Ken Schmidt calls them "mulch volcanoes." And while they don't spew molten lava or stand thousands of feet tall (like Mount Hood, above), they definitely spell bad news for trees in the landscape.

A landscape architect with the prestigious landscape architecture firm of Mahan Rykiel Associates in Baltimore, Ken bemoans the widespread practice of piling up ever-growing "volcanoes" of mulch around trees. Instead of helping trees grow, overdoing the mulch does just the opposite.

Mulchy See, Mulchy Do

Why do so many people pile up mulch around trees every year without thinking? It's a copycat crime. Nothing is more persuasive than seeing your neighbor do something with confidence. It's even worse when you see a landscape contractor do it.

"Right now, I'm looking out my window across the street at a willow oak that has 8 inches of mulch piled against the trunk for no reason," he says. "It sends the wrong message to the community, because they think it's the right way to do it."

The Right Way to Mulch

Mulching with organic matter – whether it's shredded hardwood bark, shredded cypress, ground pine bark, or pine straw – can be a boon to trees. It keeps down weeds (preventing damage to trunks from lawn mowers and trimmers), cools roots, and helps the soil retain moisture. Plus, as mulch slowly decomposes, it adds organic matter to the soil, enriching it.

Grumpy recommends planting a tree so that the top of the root ball is ½ to 1 inch above the soil surface. When the loosened soil settles in the hole, the top will be about even with the soil surface. (Planting too deeply, so that the top of the ball is below the soil surface, is a big no-no.) Then cover the ball with a layer of mulch 2 to 3 inches thick. No more. Depending on the size of the tree, the diameter of the mulched area around the tree could range from around 3 to 6 feet. Mulch should not come in contact with the trunk.

tree mulch
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| Credit: Grumpy Gardener

Grumpy Gardener

A properly mulched tree lets air and water reach the roots, while keeping down weeds.

The Wrong Way to Mulch

Many people mulch too deeply. They either apply way too much to begin with or pile new mulch atop last year's mulch that hasn't fully decomposed. So instead of the desired 2 to 3-inch layer, the layer grows to 6 inches, 8 inches, and even 10 inches deep.


Here's a typical "mulch volcano." Notice the mulch piled up against the tree trunk.

What harm does deep mulch do? Let Grumpy count the ways.

1. Roots need oxygen in order to work. Burying them deeply under mulch decreases the oxygen supply.

2. Lack of oxygen prevents beneficial soil microbes from breaking down the mulch. Instead, harmful soil microbes produce substances that are toxic to roots.

3. Trees surrounded by mulch volcanoes may send shallow roots into the mulch, instead of the soil. This makes them more susceptible to drought, stress, and wind.

4. Piling up mulch against the trunk provides a safe haven for chewing voles and harmful insects and can promote rot due to excess moisture retained by the mulch.

Can We Talk?

While we're on the subject of bad mulching, here are some other heinous practices you should avoid at all cost.

1. Using red-dyed mulch (below). As Ken notes, "The color draws your attention to the mulch instead of the plantings." Think of it this way. How many of you have a bright orange toilet in your master bedroom, so it's the first thing that folks look at when they enter? (Although to be fair, orange would help at night.)


Notice how the circle of red mulch draws attention away from this stunning fire hydrant.

2. Using black-dyed mulch. Black absorbs heat in summer. So instead of cooling the roots, your mulch does the opposite.

3. Using shredded rubber mulch. Yep, it's recycled. But do you really want your yard smelling like a NASCAR track on a hot summer day?

4. Using lava rock mulch. Where do you think you live? On the slopes of Krakatoa? Now that's what I call a mulch volcano!


Thanx to giveawayguy for the cool fire hydrant shot and Mahan Rykiel for the mulching pix.