weeds, insulate plant roots from the summer heat, and reduce the need for watering during dry spells by as much as 50 percent. The fertility the leaves release into the soil as they decompose is modest, but the cumulative effect over a period of years is considerable. Besides, the release of nutrients is slow so that plants aren’t overwhelmed with a sudden flush of nutrients such as that released by synthetic fertilizers. Shredding the leave is an essential step in this sort of re-use because if left intact the leaves will matt and smother the plants beneath them.
A couple of years ago, environmental organizations began to attack the idea of removing and recycling leaves. As they pointed out, the fallen leaves are where most of our butterflies and moths overwinter. Many pass the winter down among the leaves as eggs, caterpillars, or chrysalises, although a few adult butterflies also shelter there. Raking and shredding the leaves destroys these shelter-seekers. Likewise, when I consulted recently with Rich Hatfield, a senior conservation biologist for the Xerces Society, about ways to foster bumble bees (which are essential native pollinators) he told me to stop raking because bumble bee queens, which hibernate just an inch or two beneath the surface of the soil, need the extra insulation that a layer of leaves provides. Other, less charismatic creatures that shelter in the leaf litter include spiders, snails, worms, beetles and millipedes, which in turn provide food for birds, chipmunks, turtles, and amphibians.
I reached a compromise with Dr. Hatfield. I will continue to rake and shred the leaves that fall on my garden beds and lawn, but I will also reduce the area that I rake or blow by converting the periphery of the yard to an invertebrate sanctuary where I leave the leaves alone. The autumnal smell of burning leaves, alas, I will enjoy only in memory.
Thomas Christopher is the co-author of “Garden Revolution” (Timber Press, 2016) and is a volunteer at Berkshire Botanical Garden. berkshirebotanical.org Be-a-Better-Gardener is a community service of Berkshire Botanical Garden, one of the nation’s oldest botanical gardens in Stockbridge, MA. Its mission to provide knowledge of gardening and the environment through 25 display gardens and a diverse range of classes informs and inspires thousands of students and visitors on horticultural topics every year. Thomas Christopher is the co-author of Garden Revolution (Timber press, 2016) and is a volunteer at Berkshire Botanical Garden. berkshirebotanical.org.