Americans love mulch — and many of us are misusing it
Adrian Higgins writes about mulching for Local Living (Marvin Joseph/TWP) By Adrian HigginsAdrian Higgins Gardening columnist September 13, 2017
Autumn is knocking on the door, an optimal time for certain annual tasks in the garden — reseeding a threadbare lawn, planting trees and shrubs and dividing tired perennials.
But these tasks seem to spur the most fastidious among us into further action by cutting back every waning perennial and annual and then opening the floodgates for a sea of mulch. This yard scrubbing leaves the neatnik poised and ready to intercept the very first leaf to yield to gravity.
Mulches come in many forms, but they all seem to share a power of mind control over otherwise sensible folk. Each year, we spend an estimated $1 billion to cover soil with decaying organic matter, and I can’t help but think that much of it is unnecessary and even harmful to our plants.
For sure, mulch has its place. A thin layer, an inch or two, will go a long way toward preventing weed germination, retaining moisture in the soil and supporting moderate soil temperatures. As it decays, it adds beneficial humus to the earth.
If I go to my rest having eliminated the phenomenon known as the mulch volcano, I feel my life will have been worth it. I’m joking, of course; no one will bring that practice to an end.
The volcano is that mound of mulch piled at the base of a tree trunk. Widely embraced by many landscapers and homeowners who emulate them, the mulch volcano has no relationship to the needs of the tree. Instead, mulch piled against the lower trunk promotes decay of the tree’s protective bark, harbors gnawing rodents and causes roots to grow detrimentally.
Where did the volcano come from? When a tree is newly planted, the root flare at the base of the trunk should be set an inch or two above grade, and the soil around it should be formed as a dish with a circular lip about three feet in diameter. This traps water and feeds it to the tree’s newly planted root ball. You then place mulch over this saucer to retain moisture, but not with any mulch against the trunk. At some point, this morphed into a trunk scarf.
We picked on a redbud tree to show a mulch volcano, which if left in place will damage the base of the trunk. (Adrian Higgins/TWP)
The correct method, forming a rain-catching dish over the roots but with no mulch against the bark. (Adrian Higgins/TWP)
Casey Trees, the nonprofit organization founded to help restore and maintain the District’s urban forest, uses a mantra of “three-three-three,” said its executive director, Mark Buscaino. That is, a mulch circle that is three feet in diameter, three inches thick and three inches away from the trunk. “For some reason, people think mulch volcanoes are aesthetically appealing, so landscape companies aren’t willing to change their practice,” Buscaino said.
After a few years, tree roots extend way past the tree’s canopy, so the only value in mulching around the trunk would be to keep away lawn mowers and string trimmers. Yes, it’s hard being a tree.
The other big but less obvious mistake with mulching is in laying the stuff too thickly. When you do this, roots of trees and shrubs either become smothered by the mulch and clamor for air and water or begin to grow into it. In addition, researchers have discovered that excessive amounts of hardwood mulch cause manganese and other elements to build up to levels that are toxic to plants.
Because mulches fade in sunlight and as they decay, some people replenish mulch too often, increasing the thickness of it and compounding the blanket effect.
There is an art to laying mulch, a skill that goes hand in glove with not applying it too thickly. In addition to keeping it away from tree trunks, you should make sure mulch doesn’t build up at the base of shrubs, especially such surface-rooted bushes as azaleas and boxwood. Mulch piled against shrubs will cause them to grow roots into the mulch. When the mulch decays, the roots are left high and dry.
The top growth of perennials withers in the fall and winter, but the crown of the plant remains year-round at or just below soil level. This is where the buds for next year’s growth are generated. If you smother a perennial in mulch, you upset the plant’s biology and run the very real risk of keeping the crown wet during winter dormancy, and it will rot.
I avoid organic mulches on perennials that need free drainage and good air circulation, such as bearded irises, lavenders, rosemary and sages.
In recent years, consumers have become mesmerized by mulch that has been dyed. The most popular colors are brown, red and black. What’s the allure? Apparently, some people don’t like the idea of their mulch growing pale. Some brands guarantee that their colors won’t fade for a year.
There is a precedent for this taste. At certain periods in landscape history, gardens were decorated with colored glass, pebbles and stone chips, to form embroideries on the ground. I think of dyed mulch the same way, as a way of having unchanging patterns and hues upon the land. It seems a perfect solution for those who want a garden but don’t want to fuss with plants.
Most gardeners I know are uncomfortable with dyed mulch, and not just because it subordinates the role of plants. While traditional hardwood, softwood and bark mulches are byproducts of the timber industry, some dyed mulches rely heavily on recycled wood as their source. The worry is that there may be preservatives and other contaminants in the mulch that you wouldn’t want in your garden.
Robert LaGasse, executive director of the trade group that certifies mulch products, the Mulch & Soil Council, said that “there’s nothing wrong with recycled material if you do it correctly.” Bags of mulch that bear the council’s seal have been tested to ensure that they don’t contain lead paint, chromated copper arsenate and other contaminants found in old building materials. Look for the council’s imprimatur.
I asked Roger Davis, a senior gardener at Longwood Gardens in Kennett Square, Pa., what his ideal mulch would be. He offered two. He likes triple-shredded hardwood bark, which is dark and fairly fine but doesn’t break down as quickly as simple shredded hardwood. He also likes leaf mold, which consists of tree leaves that are halfway toward becoming dark, crumbly compost. You can buy bagged leaf mold or, in many jurisdictions, get it from your local government in the spring as the previous fall’s leaf collection. Downside: It may have some trash in it.
Better yet, you can make your own leaf mold. Gather the fallen leaves of autumn, shred them with your mower and either put them in a pile to compost or in plastic bags to store. The resulting leaf mold can be used as a mulch next spring.
Alternatively, just spread the chopped up leaves on your garden beds this fall. By mincing them, you get a neater look, and the shreds are less likely to blow around. Everything about this practice seems right. The soil creatures convert the leaves into humus, you’re keeping yard waste on site, and the price is right.
Mulch to consider
Mulch is a broad term for a soil covering and includes anything from straw to sheets of plastic landscape fabric to, egad, recycled automobile tires.
I like pea gravel for plants that need great drainage and a dry soil surface. Elsewhere, organic mulches are the way to go, but what to choose? Before we dig into them, we should mention wood chips, which are not pictured. These are the large, pale shards of freshly cut trees right out of the commercial chipper-shredder. They are good for woodland and garden paths but not as a soil covering. They are too coarse to use as a mulch or soil amendment and rob plants of soil nitrogen as part of their decaying process. They take at least a year to break down.
Pine bark nuggets (Marvin Joseph/The Washington Post)
Pine fines (Marvin Joseph/The Washington Post) Pine bark nuggets
What: Chunks of red-brown chipped pine, often sold in different sizes.
How: This is a byproduct of the softwood lumber mill.
Why: Effective, attractive and long-lasting natural mulch.
Verdict: The largest nuggets read as too big in a small bed of fine-leafed plants, and they tend to wander on sloped beds. Otherwise, we like them. What you see is what you get.
What: Small particles of aged pine bark.
How: Shredded to resemble coffee grounds, though the fineness differs by brand.
Why: Small, dark and handsome.
Verdict: An appealing mulch of soil-like texture that avoids the clunky mulch look in small urban gardens. Perfect for acid-loving plants such as azaleas, hollies and rhododendrons. Not as effective as other mulches in weed control but can double as a soil amendment.
Shredded hardwood mulch (Marvin Joseph/The Washington Post)
Brown mulch (Marvin Joseph/The Washington Post) 3. Shredded hardwood mulch
What: The wood of deciduous trees ground into shreds. Double-shredded hardwood has been through the grinder twice.
How: Whole logs are shredded in 1,000-horsepower machines called tub grinders. Aging gives the mulch a darker appearance.
Why: An attractive workhorse.
Verdict: The shards knit together, making it a good choice for hilly beds. It decays more quickly and loses its color faster than shredded hardwood bark (not pictured). Stinky mulch, improperly stored, may harm plants.
4. Brown mulch
What: Shredded wood dyed to stay ageless.
How: Colorants are added during the shredding process.
Why: It will look fresh for a year.
Verdict: The Dorian Gray of mulch. It may contain recycled products such as wood pallets. Reputable companies avoid demolition debris containing harmful chemicals. Look for the Mulch & Soil Council seal.
Red mulch (Marvin Joseph/The Washington Post)
Black mulch (Marvin Joseph/The Washington Post) 5. Red mulch
What: Shredded wood dyed to look red.
How: The pigments are added during the shredding process.
Why: Good question.
Verdict: May clash with your purple azaleas.
6. Black mulch
What: Shredded wood given the ebony touch.
How: Pigment added during grinding.
Why: Brings a touch of the noir to the landscape.
Verdict: The color seems likely to increase soil temperatures, not necessarily a good thing in hot-climate states. Still, the Johnny Cash of mulch.
Leaf compost (Marvin Joseph/The Washington Post) 7. Leaf compost
What: Leaves converted to humus by time and microbes. Buy it bagged or in bulk or make your own.
How: Shredded and aged in piles.
Why: Nothing more natural, and the soil creatures love it.
Verdict: Leaves can be added fresh (but best shredded) in fall, as half-rotted duff called leaf mold, or fully composted and soil-like. The last will form a barrier to weed seeds in the soil but not those that blow in later. Excellent for use in vegetable and annual beds, which are turned at least once a year.
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