Sawdust, Great Stuff

Some years one of my last gardening activities for the season is ‘dusting. ‘Dusting can always wait till spring, but then the weather rushes me along with so many other activities. Now I’m not talking about ‘dusting in the usual sense, which is getting rid of it. The kind of ‘dusting I’m talking about is adding dust — as sawdust.

Sawdust is a terrific material for enriching the soil with humus, which sawdust eventually becomes. Leaves, straw, hay, and other once-living things can also do this and do it well, but sawdust has the advantage that it is weed-free and the small particles mix intimately with the soil. Another advantage of sawdust is that its beneficial effects in the ground last a relatively long time.

Sawdust mulched blueberries

Sawdust is also cheap, often free for the hauling, from sawmills, lumberyards, or from your or a friend’s workshop. If you cannot beg use of a friend’s pickup truck to haul the stuff, sawdust is easy and light enough to shovel into garbage pails or bags, then throw into the trunk or backseat of your car. All kinds of sawdust are suitable except, of course, sawdust from plywood or any wood chemically treated to resist decay. And although there are mixed opinions (few facts) on the use of black walnut sawdust on plants, I have used it as a mulch with no ill effect.

On Top, or Under?

Once you get a load of sawdust, there are two things you can do with it: spread it on top of the soil, or dig it in. I’m generally not in favor of digging soils for any reason (for many reasons spelled out in my book Weedless Gardening because it exposes buried weed seeds to light and air so they germinate and become weeds, it breaks up capillary channels that move air and water around in the soil, and it burns up humus. (My book is currently out of print but an extensively revised and updated version will be available April 2027.) Better to lay out the garden with permanent walkways and planted areas, then blanket planted areas with sawdust as mulch. There, the sawdust will modulate soil temperatures in winter and summer, seal in moisture, soften the impact of raindrops so water percolates into the ground, and gradually be transformed into humus.

There is an exception to this “no dig” rule, and that’s for a very, very sticky clay soil, one that’s rock-hard when dry and sticky goop when wet. Such a soil needs ‘dusting, with as much as a 6 inch layer of sawdust mixed into the top foot or so. Peat moss would also do the job, but would cost a fortune at this rate of application and the environmental impact of harvesting peat moss is questionable. Note, though, that this is a one time digging. You lay out planted areas, dig once, then never dig or walk on planted areas again.

Nitrogen Starvation?

I’m sure many of you knowledgeable gardeners are now muttering about sawdust temporarily robbing plants of nitrogen. You’re right, nitrogen starvation of plants can result because microorganisms that transform sawdust to humus need nitrogen to do so, and these microorganisms are better than plants at getting that nitrogen from the soil.

N deficiency symptom, older tomato leaf

The solution to this problem is not to shun sawdust, but simply to add some extra nitrogen when you do use it. A rough guide would be to sprinkle 4 pounds of any fertilize having about 10 percent nitrogen over every 100 square feet of ground which is going to have a 6 inch depth of sawdust dug into it. Throw on a pound and a half of limestone also, to offset the temporary acidity from the sawdust.

Sawdust that has aged and darkened should need no additional amendments when dug into the ground. That aging darkens the material and, over time, has “burned off” some excess carbon.

No need for any concern about nitrogen when you use sawdust as a mulch. Laid on top of the soil, sawdust is slowly transformed to humus at the thin interface where the soil and sawdust meet.

The best testimonial to the benefits of ‘dusting are my blueberries, under which I have renewed a 3 inch blanket of sawdust every year for decades. Besides other benefits, the sawdust mulch has buried disease spores, so my plants get little mummy berry disease. It also keeps the soil moist. Rhododendrons, azaleas, mountain laurels, and heaths, like their blueberry kin, are also especially fond of sawdust mulch.

Sawdust mulched blueberries