Mulch helps trees, but avoid forming ‘volcano’ around trunk
Although they don’t spew ash or ooze lava, mulch “volcanoes” wreak their own kind of havoc.
Mounded against tree trunks in sloping piles suggesting a miniature Mount St. Helens, too much mulch can injure or even kill a tree.
That’s just one of the morsels about mulch that Eric Brownlee, a certified arborist and manager of the Columbus office of Bartlett Tree Experts, offered during a recent phone conversation.
The takeaway: Applied correctly, mulch can be a tree’s best friend; incorrectly, the tree will wane.
“Trees that have volcano mulch will produce roots that circle the tree,” he said — roots that might eventually choke the trunk.
Because such roots can end up impeding the flow of water and nutrients, “it’s just like if you have a rubber band really tight on your wrist.”
In addition, volcano mulching prevents air from reaching the bark.
“The trunk of a tree isn’t designed to be consistently moist,” he said.
Chronically damp bark can rot, inviting pests and disease.
What’s a well-meaning, mulch-happy homeowner to do?
“Make sure that the root collar is exposed,” Brownlee emphasized.
He called this segment the “ankle” of a tree, the point where the trunk flares out near the surface of the soil.
A layer of mulch should actually thicken as it transitions away from the trunk, not toward it.
If a tree already suffers from circling or girdled roots, a homeowner can carefully remove built-up mulch or soil, he said.
Or a professional can perform what’s called root-collar excavation, using an “air spade,” a device that uses air pressure to blow away mulch and soil without damaging the tree.
The question of when to apply mulch is a simpler matter.
“There’s never a bad time,” he said, although “the ideal time is in the spring months.”
First, soil moisture is usually high, so mulching now can help preserve it before a dry summer.
Second, mulch helps suppress weeds, many of which sprout in spring.
Whenever you apply it, “there are a lot of other benefits to mulch,” Brownlee added.
As it decomposes, it slowly adds nutrients to the soil.
In addition, a large circle of mulch means no mowing or weeding near the trunk, thus lessening the risk of damage from a lawn mower or string trimmer.
Finally, more mulch means less grass.
“Turf and trees compete pretty hard for the same nutrients and water within the soil,” he said. Without grass vying for the same substances, trees can thrive.
Of course, other plants also benefit from mulch, including flowers, vegetables and shrubs. The reasons are the same: moisture retention, weed suppression and improved soil.
For trees, mulch should be maintained at a depth of 2 to 3 inches — no deeper, Brownlee said. If you want to add mulch for a fresh look, “remove the top layer if needed. You can put it in your compost pile.”
As for the width of a mulch circle, “You want the tree ring to be as big as possible,” he said.
Ideally, mulch should extend to the edge of the drip line, aka the tips of the branches.
“Proper mulching and maintenance of the root collar is a great way to protect your asset,” he added. “It’s a maintenance item just like you would maintain your gutter or your roof.”