As spring returns to Northern Virginia, homeowners busy themselves with garden chores. Tune up the lawnmower, rake leftover leaves, and sneeze at the tree pollen. But there is one job that many undertake that can cause more harm than good — mulching.
"Without a doubt, mulching is the most misused horticultural practice in the country," said Dean Norton, chief horticulturist at the Mount Vernon Estate. "Improper mulching will shorten the lifespan of trees and shrubs by half."
James Gagliardi, the horticulturist at the American Horticultural Society’s River Farm, agrees. He blames poor mulching habits on homeowners copying what they see commercial landscaping companies do on public and private land. "If people see a ‘mulch volcano’ around the base of a tree, they are inclined to do the same at home."
Gagliardi explains that there are benefits to mulching:
* Conserve moisture in the soil.
* Moderate temperatures — it helps cool the soil in summer, warms it in winter.
* Suppresses weeds.
* Provides a unifying look in a garden.
But given Mr. Norton’s fairly direct assessment, a few mulching "Dos" and "Don’t" gathered from local experts might help folks as they prepare to mulch.
* Apply only two or three inches of mulch around trees and shrubs.
* If planting a young tree, create a donut-shaped mulch well around the trunk to concentrate water on the root ball.
* If planting annuals or vegetables, the U.S. Department of Agriculture recommends waiting for soil temperatures to rise before mulching.
"Never, ever let the mulch contact the base of a tree or shrub," said Jim Luby, manager of Holly, Woods, and Vines, an Alexandria nursery. A retired supervisor at the U.S. Botanic Garden, Luby says piling mulch up the trunk promotes rot by holding moisture against the bark, deprives the plant’s stem of oxygen, harbors disease, and provides a habitat for rodents intent on eating the tree bark. Norton says the practice also invites the tree to seek moisture from the mulch, creating shallow girdling roots instead of spreading the root structure outward.
* Using raw, unseasoned mulch or wood chips without adding fertilizer to the soil creates problems. "Organic mulches such as shredded hardwood, need to decompose for a year," Gagliardi says. "If the material is too green, it robs nitrogen from the soil as it decays, thus depriving the plant of the nutrient."
* If applying grass clippings as mulch, Luby recommends spreading them thinly. Thick, matted clumps prevent water from reaching the plant, and smell badly when oxygen can’t penetrate the mass to aid in decomposition.
* Stop piling new mulch over the old. It’s true that organic mulches will gradually decay and improve the underlying soil. However, the International Society of Arboriculture recommends removing old mulch around a tree base before adding new material. If not, repeated applications create the volcano shape. The trunk stem becomes buried and the tree dies.
"It’s discouraging that some landscaping companies are creating bad mulching practices," Norton said. "They should be setting a better example."
Mount Vernon resident Sandy Bain has seen it the same way, recently firing her landscaping company for over-mulching. "Look at that mound," she said, pointing at a tree in her front yard. "That’s ridiculous."
Lastly, the Landscape Taste Police caution against artificially dyed mulch. They suggest staying with Mother Nature’s paint pallet.
For more mulching information visit Virginia Tech’s Web site, http://www.ext.vt.edu/resources/, and click on Home Gardening, and then Landscape Maintenance. Also, see USDA at http://www.nrcs.usda.gov/feature/backyard/Mulching.html.