Enough with the ‘mulch volcanoes’!


On a recent consultation, I got to see a dying tree in a fairly new Pearland subdivision, where the homes were no more than seven years old.

For the record, it wasn’t my client’s tree. It was his next-door neighbor’s.

I walked over to show my client how unstable it was, even after being in the ground for at least five years. In my opinion, the cause was a “mulch volcano” – a mound or cone of bark mulch two or three feet deep, piled up around the trunk.

This is a horticulturally heinous practice.

I am not sure how this custom developed, but those deep mulch piles can cause considerable damage to trees and shrubs. I suspect that uneducated or untrained landscapers (can’t really call them professionals, if they do this) are the originators, and sometimes the worst culprits. The pattern is especially bad at builders’ model homes throughout the region. And I suspect some new homeowners see the style on models and decide to copy it.

A mulch volcano

A mulch volcano

Now, dams or barriers built up around newly planted trees are okay – they’re intended to keep water in the root zone for the first 12 months or so. But after about a year, those dams should be broken down and replaced by a thin covering of mulch. If the dam is left in place, and a landscape company starts adding more mulch two or three times a year, we wind up with a horrid pile that looks like a volcano with a tree stuck in the middle.

And since many landscaping companies simply do what their customers request, when asked for fresh mulch a couple of times a year, they wind up piling up several inches in an area that might just need one inch.

What’s so bad about this if you like how it looks? As the mulch accumulates, it starts to cover the bark on trees, including the much-needed root flare.

Root Flare

Root Flare

That bottom section needs air and light. Too much mulch keeps it in darkness and moisture. Bark continually in too much moisture will rot, and rotted bark cannot protect a tree from diseases. In fact, diseases develop far better in the dark moisture of mulch. Trunks affected by diseases can allow harmful insects to inflict damage.

And a weakened tree can snap in a heavy wind, right there at the rotten part of the trunk.

Some trees, such as maples, also have shallow roots. If bark mulch volcanoes are piled around the trunk, the roots will start to grow into it. Those roots tend to stay in the mulch volcano, then grow around the trunk itself. As the root grows in diameter, it pushes against the trunk, which is also trying to grow bigger. It will keep growing wider above and below the those encircling roots - called “girdling roots” - and may actually encase the root. This situation prevents tree or shrub roots from expanding out in a healthy fashion. Healthy root growth has many agronomic advantages, but it also aids in stability.

Girdled roots

Girdled roots

Shredded native mulches are best when applied to a depth of 2-3 inches, if you are starting from scratch. Even though bark mulch does break down some, after 2-3 years of annual mulch on top of mulch, it often accumulates to unhealthy levels.

So, what do you do instead? Start using compost instead of mulch. You’ll get the same weed protection and moisture preservation but with a more accelerated breakdown. Just add an inch or so per season, and you won’t achieve volcano status.

If your mulch is more hardwood shards than shredded, consider removing a couple of inches and change to native mulch or compost. If you have a dam still in place on a tree that has been in the landscape for a year or more, break it down and level out the area. Then, add minimal mulch or compost.

Also, if you’ve been planting flower beds in a mulch volcano or a mound higher than four inches, stop that immediately. I’ll cover the bone-headed idea of planting flowers around the bases of trees in another tip sheet at another time.

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PHOTOS: Getty Images, Randy Lemmon, Davidson County Tennessee Extension Service