Yard waste includes grass, leaves, and tree and brush trimmings. By weight, grass is the biggest component of yard waste, averaging half of all yard waste. Leaves and brush each provide one quarter. By volume, leaves are the biggest component.

Yard waste is the largest single component of generated municipal solid waste by weight but is a relatively small component of landfilled MSW by volume.

Composting is the controlled decomposition of organic matter by microorganisms into a humus-like product. Waste and carbon dioxide dissipate into the atmosphere. Up to 75 percent of the volume and 50 percent of the weight are lost through composting.

Aeration, temperature control (132-140 F), moisture content (40-60 percent) and an adequate carbon-to-nitrogen ratio are required for composting. Improper operation can cause odors and allow the growth of a fungus (aspergillus fumigatus), which causes health problems.

The amount of yard waste and its MSW disposal market share declined dramatically in the last four decades while the composting rate soared. Backyard compost piles and grasscycling programs have reduced yard waste generation. State and local composting requirements increased the number of commercial composting operations.

Chaz Miller is state programs director for the National Solid Wastes Management Association, Washington. E-mail him at: cmiller@envasns.org.