Home & Garden

A spring clean-up: Getting started with composting

gardening by Mary Robson, Area Horticulture Agent, WSU
Mixed in with all the abundant new growth of spring, gardens also have winter and spring weeds, spent tops of perennial plants, fallen leaves, grass clippings, and other bits of plant residue. Cutting off the dead parts of plants and pulling weeds are both part of the spring garden clean-up. What's to be done with the soggy piles of spent vegetation? Lots of gardeners make compost, creating a valuable garden soil addition from waste material.

Making a compost pile is simple
   The compost process involves mixing and stacking organic materials and allowing them to break down naturally into a crumbly brown substance. People who are new to composting often worry that it's a messy, smelly process. With a little bit of knowledge and proper management, compost-making can be unobtrusive and easy.
   Select an area that's large enough to make a pile at least 3 feet in width, depth, and height. Smaller piles don't generate enough heat to compost properly. An enclosure for the material isn't necessary, but many gardeners like the tidiness of creating a simple bin. Used wooden pallets can be recycled into a sturdy, essentially free bin, and there are many more elaborate plans available through books and local county sources. Pre-built bins, many of them made of recycled plastic, are sturdy and good-looking, especially for very small gardens.

What's happening in a compost pile?
   The mixed stack of grass clippings, weeds, and non-food parts of crops (carrot tops, etc.) will gradually decompose as soil microorganisms and larger soil-dwellers, including worms, work on it. The secret to composting is to blend a mix of "browns" and "greens." The "greens" are high in nitrogen, including grass clippings, non-food vegetable trimmings, and manure. The "browns" are fallen leaves, twigs, and sticks, pine needles, sawdust, straw, hay, wood shavings, and shredded newspaper, which are higher in carbon.
   Without a balance of "browns" and "greens," the composting process doesn't proceed efficiently. Anyone who has ever left a stack of fresh wet grass clippings in a pile without any other additions knows that the result is a stinky slimy mess. But if the same grass clippings are tossed with an equal volume of sawdust, shavings, or fallen leaves, the breakdown leads to the desired brown, rich, and fresh-smelling soil amendment.
   At the other extreme, a pile of leaves or sawdust without any "greens" will break down very slowly and be of little use as compost. So think of the process as a mixing of nitrogen-rich and carbon-rich elements. Toss the ingredients together with a fork as you build the pile.

Compost ingredients
   Fresh manure is an excellent addition to compost piles, though using refuse from the family cat or dog or any carnivore isn't recommended. Bedding from horse and cow stalls may contain a mixture of wood shavings and manure, and can be an excellent addition to a compost pile. Besides area horse boarders and farms, the Woodland Park Zoo, and even the Seattle Mounted Police horses produce valuable manure that can often be picked up.
   It's not necessary to have the manure, but for success you need some "greens." Collect grass clippings. Freshly-cut grass clippings work as well in the composting process as manure does, especially when mixed with an equal volume of fallen leaves. My three compost bins, in a small garden in Seattle, are filled year-round with just this mixture, from grass clippings dropped off regularly by a lawn service and fallen leaves that I collect and bag in autumn. With no other additions, this mixture produces excellent usable compost.
   If there are lots of grass clippings in your immediate future, get a few bags of sawdust to mix with them. This combination makes a fine soil amendment when broken down. The mix, when first tossed together, will begin to heat from the effect of soil microorganisms growing, metabolizing, and multiplying. The pile may heat up to about 130 degrees Fahrenheit in the center. Commercial compost manufacturers are able to maintain high heat by regular turning of compost; home gardeners are less likely to keep a high temperature for long. The heat accelerates the breakdown of materials.
   Be cautious when using grass clippings from lawns treated with weed-killers. The herbicide residue must be thoroughly broken down before the compost is spread on the garden; opinions vary about how long this takes, but to be safe, allow at least 9 months.
   Heat can also kill weed seeds and disease organisms, but many of them slip through in a home compost. For this reason, we recommend not composting weed seeds, or any leaves or other plant materials with diseases such as rose black spot or dogwood anthracnose. Kitchen food wastes don't make good additions to compost. They attract rodents and other wandering animals, and may develop flies. If you want to compost kitchen scraps, build a worm bin, or get a bin such as a "Green Cone," which is completely enclosed to repel rodents. Never add meat, dairy, or fatty food wastes to any type of compost.
   It's also not necessary to buy packaged "compost starters." The microorganisms which do the work are present on each leaf and grass blade. Some gardeners add a scoop of garden soil to the mix now and then, but this too isn't necessary. If there are more "browns" present than "greens," gardeners may add nitrogen fertilizer like blood meal, cottonseed meal, or ammonium nitrate. Experience with the process will determine how to mix the ingredients for best results.

The finished product
   The end product may take six weeks, several months, or a year to be finished, depending on how good the green-brown balance is and whether you turn the piles to keep the heat up. Keeping the pile covered helps with the process and keeps the pile from leaching.
   My own compost bins are filled in fall with leaves and grass clippings, and dug out in spring, with only one turning. That one turning wouldn't be necessary, but it's enjoyable! The finished compost is dark, earthy-smelling, and a terrific soil amendment. Another clue to finished compost is that the original materials are unrecognizable.

Uses for compost
   Compost isn't fertilizer, though it does contain some plant nutrients. When dug into the soil, it improves water-holding capacity, and helps with improved soil structure (it will, for instance, added over a number of years, help to "open" clay soils).
   Put through a screen, it makes a good addition to potting soils (use about 1/4 screened compost to 3/4 potting soil). Add some perlite or pumice for enhanced drainage. Spread it on the garden as a mulch at 2-3 inches per year.
   Gardeners call compost "black gold," which might seem an exaggeration until you've worked for a while with the riches of decomposition. For the King County Compost Hotline, call (206) 296-4466. The Master Gardener information line can also help. Call (206) 296-3440.

Visit WSU's Agriculture Site on the World-Wide Web at http://www.cahe.wsu.edu.