Mulch Volcanoes Revisited


In recent months, I have conducted lots of on-site consultations, and I’ve noticed that many people still don’t understand why mounds of mulch should not be built up around the bases of trees. I call them “mulch volcanoes.”

I suspect 50% of my consults lately have involved dealing with these things, and I promise they’ll harm the trees in the long run.

So, here’s an update on my 2019 tip sheet on the subject. And, as I did back then, I’ll ask you to take a look at the pictures, so you know what NOT to do. You can see the problems that mounds of mulch piled up around the trunks of trees and shrubs can cause. Mulch up to three feet deep can cause immense damage.

I am not sure how this practice developed but I suspect uneducated and untrained landscaping companies are the originators. We can't really call them professionals if they do this, right? And the idea seems to be especially pronounced at model homes in new residential communities. I further suspect naive or first-time homeowners decide to copy the style.

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 (below). 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 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.

On a related topic, I have a buddy in the landscaping business, and I’ve referred many people to him. Recently, though, he showed me a completed project he’d done with red dyed mulch, and I was aghast that he was proud of it. I reminded him that soil scientists say dyed mulch releases terrible toxins into the soil. He came back with the phrase that always gut-punches me – “But the customer asked for it, Randy.”

All landscapers should educate their customers as to why NO DYED MULCH SHOULD EVER BE USED in landscapes, especially around trees! So, here’s …

What landscapers should teach customers about dyed mulch

1. Dyed mulch doesn’t look natural: It looks artificial, and anyone with horticultural experience will tell you it’s the equivalent of putting fake plastic flowers in a landscape – a phony look people will make fun of. Mulch for a landscape should be part of nature, reflecting at natural aspect.

2. Most dyed mulches are made of recycled waste wood: It’s almost always trash wood from broken pallets, old decking, demolished buildings or … worse yet … CCA-treated lumber. CCA stands for chromium, copper and arsenic - chemicals used to preserve the wood. Processors grind up the trash wood, then spray it with a dye to cover up inconsistencies and give it a uniform color. Most will claim their dye is “organic,” but they avoid revealing the wood source.

3. There’s a negative effect on the soil: Dyed mulch doesn’t break down the way native and shredded mulches do. Instead, dyed mulches - especially those that are midnight black - leach the dye and possible CCA contaminants into the soil, killing beneficial bacteria. It can also kill off helpful insects and earthworms.

4. Nitrogen fixation almost always occurs: This produces yellowing leaves on annuals and perennials. Nitrogen in the soil works so hard to break down the wood, roots don’t get enough to keep the plant green.

5. Shredded native mulches with compost break down in the soil: The best mulches and composts become part of the soil’s organic content over time. That makes for more beneficial soil bacteria and enhances the environment for earthworm production. Composted or naturally aged mulches actually release nitrogen into the soil, thereby helping plants rather than robbing them of that important element.

If your landscaper doesn’t agree with these principles, get a new landscaper.

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