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Why Mulch Is Killing Your Trees

mulch volcano

Drive around any suburban neighborhood in Alexandria, Arlington or Fairfax counties and in many yards you’ll see one of the biggest tree killers – mulch volcanoes.

Many landscapers and homeowners pile mulch up around tree trunks, sometimes as high as 3 feet! And each year they add more mulch to the pile.

I don’t know why mulch volcanoes became a “thing” when they’re so clearly destructive. Perhaps it’s a mistaken belief that if some mulch is good, more must be better. Unfortunately, that’s not the case when it comes to mulching your trees.

What Is Mulch?

Mulch is a generic term for material that’s used to cover the soil surface. There are two general types of mulch:

  • inorganic mulch, like gravel or shredded rubber
  • organic mulch, such as shredded bark, grass clippings, wood chips, pine straw, compost, cocoa hulls or chopped leaves

The Benefits of Mulch

The benefits of a layer of mulch include weed suppression, moisture retention, temperature regulation and erosion control. Organic mulch has the additional benefit of improving the condition of the soil beneath it as it breaks down over time.

If you can, opt for organic mulch as it’s typically better for plants’ health.

How Much Mulch Should You Use?

Trees and shrubs generally need 2 to 4 inches of mulch. For poorly drained soil, use only 2 inches of mulch.

This is the range in which you get the benefits of mulch without causing the problems associated with over mulching.

Each year, before you spread more mulch around your plants, check to see how deep the existing mulch is. If it’s still in the recommended range there’s no need to add more – just fluff it up so it looks nice and to let water through more easily. If some of the mulch has decomposed, add just enough to bring the depth back to 2-4 inches.

The color of many organic mulches fades over time. If you really want that “fresh” look (and raking or turning the mulch doesn’t improve the look), remove some of the faded mulch before adding new.

Where Should Mulch Be Placed?

Spread mulch all the way out to the edge of the tree’s canopy (that’s the area where most of the tree’s roots are found). Creating just a small circle of mulch around the trunk doesn’t do much good, other than to prevent lawn mower and string trimmer damage to the tree.

Do not put mulch directly up against the tree trunk. Instead, pull the mulch away from the trunk so there’s none touching it. You want to be able to see the point where the trunk flares out as it goes into the ground.

The Problems With Mulch Volcanoes

Piling mulch directly around a tree trunk causes a number of problems.

  • It creates a moist environment that’s perfect for fungus and other pathogens to flourish and attack the tree. You’ll often see tree trunks completely rotted out under the mulch.
  • Piling mulch against a tree makes a wonderful home for rodents and other pests. Plus it gives them a ready food source – they’ll eat all the bark off the tree under the mulch.
  • As the mulch decomposes, it heats up (just like a compost pile), reaching temperatures well above 100F. That’s enough to kill the inner bark in young trees.
  • Trees respond to the dark, moist mulch by putting out new roots directly into the pile of mulch. When the roots reach the edge of the mulch pile they turn back toward the center, eventually wrapping around the tree and girdling it. The tree becomes unstable and less able to take up moisture and nutrients as the main roots are killed off.
  • Over time, the mulch compacts and instead of letting water through, the mulch pile may repel rain and irrigation. The ground under the mulch pile dries out, leaving the tree without the moisture it needs.
  • If the ground around the tree is relatively moist to start with, a big pile of mulch will just make it wetter. Eventually the tree “drowns” from too much water and lack of oxygen.

None of these problems are immediately apparent when mulch is piled around a tree. But make no mistake about it – mulch volcanoes are a death sentence for trees.

What To Do If You Have Mulch Volcanoes

If you have a large pile of mulch around any of your trees, remove as much as you can.

Then take a good look at the tree trunk. If you see signs of rot, bark damage or roots above ground level give us a call.

Trees with extensive damage at the base can be unstable and dangerous so should be removed. But some may be saved (for example, by cutting out girdling roots). Our experienced tree professionals can recommend the best course of action for trees that have been compromised by volcano mulching.