I got to see a dying tree in Pearland in a fairly new subdivision in which the houses were a maximum of 7 years old while doing a consultation. For the record it wasn’t my client’s, rather his next-door neighbor’s. I walked over to show him how unstable the tree was, even after being there for at least 5 of those 7 years. And in my professional opinion, it was all caused by a “Mulch Volcano.”
As you might suspect it also gives me pause to warn everyone not to pile up the mulch too high around trees, so we aren’t perpetuating this horticulturally heinous activity. For the record, mulch volcanoes are the mounds or cones of bark mulch that are two or three feet deep, and piled up along the trunks of trees and in some cases larger shrubs.
I am not sure why this habit has developed, but these deep mulch piles can cause immense damage to trees & shrubs. I suspect uneducated/untrained landscaping companies (can’t really call them professionals, if they do this) are the originators and sometimes the worst culprits. Not all of them, but many of them.
It’s especially bad on new builder model homes in communities throughout the region. So, here’s what I suspect happens: the uneducated and unsuspecting new homeowner/potential new homeowner sees it on the model homes and does likewise thinking they are following a “professional” technique. I also suspect this practice is self-perpetuating, because of the dam/barriers that are built on newly planted trees to keep the water in the root zone for the first year or so. After a year or so these damns can and should be broken down and then a thin area of mulch can be applied. But when the dam stays in place, and several times a year a landscape company starts mulching from the inside and building and building a mound, then we get these horrid mulch volcanoes.
Since professional landscapers (uhm, again, not sure they are really pros) get paid to do work, when customers ask for fresh mulch a couple of times a year (as everything I’ve ever written recommends) sometimes they are piling up several inches in an area that might just need one inch. So, you may be asking, “what’s so bad about this Randy, if it looks good?” As that accumulates, especially on trees, it starts to cover the bark, and stops allowing for the much-needed root flare. However, that bottom section needs air and light. If you pile too much mulch onto the bark, it’s then covered into darkness and moisture.
As the bark continues to be in continuous moisture, it will begin to rot. Rotted bark cannot protect the tree from diseases, and in fact, diseases grow better in the dark moisture of the mulch. Hence, the affected trunks are more likely to get diseases and allow harmful insects to damage trees. Now the tree can snap in a heavy wind, right there at the rotten part of the trunk.
Some trees have shallow roots, especially maples. If bark mulch volcanoes are piled around the trunk, the roots will start to grow up into it. These roots tend to stay in the mulch volcano and will then grow around the trunk in the mulch. As the root grows in diameter, it pushes against the trunk, which is also trying to grow bigger. These roots will eventually strangle the trunk. This type of root is called a “girdling root”. The trunk will keep growing wider above and below the girdling root, and may actually encase the root. It also doesn’t encourage the tree and shrub roots to expand out in a healthy fashion. Healthy root growth has many agronomic advantages, but it also aids in stability.
So, here are some basic rules:
Shredded Native Mulches are best when put down to a depth of about 2-3 inches if you are starting from scratch. Even though bark mulch does break down some, after 2-3 years of annual mulching on top of mulch, it often accumulates to unhealthy levels for your plants.
What do you do instead of annually piling more mulch on top of mulch? Start using compost instead of mulch. You get the same weed prevention and moisture preservation, but you get a more accelerated break down and thus adding an inch or so per season, you don’t achieve mulch volcano status. If the mulch is obviously more hardwood shards as opposed to shredded, consider removing a couple of inches and make the change to native mulches or compost.
If you know there’s a dam/barrier still in place and the tree has been in the landscape for a year or more, break that down and level out the areas so you can add minimal mulch or compost. Also, if you’re planting flower beds in these mulch volcanoes or a mound of mulch higher than 4 inches, stop that practice immediately. Unfortunately, that bone-headed practice is a whole other tip sheet for a different time.