Dear Mike: I generally listen to your sage advice as I perform the weekly cleaning duties in my furniture shop. As you can imagine, the shop generates a LOT of sawdust. Can it be used for anything useful in a garden way? I mostly work with Cherry, Walnut and Ash and currently dump the sawdust in randomly located piles in the woods behind my shop. It would be great to find some better use for the stuff. Thanks for your input,
- ---Laron (rhymes with Aaron) in New Carlisle, OH
- ---Brent in Gaithersburg, MD
And don't use any sawdust! If you mix sawdust into your soil, nothing will grow there for a year or more. Pure wood materials like sawdust and wood shavings are super-high in carbon, and their carbon will absorb all of the plant-feeding nitrogen in your soil in its quest to decompose. After it DOES decompose, the soil WILL be richer, but for that first year or two it'll be a plant graveyard.
It's even tricky to compost the stuff. As I often explain, the best compost is made by combining carbon-rich "dry brown" material, like shredded Fall leaves, with "wet green" sources of nitrogen, like grass clippings and kitchen waste. Sawdust is a 'dry brown' material, but it's a much more highly concentrated form of carbon than leaves.
When you combine the recommended four parts of shredded leaves and one part green waste, it's fairly easy for most of the dry brown material to come into contact with most of the green waste. But when you're talking sawdust, you'd have to limit yourself to VERY small amounts to avoid going way out of whack on approximating the correct 30 to 1 carbon-to-nitrogen ratio.
I'd guess that if you were using kitchen waste for your green material, the correct ratio would be a cup or two of sawdust to about five gallons of garbage. It just wouldn't work; there isn't enough carbonaceous material to 'touch' most of the garbage and start the composting reaction going.
Theoretically, you could mix up equal amounts of sawdust and a very hot nitrogen material like blood meal and get it to cook. But it doesn't make much sense financially, and you wouldn't get much compost for your trouble. And I can't imagine trying it with kitchen waste or other low-nitrogen materials. Many listeners have told me they tried mixing saw dust or wood shavings with green waste in one of those tumbling drum systems that allows for easy turning and it just sat there, despite their giving it a good tumble several times a day.
BUT this does NOT mean that you can't compost your wood waste! Wood IS a natural substance and it will become a soil-like material; just not in the average home compost pile or drum system. The easiest answer is to just pile it up and allow it to break down naturally, which will take several years. As with all compost piles, the stuff on the bottom will be ready first, so I'd check things out close to the ground after a year or two.
You should be able to reduce the time involved by mixing in some nitrogen-rich material and turning the pile on a regular basis. Coffee grounds are a great choice; they're nitrogen (and calcium) rich and should be investment-free. Any café you frequent should be happy to slip you some, and Starbucks stores have a "Grounds for Gardeners" program where they give away their used coffee grounds in five-pound sacks.
Other high-nitro items you could use include blood meal (available bagged at garden centers), crab and shrimp shells (free for the taking but difficult to work with and very attractive to varmints) and bat and sea bird guanos (also available bagged; or you can offer to shovel out the Bat Cave while Alfred is on vacation).
If you have lots of sawdust, you should definitely give it a try. Experiment and be patient; and remember that no matter what you do, it will take a while to break down, and only very rich sources of nitrogen will help it do so faster. This is no place for kitchen scraps.
Oh, and I would hope that this is obvious, but don't use pressure treated wood, old railroad ties or other toxic wood in any form. Any sawdust from treated wood should be disposed of safely and legally—not in your landscape or even in the woods.
And finally, I think it would be a real good idea to separate out the sawdust from that walnut wood. As you probably already know, black walnut—the type most often used in woodworking—contains juglone, a naturally occurring compound that stunts the growth of (or just plain kills) many other plants, especially tomatoes and other popular backyard crops. Although the concentrations are highest in the roots, there is some juglone in every part of the tree, and compost made from black walnut sawdust might send some of your most prized plants to sing in the Choir Invisible.
Keep a bucket labeled for walnut handy, and use that sawdust as a killing mulch on plants you want to eliminate.