Wood Chip Mulch: A Boon for Gardeners

Even before the soil could be worked this spring, I indulged my impulse to dig – shoveling into a pile of wood chips that an arborist delivered last fall. The chips were sodden from snowmelt and did not yet have much visible mycorrhizal fungi, those delicate white veins revealing the amazing fecundity of microscopic life that colonizes decomposing wood. But even relatively fresh wood chips are chock-full of potential soil nutrients and will function like time-release vitamins to nourish trees, shrubs and deep-rooted perennials.

Arborist wood chips are a remarkable commodity that requires no commerce, being available free – either delivered as a courtesy by arborists (including those contracted to clear utility right-of-ways) or as a byproduct of on-site tree work.

We first began mulching with wood chips soon after acquiring our land and clearing a former pasture that was overtaken by alder and saplings. A grueling day spent with a rented chipper produced a mountainous chip pile. We kept our fingers intact that day but, not wanting to press our luck, have left chipping to the experts ever since.

In the intervening years, we’ve welcomed many loads of arborist wood chips and can attest to how satisfied the botanical recipients have been. Our modest-sized berry patch, which gets a fall infusion of manure and wood chips, delivered 32 quarts of raspberries last season.

But don’t take my word for the efficacy of wood chips; they have been field-tested by scientists. Linda Chalker-Scott, a longtime Cooperative Extension professor at Washington State University, compiled a review summarizing studies on the comparative benefits and drawbacks of various mulches. While many homeowners think of mulch primarily in terms of weed control and yard aesthetics, she points out it can serve a multitude of functions: help maintain optimal soil temperature (by buffering extremes); improve soil moisture and nutrition; reduce erosion; and even diminish the toxic effects of roadside salt.

Chalker-Scott cites one study of 15 different organic mulches for landscape trees and shrubs that found local arborist wood chips to be “one of the best performers in terms of moisture retention, temperature moderation, weed control and sustainability” (being renewable, and involving no packaging and minimal transport). Due to their slow decomposition and coarse texture, which allows water to permeate but does not compact as finer-textured mulches do, she notes that wood chips are “cited as superior mulches for enhanced plant productivity.”

Traditional gardening guidance may suggest that wood chips can acidify soils, lock up nitrogen, damage established plants or introduce pests. Research trials refute each of these claims as Chalker-Scott elaborates in the review. The concern with nitrogen deficiency, for example, applies only in the top inches of soil just beneath the mulch. As long as gardeners avoid using wood chips on plants with shallow root systems (such as annual flowers or vegetable crops), tilling it into soil or using it in backfill when planting, there’s no problem. As for drawing ticks, she writes, “dry-surfaced mulches, such as gravel and wood chips, are (actually) recommended as deterrents to deer ticks.”

Some savvy gardeners, like Sharon Turner of Washington, Maine, base their entire gardening philosophy and practice around mulching, much of it with wood chips. Turner grows shrubs and trees for Fedco and a variety of plants for direct retail, and she teaches classes in landscape design. (I’ve bought plants from Sharon and visited her land, which is reminiscent of “The Secret Garden” with pink rose bushes that spiral up into the branches of a tall peach tree.)

“My mulching fetish goes all the way back to Ruth Stout” (author of the 1955 classic “How to Have a Green Thumb without an Aching Back,”) Turner explains. “Permanent mulching is the way to go. No one tills the forest.” The idea is to mimic in gardening the accumulation of surface material that so perfectly nourishes natural habitats, without any unnecessary disturbance of the soil.

Turner is careful about the source of her chips, getting them from a local sawmill or from arborists rather than relying on sources that might incorporate construction debris with potentially toxic contaminants.

The benefits of mulching, she adds, depend on applying it right. Avoid the volcanic mounds favored by yard-care companies; pull chips several inches away from any trunk and extend them in a level layer out to the drip line (below the outermost edge of the plant’s canopy).

The appearance of wood chip mulch may not appeal to everyone but I have grown to like the texture and silvery gray tones it assumes with time. To me, it looks like a healthy ecosystem in the making.

© Marina Schauffler, 2018. All Rights Reserved. Column reprints available upon request.