Recycle the nutrients in fallen leaves

Recent night temperatures remind us that autumn is near, that leaves soon will be on the ground. Sadly, too many of those leaves will end up in plastic bags at curbside destined for landfills. Too many will be burned, polluting the air and creating numerous health hazards. Neither fate is necessary.

The red oak in Marjorie’s garden put on several inches of new growth this year and its branches bear an abundant crop of acorns, the young tree’s first. It managed this vegetative and reproductive growth without our help – we did not apply fertilizer in any form – so where did the nutrients that supported this growth come from?

Part of the answer came to light when I set up Lynne’s new trampoline under the shade of two tall yellow birches, the only expanse of level ground on the property. I did not anticipate the nightly rain of caterpillar frass that must be swept off the trampoline’s mat every morning before jumping can commence.

Like earthworm castings, caterpillar frass contains nutrients essential for plant growth. I have no doubt that the caterpillars have always been there, every year feeding on the birch leaves while returning unused nutrients to the soil. Lying on the mat, looking up into the canopies, I can see countless partially eaten leaves. There is a lot going on in the garden that you miss without the proper vantage point.

Then there are the leaves that do make it to the ground in fall, adding their nutrients to the pool. Fallen leaves of deciduous trees such as maple, beech, ash and oak contain about 0.5 percent nitrogen, 0.1 percent phosphorus and 0.5 percent potassium, along with substantial amounts of calcium and magnesium, all essential nutrients for plant growth.

Because decomposition of leaves is a slow process, these nutrients are released gradually – nature’s version of a slow-release fertilizer. Indeed, annual top dressing with decomposing leaves – along with help from earthworms and caterpillars – is all the fertilizer that trees, shrubs and many perennials need for healthy growth. Even lawns are healthier for the nutrients released from fallen leaves by a mulching lawn mower.

Nutrients, however, are only part of the garden worth of autumn leaves; shredded leaves also improve both the structure and water-holding capacity of the soil. While rich topsoil can hold 60 percent of its weight in water (compared to 20 percent for subsoil), decomposing leaves can retain 300 percent of their weight, or more.

The question then becomes not what to do with autumn leaves, but how to do it. Start by shredding the leaves into small pieces that will break down quickly. This can be done with special grinders designed for the task or with a lawn mower. The resulting mulch can be immediately spread in the garden walkways to control weeds and around your trees and shrubs as well.

Some of the shredded leaves can be added to the compost pile along with high-nitrogen materials such as grass clippings or stable manure. Too many leaves and the composting process slows, while a mixture of five parts leaves to one part manure will decompose quickly.

Autumn leaves are too valuable to be treated as trash or burned. If you are not able to recycle all of the leaves that fall on the lawn in your own garden, find a gardener who will use them. And check with local government officials to see whether your community operates a leaf pickup and composting program for its residents, as do Bangor and Brewer.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@ptc-me.net. Include name, address and telephone number.