Sometimes it’s damn hard to keep up with the current best thinking on gardening practices, and the question of whether wood chip mulch is good or bad for our plants is a case in point. After what thought was a lot of research, I came down against it in my page about mulch and mulching, recommending instead the use of shredded pine or leafmold mulch. (Around plants, that is. Wood chips on paths are indisputably okay.)
Now here comes Master Gardener Magazine with an article about wood chip mulch by Linda Chalker-Scott (Ph.D., Extension Urban Horticulturist and Associate Professor, Washington State University) to contradict conventional wisdom. (Those durn scientists can be SO annoying, with their namby-pamby reliance on the scientific method and peer review and all that stuff for sticklers.)
Here’s what she has to say: In tests (something that apparently scientists are still doing) wood chips perform with the best of all possible mulch materials for moisture retention, temperature moderation, weed control,sustainability and enhanced plant productivity. And what’s more, in urban areas they’re often FREE.
Drawbacks? What drawbacks?
Referring to the reported drawbacks of wood chip mulch, she calls them "much ado about nothing". The concern that wood chip mulches can tie up introgen and cause deficiencies in plants, it turns out that studies show that it actually increases nutrient levels in soils and the foliage of plants. "My hypothesis is that a zone of nitrogen deficiency exists at the mulch/soil interface, inhibiting weed seed germination while having no influence upon established plant roots below the soil surface." For that reason, she recommends against wood chip mulch around plants with shallow roots – annuals and vegetables.
Even if you remain unconvinced by the research, you can still use wood chips on top of a more nutrient-rich underlayer (say, of compost). This "mulch sandwich" approach mimics what you’d see in the mulch layer of a forest.
Here’s what surprised me – her recommendation that 4-6 inches of the stuff be used. That’s because "A review of the research on coarse organic mulches and weed control reveals that shallow mulch layers will promote weed growth and/or require additional weed control measures." Again, a divergence from lots of other writers recommending 2-4 inches, or even a maximum of 2 inches.
But what about soil structure?
Now here’s something the article didn’t addres – the impact of mulch on ability of the soil to hold moisture and other benefits of what we call good soil structure. Do wood chips decompose fast enough to improve soil within, say, a year? I’ll shoot the link to this post to the good folks at Master Gardener Magazine and see if we can get a response.