Mow or Compost Leaves for Best Results

Some gardeners take a hands-off approach to leaves in the fall. But leaves allowed to remain on lawns can pack down to form a tight mat that prevents sunlight from reaching the grass, said University of Missouri Extension horticulturist David Trinklein. Leaves also trap and hold moisture, which increases the potential for disease.

Rather than let leaves languish on the lawn or send them to the landfill, Trinklein recommends two ways of using leaves effectively: Either mulch the leaves by mowing or make a compost pile.

To mulch, clear the lawn by mowing damp leaves. Adjust your lawn mower to its highest setting and mow in a crisscross pattern. Mow twice to cut leaves to confetti-size pieces. These small pieces will filter into the lawn, decompose and release nutrients for the grass.

When used to start a compost pile, tree leaves can provide an inexpensive source of valuable organic matter. Compost added to soil binds small clay particles to make larger particles. This improves soil aeration, root penetration and water flow in clay-type soils for which Missouri is notorious.

Soil microbes need two elements to form compost: carbon and nitrogen. Most leaves are high in carbon but low in nitrogen. Adding nitrogen to the compost pile can hasten the decomposition process. Add a general-purpose fertilizer such as 12-12-12 at the rate of about 2 pounds per bushel of leaves, or add 1-2 inches of animal manure or green plant material such as grass clippings.

Construct the compost pile in layers by placing about 4-6 inches of well-shredded leaves followed by a layer of the nitrogen source. This should be repeated until the pile is about 36-48 inches deep and covers an area of about 25 square feet. If you want to enclose the material being composted, you can buy specialized composting bins, but wire mesh, concrete block or treated lumber enclosures also work well, Trinklein said.

Water each layer as you add fertilizer. “A properly constructed compost pile should feel damp to the touch but not wet,” Trinklein said. If using manure as a nitrogen source, make certain of its history. Some herbicides applied to pastures and hayfields pass through the digestive systems of animals and remain strong enough in the manure to damage sensitive species like tomato.

When soil microbes encounter an abundant amount of carbon along with adequate nitrogen, they “go bananas,” according to Trinklein. Microbial activity heats up the compost pile. When composting is done correctly, the temperature at the center of the pile should reach about 135-140 F, hot enough to kill weed seeds, insects and most pathogens.

The time required to achieve this temperature depends on variables such as carbon-to-nitrogen ratio, moisture, aeration and outside air temperature.

As decomposition wanes, the compost pile begins to settle and cool. This means the microbes have run out of “food,” and the pile is ready to be turned. Use a rake or shovel to move partially decomposed leaves from the outer edges to the center of the pile. This renews microbial activity, causing the pile to heat up again. Once the pile fails to heat after being turned, decomposition is complete.

Because of the late start in autumn, leaves may not decompose completely before cold weather sets in. However, the process begins again when air temperatures rise the following spring.

Some leaves also may be used as mulch for tender plants such as azaleas and rhododendrons. Trinklein said stiff leaves such as those from oak trees do not mat and work best for mulch. Put leaves in a wire cylinder around the plant to keep them in place.