Last weekend, as I encouraged listeners to take advantage of a great deal and stock up on compost as a mulch, I was again moved to warn about piling it up too high around trees, creating what I refer to as "mulch volcanoes."
Those are mounds or cones are sometimes two or three feet deep, and that can cause very serious damage. I'm not sure how this practice developed, but I suspect uneducated and untrained landscaping companies (can't really call them "professionals") are the originators. And it seems to be especially pronounced at model homes in new residential communities. So, I'm betting that naive homeowners spot the practice and decide to copy it.
Professional landscapers get paid to do the work their customers order, and when homeowners ask for fresh mulch a couple of times a year ... as they should ... several inches sometimes get piled up in an area that might just need one inch. As it accumulates, it starts to cover the tree bark and the very important root flare.
That bottom section needs air and light, but with excess mulch, it's forced into darkness and subjected to moisture. Bark that's too moist for too long will rot, and rotted bark cannot protect a tree from disease. In fact, diseases grow better in the dark moisture. Then the trees become susceptible to insects.
Some trees, such as maples, have shallow roots. If mulch is piled high around their trunks, the roots will start to grow into the mulch and encircle the trunk. (ABOVE) This is called a "girdling root." As the root grows, it pushes against the trunk, which is also trying to grow bigger. Eventually the root strangles the trunk which will keep growing wider above and below the girdling root.
Sometimes, the trunk may actually encase the root. This prevents the tree or shrub roots from expanding out in a healthy fashion, missing out on many agronomic advantages and affecting the tree's stability. (BELOW)
So, here are some basics:
Shredded native mulches are best when at a depth of 3-4 inches if you are starting from scratch. Even though mulch will break down over time, it often accumulates to unhealthy levels after two or three years of annual applications. So remove a thick layer and begin again. Or, start using compost instead of mulch. With compost, you get the same weed prevention and moisture preservation, but a more accelerated breakdown. So by adding an inch or so per season, you avoid achieving volcano status.
If your current mulch is more hardwood shards than natural shredded, consider removing a couple of inches and change to native mulch or compost with your next application.
If you're new to GardenLine, our email tips or our Facebook pages, you may also need to review GardenLine's 10 Commandments of Mulch.