Mulch Volcanoes 2019

Last week, I did an on-site consultation for a prize winner from the recent Arbor Gate Art in the Garden Party. It allowed me to visit my old Fairfield stompin’ grounds in Cypress. The winner’s property was in pretty darn good shape, but I was aghast at the number of homes in the subdivision that were being endangered by “mulch volcanoes!” I predict the disturbing amount of mulch piled up on young tree trunks there will result in numerous dead, sick and felled trees 5-10 years from now.

My consultation client didn’t have any mulch volcanoes, but he asked why I discouraged the practice. If you’re unfamiliar with the term, mulch volcanoes are the mounds or cones of mulch, one, two or three feet deep, piled up around the trunks of trees and shrubs. I am not sure how the practice developed, but I suspect uneducated or untrained landscaping companies are the originators. They really shouldn’t be called “professionals” if they do this. Such deep mulch can cause very serious damage, and the idea seems to be especially pronounced at model homes in new residential communities. There, I think that naive or first-time homeowners just copy it, creating a domino effect in a subdivision as neighbor after neighbor picks it up.

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 bark of the tree trunk 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 and diseases.

Some trees, such as maples, have shallow roots. If bark mulch is piled high around their trunks, the roots will start to grow into the mulch. These roots tend to stay in the mulch where they grow to 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 root strangles the trunk which 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, and it aids in the tree's stability.

So, here are some basics:

Shredded native or 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 time, after two or three years of annual mulching, it often accumulates 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 you get 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.

Please share this article with as many people as you can, because this practice has got to stop.

And if you’re new to GardenLine, our email tips, or our Facebook page, you may also need to learn my 10 Commandments of Mulch.

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