Mulching Trees in Central PA: How to Avoid the “Volcano” Mistake That Kills Trees Slowly

Mulching Trees in Central PA is one of the best things you can do for them: or one of the worst, depending on how it’s done. Drive through almost any neighborhood right now and you’ll see the wrong way: trees with mulch piled high against the trunk like a little brown volcano. It looks tidy. It looks like someone cared. And it’s slowly killing the tree

Mulching trees is one of the best things you can do for a tree, when it’s done right. Done wrong, it’s one of the most common and expensive landscaping mistakes a homeowner can make, and the damage often doesn’t show up for years.

This article covers what good mulch actually does, why the “volcano” is so harmful, the correct depth and shape, which mulch types are worth your money, and how to fix a volcano you may already have.

What good mulch actually does for your trees

Done correctly, mulch is genuinely one of the highest-value, lowest-cost things you can do for the trees and shrubs on your property. A proper ring of organic mulch retains soil moisture so you water less, moderates soil temperature to protect the fine feeder roots from heat and cold, suppresses weeds that would otherwise compete for water and nutrients, and improves soil structure as it slowly breaks down.

It also creates a safety zone around the trunk, a mulched ring keeps lawnmowers and string trimmers away from the bark, which prevents the kind of repeated wound damage that quietly weakens trees over time.

So the goal here isn’t “don’t mulch.” Mulch is good. The goal is to mulch in a way that helps the tree instead of suffocating it.

The volcano: why piling mulch against the trunk is a slow killer

A “mulch volcano” is exactly what it sounds like, mulch mounded up in a cone around the base of the tree, pressed right against the bark. It’s everywhere because, at a glance, it looks neat and intentional. The problem is what it does underneath.

When mulch is piled against the trunk, it traps moisture against the bark. Tree bark is not designed to stay wet. Constant moisture softens it and opens the door to decay, fungal disease, and insects that bore into the weakened wood. The bark is the tree’s skin and its protective barrier, keep it wet long enough and that barrier fails.

The deeper problem is what happens to the root flare, the part of the trunk that widens out as it meets the soil. A healthy tree should always show its flare. When you bury it under a mound of mulch, two things go wrong. The trunk starts to rot where it’s covered, and the tree responds by sending roots upward into the mulch looking for oxygen. Those roots can circle the trunk and eventually strangle it, a problem called girdling that can take years to kill the tree but is often fatal once it sets in.

And here’s the part that makes it almost absurd: deep mulch doesn’t even do its job better. It wastes time and money while harming the tree. Penn State Extension’s own demonstration showed that the exact same volume of mulch used to build a volcano was more than enough to mulch the tree correctly when simply spread out flat and pulled back from the trunk. You’re not saving anything by piling it up. You’re paying more to do damage.

Planting a new tree? Our guide to planting a maple tree in PA covers species choice, placement and setting the root flare at grade.

How deep, how wide, how far from the trunk

The correct way to mulch is simple enough to do in an afternoon. The specs that actually matter:

The “go for distance, not height” rule is the one most people get backwards. A wide ring of mulch only two or three inches deep does everything you want, moisture, weed suppression, temperature control, across the whole root zone. A narrow mound piled high does none of it better and suffocates the trunk.

When to mulch, and the spring timing most people get wrong

One more timing note: if you already have mulch from last year, check the existing depth before adding more. Piling fresh mulch on top of old mulch every spring is one of the main ways volcanoes form in the first place. If you have two or three inches still in place, you don’t need to add a full layer, just rake and refresh the surface, or add a thin skim of new material for color. Adding new mulch over old without checking is the same as over-mulching.

Which mulch is actually worth your money

Not all mulch is equal, and the most expensive bag isn’t always the best choice. Here’s the honest breakdown for trees and shrubs.

If you want the simplest answer: aged shredded hardwood or aged wood chips, in a natural color, applied two to four inches deep in a wide donut. That covers the vast majority of central PA properties and costs less than the fancy dyed bags.

How to fix a volcano you already have

Good news, if you’ve been building volcanoes for a few years, the fix is usually straightforward and you don’t need to buy anything. You’re not removing mulch so much as redistributing it.

If the volcano has been in place for a long time, or the tree already looks stressed, thinning canopy, dieback in the upper branches, fungus at the base, pulling the mulch back is still the right first move, but the tree may need a closer look. Years of a buried flare can mean girdling roots or trunk decay that isn’t visible until you know what to look for.

What gets done right vs. what to skip

Sources: Penn State Extension, “Mulching Landscape Trees” and “Mulch Volcanoes Are Erupting Everywhere”; The Morton Arboretum; University of Maryland Extension. Specifications current as of 2026.

Written and reviewed by Thayne Muthler, owner of Muthler Landscaping & Construction (PA HIC #PA212584). How we check facts: editorial policy.