Recently on GardenLine, we discussed using compost instead of mulch around the base of trees, and I was once again moved to warn about piling it up too high to make a "mulch volcano" – a mound or cone of mulch, a foot or more deep, piled up around trees and shrubs.
Too much compost or mulch can be as detrimental as not enough. The pictures here show what NOT to do.
I’m not sure how this practice developed, but deep mulch can cause considerable damage. I suspect uneducated, untrained and unprofessional landscape companies are the originators. And it seems to be especially pronounced at model homes in new residential communities. I further suspect that naive or first-time homeowners spot the practice and decide to copy it.
Professional landscapers get paid to fill their customers' requests, and when homeowners order (as they should) fresh mulch twice a year, several inches sometimes get piled up in spots that might need just one inch. As a result, it accumulates to cover the base of tree trunks and the very important root flare.
That bottom section needs air and light. 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 the tree from diseases. In fact, diseases grow better in the dark moisture of the mulch, and 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.
This is called a "girdling root," and as it grows in diameter, it pushes against the trunk, which is also trying to grow bigger. Eventually, the trunk will grow wider above and below the girdling root, and may actually encase it. It also prevents tree and shrub roots from expanding out in a healthy fashion. Healthy root growth has many agronomic advantages, and it aids in a tree's stability.
So, here are some basics:
Shredded Texas 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 two or three years, bi-annual applications often accumulate to unhealthy levels. So start using compost instead of mulch. Or remove a thick layer and begin again. With compost, you get the same weed prevention and moisture preservation, but with a faster breakdown. And by adding just 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.
You may wish to review GardenLine’s “10 Commandments of Mulch” for more information.