Why Is Mulch Killing Your Trees?

Drive through any suburban area in Alexandria, Arlington, or Fairfax counties, and you’ll notice one of the deadliest tree killers – mulch volcanoes – in many yards.

Mulch is often piled up around tree trunks by landscapers and homeowners, reaching heights of up to 3 feet! Every year, they add to the mound of mulch.

I’m not sure why mulch volcanoes have become so widespread, given how blatantly harmful they are. Perhaps there is a misguided idea that if some mulch is good, then more must be better. Unfortunately, when it comes to mulching your trees, this is not the case.

What Exactly Is Mulch?

Mulch is a broad word for any substance used to cover the soil’s surface. Mulch is classified into two types:

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

Arborist wood chips (what’s left over after a tree or branches have been removed and chipped) are one of the greatest forms of mulch.

Mulch’s Advantages

Mulch provides several advantages when utilized correctly.

Mulch has the following advantages:

•Weed management
•moisture retention
•soil temperature regulation
•erosion control

Organic mulch has the added advantage of enriching the soil underneath it as it decomposes over time.

If possible, use organic mulch since it is often healthier for the health of plants.

How Much Mulch Do You Need?

Mulch should be applied to trees and shrubs at a depth of 2 to 4 inches. Use just 2 inches of mulch on poorly drained soil.

This is the range in which you may get the advantages of mulch while avoiding the issues associated with over-mulching.

Before you lay new mulch around your plants each year, check to see how deep the previous mulch is. If it’s still within the approved range, there’s no need to add more; just fluff it up to make it look good and let water to pass through more readily. If part of the mulch has decomposed, just apply enough to restore the depth to 2-4 inches.

>> Spring Yard Cleanup Tips – Simple things to do to improve your landscaping and trees when spring comes in Northern Virginia (including mulching).

Many organic mulches lose their color over time. Remove part of the faded mulch before adding new if you truly want that “fresh” effect (and raking or rotating the mulch does not help the look).

When Should Mulch Be Used?

Spread mulch all the way to the tree’s canopy edge (this is where the majority of the tree’s roots are situated). Making a tiny circle of mulch around the trunk serves no purpose other than to protect the tree from lawn mower and string trimmer damage.

Do not apply mulch straight to the tree trunk. Pull the mulch away from the trunk so that none of it touches it. You should be able to see where the trunk widens out as it sinks into the earth. Instead of a mound, consider spreading mulch in a donut-like ring.

Mulch Volcanoes Have Issues

Mulch piled directly around a tree stem presents a variety of issues.

•Rotten tree trunks – Mulch heaps provide a damp environment in which fungus and other infections may thrive and harm the tree. Under the mulch, tree trunks are often entirely rotted away.
•Rodent and pest concerns – Piling mulch against a tree provides an excellent habitat for rats and other pests. It also provides them with a convenient food supply, since they will consume all of the bark off the tree beneath the mulch.
•Cooked, dead bark – As the mulch decomposes, it warms up (similar to a compost pile), reaching temperatures far beyond 100 degrees Fahrenheit. That is sufficient to harm the inner bark of young trees.
•Girdling roots – Trees react to the dark, wet mulch by growing new roots right into the pile. When the roots reach the border of the mulch pile, they return to the middle, finally winding around and girdling the tree. As the primary roots die, the tree becomes unstable and less able to absorb rainfall and nutrients.
•Dried out soil – Mulch compacts with time, and instead of allowing water to pass through, the mulch pile may reject rain and irrigation. The earth underneath the mulch pile dries up, depriving the tree of the moisture it need.
•Drowned trees – If the ground surrounding the tree is already damp, a large quantity of mulch will just make it wetter. The tree eventually “drowns” as a result of too much water and a lack of oxygen.

When mulch is heaped around a tree, none of these issues are immediately visible. But make no mistake: mulch volcanoes are a tree’s death sentence.
What Should You Do If You Have Mulch Volcanoes?

Remove as much mulch as possible from around your trees if you have a significant pile.

Then examine the tree trunk closely. Please contact us if you see any symptoms of rot, bark damage, or roots above ground level.

Trees with substantial damage at the root are unstable and should be removed. However, some may be preserved (for example, by cutting out girdling roots). For trees that have been harmed by volcanic mulching, our expert tree consultants can advise you on the best course of action.

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