Autumn Leaves: From Trash to Treasure

Published: November 2, 2021

Leaves on trees sporting the vivid colors of autumn are quite an attractive sight. However, once gravity takes over and they accumulate in the yard, leaves become a problem. The annual ritual of disposing of autumn leaves is labor-intensive and, at times, frustrating. Especially when additional leaves keep blowing into one's yard from neighboring homes, making the process seem never ending. However, proper leaf disposal is necessary to maintain a healthy, attractive landscape.

When allowed to accumulate over turfgrass and low ornamental plants, leaves can pack down and form a tight mat, particularly during the course of a wet winter. A thick layer of leaves can block sunlight from reaching turfgrass, thus reducing the ability of plants to manufacture food in the fall. The latter is important for cool season turfgrass species such as Kentucky bluegrass or fescue. Additionally, leaves tend to trap and hold moisture in the plant leaf canopy, increasing the potential for disease outbreak.

When allowed to accumulate, leaves can harm turfgrass and small perennials below them.

On the other hand, fallen leaves represent a valuable resource that many gardeners overlook. In addition to containing modest amounts of certain essential mineral elements (e.g., nitrogen, phosphorus and potassium), they are a rich source of organic matter. The latter is invaluable in helping to build soil structure. Therefore, bagging and discarding autumn leaves is not a sound decision from an ecological or economic point of view.

One way to recycle autumn leaves is to mulch them into the lawn. On a dewy morning when the leaves are still damp, adjust your lawn mower to its highest setting and start mowing. By using a crisscross pattern and double-mowing, leaves often can be reduced to the size of confetti. So-called "mulching mowers" are especially proficient at shredding fallen leaves. The tiny pieces of leaves will gradually filter into the lawn and begin to decompose. The end result will be the release of nutrients for use by the turfgrass. Research has demonstrated that a layer of leaves up to six inches thick can be mulched into the lawn with no ill effects.

Fallen leaves can be incorporated into the lawn with a mulching mower.

Composting is another way to prevent damage from fallen leaves and turn them into a useful soil amendment. Compost is partially decomposed organic matter created by biological processes in which soil-inhabiting organisms break down plant tissue. Compost is very beneficial for improving soil structure, because it binds small (clay) soil particles together making them larger. This "aggregation" of soil particles helps improve aeration, root penetration and water movement. When applied to sandy soils, compost can help retain moisture and nutrients.

The soil organisms that breakdown carbon require nitrogen in the process. The carbon-to-nitrogen ratio (C:N) of a material is an estimate of the relative amounts of these two elements it contains. Most leaves are high in carbon but low in nitrogen. As a general rule, the C:N ratio of leaves ranges from about 35-85:1. This combination makes their natural decomposition rather slow.

Compost bins can be made from salvaged materials such as shipping pallets.

In cases where the C:N ratio of the material being composted is wide (i.e., greater than 30:1), the incorporation of additional nitrogen will help greatly to hasten the composting process. Additional nitrogen can be added in the form of fertilizer (e.g., urea) or from organic materials high in nitrogen, such as manure or grass clippings.

Constructing compost piles is a quick and effective way to turn autumn leaves into a useful garden product. If the leaves are chopped or shredded before composting, the process can be hastened even further. Other plant material can be added at the same time leaves are being composted. Specialized composting units or bins are available commercially. However, simple enclosures of wire, concrete blocks or treated lumber can be equally effective.

A backyard compost pile can be almost any size that is convenient for the space available. However, for best results, it should be no less than about 25 square feet in area at its base and three feet in height. As a rule, larger compost piles are better than smaller ones. Whatever the size, always locate a compost pile in an area that is well-drained.

After setting up the enclosure, start the pile with a layer of leaves or other plant material. The initial layer should be between six and eight inches in depth. Since nitrogen is needed by the organisms responsible for decomposition, add a nitrogen source on top of each layer of leaves. Approximately four pounds of actual nitrogen are needed for each bushel of packed or shredded leaves. This converts to about 10 ounces of a pure nitrogen fertilizer such as urea, or two pounds of a general garden fertilizer such as 12-12-12.

Water each layer lightly as it is added. A properly constructed compost pile should feel damp to the touch, but not wet. It often is easier to moisten the dry material before adding it to the compost pile than to moisten the entire pile after it is constructed.

If manure is used as a nitrogen source, use a layer from one to two inches deep over each layer of leaves. However, make certain the manure is from animals that have not been fed hay or forage treated with herbicides. Certain herbicides (e.g., plicloram) have the ability to pass through the digestive tract of an animal and still retain enough activity to damage sensitive species such as tomato.

Compost piles should be turned when their core temperature reaches 140 degrees F.

Temperature is very important in the composting process. As soil organisms decompose the leaves, their heat of respiration causes the temperature in the pile to rise dramatically. The center of a properly made compost pile should reach a temperature of 110 to 140 degrees F in four to five days. At this time, the pile begins to "settle," which is a sign that the pile is working properly. When settling starts to occur, the compost pile should be turned every few weeks until the leaves are thoroughly decomposed and are a uniform dark color. The process of turning mixes partially decomposed material back into the center of the pile while adding oxygen at the same time.

Since soil microbial activity is temperature-sensitive, leaves may not decompose completely in the fall before the arrival of cold weather. The process will resume again when the weather warms in spring. At that time, additional turning of the pile might be necessary.

Leaf mold is made by placing leaves in a compost bin without adding extra nitrogen or turning the pile.

Alternatively, instead of composting, autumn leaves can be made into leaf mold. The latter simply is an organic material consisting of partially decomposed leaves. Unlike traditional compost that undergoes a heat-generating, bacterially-driven process, leaf mold is produced through a cooler and much slower fungal-driven process. Leaf mold is made much the same way as compost, only no additional nitrogen is added. The resulting partially decomposed material is an excellent additive to soil. It can be mixed in during tillage, or used as a surface mulch for weed control.

In addition to mulching leaves into the lawn or turning them into compost or leaf mold, they also may be collected and used as a mulch to protect tender plants (e.g., azaleas and rhododendrons) in the landscape. The best leaves for this use are those are very stiff and do not collapse (form a dense mat) during wet weather. Oak leaves are excellent for use as winter mulch. When used as mulch, leaves should be enclosed in a wire cylinder placed around the plant to keep them in place.