For some reason, a pile of leaves can trigger fear in people. There are all sorts of scary rumors about what happens when you make a pile of leaves. Might the leaves attract rats, or catch fire from the heat of decomposition, or cut off oxygen and water to the ground underneath and thereby harm nearby trees? My experience provides these answers: no, no, and no. Then there are scary stories about what a leaf pile won't do. My radio alarm woke me up one weekend morning to a segment of You Bet Your Garden, the entertaining and informative gardening program out of Philadelphia. There was our normally spot-on Mike McGrath warning listeners that a pile of unshredded leaves will not decompose nor reduce in size. Come on, Mike. Just because shredded leaves are a handy item in the yard doesn't mean whole leaves are not.
Still, a scientific background will train your mind not to take any knowledge fully for granted, so I decided to put whole leaves to the test. This past spring, I "corralled" some wet, red oak leaves left over from winter in a circular corral made of green garden fencing. By early August, the pile was 2/3rds its original size.
By October, they had settled to a third the original bulk, and it was time to check inside the pile to see what sort of decomposition had taken place, and also to check what the soil and tree roots looked like underneath the pile.
On the outside, the leaves didn't appear to have decomposed, but that proved to be just a facade disguising the transformation the pile had undergone inside. Peeling away the outer layer of leaves revealed a core of rich, moist compost--enough to fill a large plant pot.
A three foot diameter leaf corral produced twenty pounds of compost, without any effort expended beyond tossing the original filling of the leaf corral this past spring. I may have tossed a shovel full of dirt on the leaves, to "seed" the pile with the micro flora and fauna that do the decomposing, but those would have migrated upward from underneath the pile anyway. There was no mixing of the leaves--the pile did all the transformative work on its own.
And did the pile of leaves, originally three feet high, cut off water and air to the soil underneath? The soil underneath the pile was moist and thick with tree roots. The leaf pile was not killing trees but instead feeding them. Just as the decompositional flora and fauna migrate upward into the pile from the soil underneath, groundwater will "wick" upwards in the soil to keep the underside of a leaf pile moist.
Red oak leaves are some of the more decay-resistant leaves homeowners encounter, and yet they broke down in an untended pile in less than a year. It may have helped that they were wet when they were made into a pile, so the next experiment may be to fill a leaf corral with dry leaves, and see what happens. Hypothesis: Rain, snow, and soil moisture will seep into the pile from above and below, and the pile will decompose in a way that is beneficial to all living things around it.
A leaf pile like this is about as sustainable as you can get. Instead of piling leaves in the street, where they must then be trucked to a composting site, ground, mixed in windrows, turned, screened, re-piled, then carted back into town, with fossil fuel burned for every step, make a pile in your own yard and let nature do all the work. And to model good behavior for your neighbors, integrate a leaf corral into your front yard landscaping rather than hide it in the back.
Make use of the resulting compost, or leave it all for the trees to feast on, so that the leaf corral becomes a bottomless channel for transitioning leaves back into soil. If enough people do this, government expense will go down, and the quality and permeability of urban soils will go up.