From The Garden: Cedar Grove fine-tunes art of turning waste into compost

If you feel overwhelmed by yard wastes after fall clean-up, take heart. Seattle area gardeners have several easy ways to cope. First, we can compost as much as possible at home. For anyone with a lawn, the place to begin is there.

Grass clippings make up much of the bulk in green waste. Keeping cut grass at home reduces curbside recycling needs and also benefits the lawn. When grass is cycled directly back into lawns, it composts quickly, dramatically reducing the need for fertilizers, pesticides and summer water. Mulching mowers are the key to efficient grass composting in our own back yards.

We also can participate in urban recycling with Cedar Grove. Back in 1989, Seattle contracted with Cedar Grove to process an estimated 13,000 pounds of curbside yard waste. To everyone's amazement, more than 43,000 pounds were collected that first season.

That enthusiastic community response gave the young Cedar Grove facility some growing pains. The technology for bulk composting was still developing, and the shredders were barely able to handle the unexpected volume.

"It was really hard to deal with the excess in those days," recalls Suzanne Leger, marketing manager of Cedar Grove. "The program was so new, and nobody really knew how to make a high-grade product. Equipment was always breaking down because all kinds of stuff was coming in. The City of Seattle helped a lot by posting monitors to eliminate inappropriate material at transfer stations."

Cedar Grove began composting with wind rows -- long mounds of material that were turned with heavy equipment. "That worked well for smaller scale composting," notes Leger, adding that "the weather here makes that process seasonally challenging. Excess moisture in winter reduces the heat each time you turn a pile. That's not exactly optimal, and you're not going to get the best product."

Today, shredded yard waste is heaped on concrete slabs perforated with air holes. An advanced aeration system draws air into the heaps with powerful fans. "Since this is an open-air facility, we use a negative air flow system that pulls air like a vacuum cleaner down through the material, which helps control odors," Leger explains.

The heaps are built with care, balancing nitrogen-rich green materials with brown or carbon-based materials and wetting them down thoroughly. Good air flow creates an encouraging environment for naturally occurring microbes that produce rapid decay. "We don't need to add anything but water in dry seasons," Leger points out. "All we have to do is make decomposition possible, and nature does the rest."

The huge piles are frequently monitored for moisture and temperature, following compost quality guidelines established by the Department of Ecology and overseen by the King County Environmental Health. Those guidelines require that bulk compost reach and hold temperatures of not less than 130 degrees for a minimum of two weeks.

"That's the established threshold to kill weed seeds and plant pathogens, and provide sufficient heat to volatilize pesticide and herbicide residuals," Leger clarifies. While homemade compost rarely achieves such temperatures for long, this prolonged heat period is vital with a commercial product, where quality must be consistently high.

Today, Cedar Grove processes between 160,000 and 200,000 tons of yard waste each year. Each 10,000-cubic-yard pile of material hosts thermophyllic microbes that prefer high temperatures (the mounds reach temperatures as high as 145 degrees) for 21 days. Soon thereafter, a dramatic drop in temperature and volume signals the end of the initial heat cycle.

The material travels by covered conveyors to secondary pads for another 21-day period. Mesophyllic microbes that thrive in moderate temperatures take over during this refining stage. The compost shrinks again and grows darker in color as it cures further.

From here, compost moves through several finishing stages until it is screened and set aside to mature. The stacks are built to shed water, so they continue to cure slowly all winter. Next spring, some of the finished compost will be mixed like an innoculant with raw material to speed decomposition.

"Our compost cures for six months before we sell it," Leger states with pride. "We've gone from having one huge batch available each spring to having 6-month-old compost at any given time throughout the year. That gives us much better quality control."

The final product is greatly reduced in volume. "A ton (coming) in is equal to a (cubic) yard (going) out," Leger says. How good is it? To judge from the batch the Friday Tidy volunteer gang I helped spread at our local library recently, it's gorgeous. Not only does it look and smell great, it qualifies for organic certification.

According to Leger, "Cedar Grove tests at established levels for toxins, and we have never even had trace amounts. We also had our potting soil tested for Legionella with no trace found."

Cedar Grove compost and potting soil is available at many nurseries and garden centers. Gardeners can also have it delivered (prices change by delivery zone) or save a bundle by picking up a load at the Maple Valley facility for $14.70 a cubic yard.

Ann Lovejoy, a free-lance food and garden writer, lives on Bainbridge Island. She can be reached via e-mail at: lovejoy60@aol.com

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