By Roger Gettig Holden Arboretum
There is a rapidly spreading plague attacking landscape trees throughout the region: mulch volcanoes (go ahead; Google it. I know you want to.)
A mulch volcano is the result of mounding up mulch high against the trunk of a tree, usually resulting in a broad pyramidal shape. This is bad for the tree. So bad that mulch volcanoes are specifically called out in Chapter 11 of "How Trees Die" by Jeff Gillman.
This inexcusable practice is erupting across commercial and residential landscapes, frequently resulting in stunted growth and most likely leading to the slow death of the tree. So stop doing it!
This is not the only way we harm trees in our landscapes, but I'm going rant about mulch volcanoes today because this practice is so easy to recognize and just as easy to avoid.
Doughnuts vs. volcanoes.
The proper way to mulch resembles a doughnut, not a volcano. Mulch should be placed 2 to 3 inches deep around the tree, ideally as far out as the branches reach (the "drip line"), but mulch should always be kept several inches away from the trunk (the hole in the doughnut).
Mulch should never touch the trunk. Ever. So if you have a lot of mulch, go wide, not deep. And this broad ring should have a rim around the edge so that it contains the water when you irrigate, giving the water a chance to soak in. Contrast that with trying to water a mulch volcano, where the water runs off and everything gets wet except the tree roots.
Never bury the tree trunk.
You should never bury the trunk of a tree with mulch (or anything). If you plant the tree too deeply, the trunk gets buried. If you mulch too deeply and/or too frequently, the trunk gets buried.
It is easy to spot the trunk, right? It is where the tree flares out (gets wider) as it gets closer to the ground. This signifies the transition zone between main stem and root zone. If it looks more like a phone pole going into a mulch pile and you don't see any root flares, either the tree was planted too deeply or the mulch is piled too high (or both).
Why is this bad?
Burying a trunk will promote bark decay and infections, and provides rodent and insect habitat. It also encourages the growth of stem-girdling roots. These are roots that grow across the trunk (either above or below ground), which will eventually strangle the tree.
All this happens slowly, which is why we are slow to react. It could take years before you start to notice the tree struggling and by then it may be too late.
Trees should last us for generations, but the average life of an urban tree is usually less than 10 years. Mulch volcanoes are definitely not the only cause of this decline, but they certainly are becoming one of the most common.
Too "mulch" of a good thing.
If this is bad, why is it so widely imitated? Many gardeners understand the benefits of proper mulching: weed suppression, modified temperature, moisture retention, increased organic matter in the root zone. So if some mulch is good, then a lot of mulch should be better, right? Bad idea.
Some people like the look of mulch, so they add fresh mulch every year, not checking to see how much they really need. If you add 2 inches per year but only 1 inch decomposes per year, then it could be 10 inches deep after a decade of dutiful mulching. And if you get paid to put down mulch every year, there is little financial incentive not to over mulch. If nothing else, it is wasteful.
So if you don't need that much mulch and it is hurting the tree, don't pay for it to happen.
Thrive, not just survive
Planting a tree is a substantial investment that can pay off for generations. Among other things, trees provide clean air, clean water, summer shade and wildlife habitat. Trees make your communities more livable.
Some trees hobbled by mulch volcanoes do survive, but they don't thrive (they will grow 2 inches per year instead of 2 feet). Protect the growth potential of your investment in trees: ban mulch volcanoes from your community. Help the trees. Please.
Gettig is director of horticulture and conservation at the Holden Arboretum. Got a question for the Ground Crew? Send email to email@example.com, with Ground Crew in the subject line. Or, mail a letter to Ground Crew, Inside and Out, The Plain Dealer, 1801 Superior Ave., Cleveland, Ohio, 44114. Include your name, address and daytime phone number on emails and letters.