mulch volcanoes

Nice lawn, but the mulch is going to harm the trees.

Way back in the mid-1980s, when my husband and I first started gardening, we had some friends who raised horses out in West Valley. They had a lot of horses, and those horses produced tons of manure. When this happens, who ya gonna call? Well, the Baranys, of course.

The horses were bedded on wood chips, and when their stables were cleaned, the glorious mess was piled up outdoors and left to rot for at least a year. Each spring, to make room for new piles, our friends would load the “black gold” into their ancient six-wheeler truck, and we’d drive it home, lumbering east down Tieton Drive at 20 mph. We emptied the load, one shovelful at a time, onto our driveway, and from there, wheelbarrowed it all over the yard.

It was a sight to behold: fluffy, dark and slightly steaming. All the weed seeds were long gone, thanks to the high temperatures generated inside the pile. Honestly, this stuff was so beautiful it was all I could do to keep my toddler daughter, Alison, from playing in it with her sandbox toys.

Some springs, we would bring home as much as three truckloads full, and end up with a surplus. My husband’s solution was to mound it up around our newly planted trees, mostly to keep me from running into them with the lawn mower. We didn’t know it then, but we had created what are now referred to as mulch volcanoes.

Even if you’ve never heard of mulch volcanoes, you’ve seen them. After all, they’re erupting all over Yakima (sorry, I couldn’t resist). They’re those symmetrical, cone-shaped piles of mulch at the base of trees. You may have created mulch volcanoes of your own, and have gone for years without noticing much of a problem, but the damage can occur slowly and silently.

One clever garden writer explained that volcano mulch looks like a muffin. Instead, the mulch around your tree should look like a doughnut. That is, a circle of mulch around the tree with a hole in the middle where no mulch touches the trunk. The tree shouldn’t look like a telephone pole sticking out of the mulch; you should see it flare out at the bottom. Aim for a mulch layer that’s no more than 3 inches deep, at least 3 inches from the trunk, and extending out to the drip line.

This is what Pennsylvania State University Extension has to say about how trees benefit from properly applied mulches.

Mulches help conserve soil moisture, which means less frequent irrigation. They also help moderate soil temperature, protecting fragile feeder roots from temperature extremes. They help keep weeds down, at least for a while. But just as weed seeds blow into the garden, they’ll find their way into the mulch, where they easily germinate. Organic mulches also add organic matter to the soil as they break down, improving soil structure, porosity and nutrient-holding capacity. And all types of mulch protect vulnerable trunks from weed whacker and mower damage.

Problems can arise when the mulch is applied more than 3 inches deep, or placed in contact with the tree’s trunk. Wet mulch in contact with the trunk can cause the bark to break down. Additionally, as the mulch starts to compost, it heats up, which can further damage the bark and the underlying vascular tissues, compromising the tree’s ability to transport water and nutrients.

Too much mulch also blocks tree roots from getting sufficient oxygen. Lack of oxygen can also cause the tree to develop adventitious roots, which grow from trunk tissue instead of true root tissue. As they grow in diameter, adventitious root can develop into girdling roots, which circle the trunk, instead of spreading out from the tree in all directions like a web. This compromises the tree’s ability to transport water and will eventually kill the tree.

If piled on too thickly, the underlying soil can remain too wet for too long, and the tree’s roots can begin to rot.

Lastly, an overly thick mulch layer can become impervious to overhead irrigation or rainfall, further stressing the tree.

As always, a picture is worth 1,000 words. University of New Hampshire Extension has a great video at

In the meantime, just try to remember: No volcanoes, only doughnuts.

Carol Barany and her husband, John, found paradise on 1 1/3 acres just west of Franklin Park, where they raised three children and became Master Gardeners. Contact her at