Be a Better Gardener: More About Mulches

by Thomas Christopher

In a recent column, I proposed mulches composed of living plants as an environmentally friendly alternative device for suppressing weeds, enhancing garden plant growth and nurturing the soil. I suspect, though that many readers prefer the look of the conventional mulches. To secure information about the best of these, the advantages and disadvantages of the different kinds, I turned to Dr. Linda Chalker-Scott of Washington State University.

Dr. Chalker-Scott is an extension specialist in urban horticulture who actually began her career as a biologist before switching to horticulture for her doctorate. She kept her devotion to the scientific method and continues to apply it to her scrutiny of horticultural practices. She has become a well-known gardening myth-buster and was a founding member of “The Garden Professors” (gardenprofessors.com), a blog that has since 2009 used scientifically derived studies to assess the science behind common gardening practices. I like Dr. Chalker-Scott not only for her meticulous accuracy but also for the fact that she is notably un-pompous.

I began by asking her about a traditional favorite. Bark had been a waste product of the lumber industry typically disposed of by burning; when clean air legislation prohibited that practice in the 1970s, the forest products industry discovered that by shredding the bark and packing it into plastic bags, it could sell the result as a covering for garden beds. Bark’s attractive appearance soon made it a gardening favorite for suppressing weeds and conserving moisture in the soil.

The irony, Dr. Chalker-Scott said, was that bark actually does these things poorly. It evolved as a protective covering for trees and is repellent to water so that it can keep the wood within trunks and branches from dehydrating. As soon as a bark mulch has settled, it actually sheds water like a raincoat. Bark mulches also inhibit the movement of gases into and out of the soil, excluding the oxygen needed by plant roots and the soil microflora and fauna from moving, and preventing the escape of carbon dioxide, a waste product of that life.

A mulch of straw is far more beneficial, and its relatively rapid decomposition means it is superior for providing plant nutrients and organic matter to the soil. This makes it ideal for vegetable gardens. Hay, Dr. Chalker-Scott warned, is full of weed seeds and shouldn’t be substituted for straw. Compost, another rich source of nutrients and organic matter, unfortunately, provides a perfect seed bed for weeds. Apply it only to badly degraded soils, Dr. Chalker-Scott advised, and then cover it with some sort of more weed-resistant mulch.

Dr. Chalker-Scott’s favorite mulch is the wood chips produced by arborists after felling or pruning trees. Applied four inches deep, it blocks weed growth, allows the passage of gases into and out of the soil, lets water infiltrate, and because the wood is typically mixed with leaf fragments, it provides a balanced blend for composting in place. Contrary to popular belief, she added, a wood chip mulch does not typically harbor tree pests or diseases and does not provide habitat for termites. Termites, Dr. Chalker-Scott said, actually prefer cardboard as food. Laying down sheets of cardboard before covering the area with wood chips – a process called “sheet mulching” – not only makes the mulch more termite attractive but also inhibits the movement of gases and water. Nor does it enhance the weed-suppressing characteristic of the wood chips.  Because wood chip mulch mimics the natural litter of a forest floor, it’s especially beneficial for woody plants.  Dyed wood chip mulch, made from grinding up old shipping pallets or construction timbers, is a close second to arborist wood chips in the benefits it provides, and its neat appearance appeals to many gardeners.

One final tip from Dr. Chalker-Scott was that “volcano mulching,” the practice of surrounding trees with cones of mulch, may be unsightly, but if the mulch is something porous such as wood chips, it doesn’t typically injure the tree as is popularly believed.  Dr. Chalker-Scott admitted that she shared this belief until she began investigating trees supposedly killed by this practice. What she found was that the death of the tree was usually the result of poor planting.  Commonly, the hole that had been excavated was too shallow so that the tree’s root ball sat up too high.  The mulch simply served as a coverup.

To listen to the rest of Dr. Chalker-Scott’s recommendations about mulch, log onto the Berkshire Botanical Garden’s free “Growing Greener” podcast at https://www.berkshirebotanical.org/

Be-a-Better-Gardener is a community service of Berkshire Botanical Garden, located in Stockbridge, Mass. Its mission, to provide knowledge of gardening and the environment through a diverse range of classes and programs, informs and inspires thousands of students and visitors each year. Thomas Christopher is a volunteer at Berkshire Botanical Garden and is the author or co-author of more than a dozen books, including Nature into Art and The Gardens of Wave Hill (Timber Press, 2019). He is the 2021 Garden Club of America's National Medalist for Literature, a distinction reserved to recognize those who have left a profound and lasting impact on issues that are most important to the GCA. Christopher’s companion broadcast to this column, Growing Greener, streams on WESUFM.org, Pacifica Radio and NPR and is available at berkshirebotanical.org/growinggreener.