Why Mulching Matters for Trees in Indiana
Mulching is the single most beneficial thing most Bloomington homeowners can do for the trees on their property. Done correctly, it replicates the conditions trees evolved in — a thick layer of decomposing organic matter covering the root zone — and delivers a stack of measurable benefits that no fertilizer or chemical treatment can fully replicate.
Here is what a proper mulch ring does for a tree. It retains soil moisture by reducing evaporation from the soil surface, which matters enormously during Bloomington's hot, dry July and August stretches when street trees and yard trees alike are under heat stress. It regulates soil temperature, keeping roots cooler in summer and moderating freeze-thaw cycles in Monroe County's Zone 6a winters. It suppresses weed and turf competition for water and nutrients in the critical root zone directly beneath the canopy. And as it decomposes, it continuously improves soil structure and feeds the mycorrhizal fungi networks that a healthy tree root system depends on.
Walk into any mature forest in southern Indiana — Morgan-Monroe State Forest, Yellowwood, or the woodland remnants scattered through Monroe County — and look at the ground beneath the trees. You will not see turf grass growing against the trunks. You will see a thick, loose layer of leaf litter and decomposing wood that has been accumulating for decades. Mulch mimics that forest floor. It is not decorative. It is the growth environment trees spent millions of years evolving to thrive in.
The difference a proper mulch ring makes is visible within a single growing season. Trees in mulched beds typically show better leaf color, fuller canopy density, and more vigorous annual growth than the same species grown with turf competing against their roots to the trunk line.
The Number One Mulching Mistake: Volcano Mulching
Drive through almost any Bloomington neighborhood — Elm Heights, the streets near Indiana University's campus, the newer subdivisions out on the east and west sides — and you will see it constantly. Mulch piled high against tree trunks in a cone shape, looking like a little volcano with the tree as its summit. It is extremely common, it looks tidy from a distance, and it is slowly killing trees across Monroe County.
Volcano mulching — piling mulch directly against and up the trunk of a tree — is one of the most damaging things you can do to a landscape tree. Here is why.
Tree bark at and below the soil line is not designed to stay wet. It is a protective layer that manages moisture transmission in a very specific way. When mulch is piled against the trunk and kept perpetually moist, the bark begins to break down. Cambium tissue — the thin layer just beneath the bark that transports water and nutrients and produces new wood — is damaged by chronic moisture and reduced oxygen. Once cambium is destroyed in a band around the trunk, the tree is effectively girdled. Water and nutrients cannot move past that point. The result is slow, steady decline over several years that often looks like a disease or pest problem when the real cause is the mulch.
Volcano mulching also harbors pests and pathogens. The warm, moist conditions inside a mulch pile are ideal for wood-boring insects, fungal pathogens, and rodents. Voles and mice use deep mulch piles against tree trunks as sheltered runways, and they gnaw bark through the winter when other food sources are limited. A trunk girdled by rodents in a Bloomington winter can decline rapidly the following spring.
Finally, mulch piled against trunks encourages adventitious root growth — roots that develop in the mulch layer above the normal root collar. These aerial roots wrap around the trunk as they grow, slowly constricting it the way a wire wrap would. This is called stem girdling roots, and it is a leading cause of slow decline in landscape trees throughout Indiana.
You will see volcano mulching in front of businesses, in municipal plantings, and throughout residential neighborhoods. Its prevalence does not make it correct. Our arborists spend part of every estimate visit quietly correcting it.
How to Mulch Trees the Right Way
Proper mulching is simple once you know the rules. There are really just three things to get right: depth, shape, and distance from the trunk.
Depth: Apply two to four inches of mulch across the root zone. This is enough to retain moisture, moderate temperature, and suppress weeds without suffocating roots or creating the anaerobic conditions that rot bark and damage mycorrhizal networks. More is not better. A six-inch mulch layer is not twice as beneficial as a three-inch layer — it is more likely harmful, restricting oxygen exchange to roots and staying soggy long after rain.
Shape: The correct shape is a donut, not a volcano. Mulch should be spread in a flat, even layer across the root zone with a clear gap — three to six inches of bare soil — between the mulch and the trunk flare. The trunk flare is the point where the trunk visibly widens as it meets the ground. This flare should always be visible. If you cannot see it, either the mulch is too deep at the trunk, or the tree was planted too deeply — a separate problem worth addressing.
Extent: Extend the mulch ring as far out as practical, ideally to the tree's drip line — the outer edge of the canopy. The effective root zone of a large tree extends far beyond what most people imagine, and mulching only the area immediately around the trunk captures only a fraction of the potential benefit. In a typical suburban Bloomington yard, a mulch ring six to ten feet in diameter is a realistic minimum for an established shade tree. Larger is genuinely better.
A newly planted tree benefits from mulching immediately at planting. An established tree with turf growing against its trunk benefits immediately from removing that turf and establishing a proper mulch ring — even a modest improvement in the mulched area creates measurable benefits within one growing season.
Best Mulch Types for Indiana Trees
Not all mulch materials are equal, and some commonly used products are actively harmful to trees. Choosing the right material is as important as applying it correctly.
Hardwood bark mulch is the standard choice for Bloomington landscapes. It decomposes at a reasonable pace, improves soil structure as it breaks down, and is readily available from nurseries and home improvement stores throughout Monroe County. It provides all the moisture retention, temperature moderation, and weed suppression benefits described above without introducing problems.
Wood chips from tree services are an excellent option — and often free or very low cost. When a tree service chips removed branches on site, those chips contain a mix of bark, wood, and green material that decomposes more quickly than shredded bark and feeds soil biology very effectively. This type of material is sometimes called arborist chip mulch, and research from Washington State University has shown it produces exceptional results for tree health compared to processed bark mulch. If you have a tree service doing work at your property, ask whether you can keep the chips.
Pine bark mulch is appropriate for acid-loving ornamentals but has limited application for the native oaks, maples, and other hardwoods that dominate Bloomington landscapes. Its pH contribution is a minor factor for most established trees but worth noting for species that are already showing iron chlorosis from Bloomington's alkaline limestone-influenced soils.
Dyed mulch is a widely available product to avoid. The dyes themselves are generally considered safe, but the wood base for many dyed mulches is recycled construction lumber, pallets, and other urban wood waste that may contain chromated copper arsenate (CCA) preservatives or other treatments. This material does not improve soil health the way natural hardwood mulch does, and its quality is inconsistent.
Rubber mulch and stone mulch should not be used around trees. Rubber mulch does not decompose and provides none of the soil biology benefits that make organic mulch valuable. It retains heat to a degree that can damage surface roots in summer. Stone mulch — river rock, lava rock, pea gravel — creates a heat sink around the trunk, drives soil pH in directions most native Indiana trees do not prefer, and prevents the organic matter exchange that builds healthy soil over time. Both look tidy in landscape beds. Both are the wrong choice around trees.
When and How Often to Refresh Mulch
Mulch breaks down over time. That decomposition is actually part of what makes it beneficial — as it decays, it feeds soil organisms and builds organic matter in the root zone. But as it breaks down, the layer gets thinner and less effective. Knowing when to refresh keeps your trees getting the full benefit year-round.
For most Bloomington properties, refreshing mulch once per year is the right cadence. The ideal timing is early spring — late March through April — before the summer heat and dry season arrive. Fresh mulch applied in spring helps retain the soil moisture from spring rains, moderates soil temperature as temperatures climb through May and June, and gives you a clean start on weed suppression before the growing season gets going in earnest.
When refreshing mulch, do not simply add a new layer on top of the old one without checking the total depth. Old mulch compacts and mats over time. If you have been adding fresh mulch annually for several years without removing the old material, probe the existing layer with a stick or trowel to check actual depth. If you are already at four inches, adding more without removing old material will push you into the problematic range. Pull back compressed old mulch, check for bark rot or root problems where the mulch meets the trunk, and then apply fresh material to bring the total back to two to four inches.
For newly planted trees in their first two seasons, check the mulch depth more frequently. New plantings in Monroe County's clay soils can have drainage challenges, and mulch that stays too wet against a young tree's vulnerable root system can cause establishment problems. Check after heavy rain events to confirm the mulch ring is draining well and not holding standing water.
If you have any concerns about the trees on your Monroe County property — whether it is a mulching question, a structural concern, or a tree that just does not look right — Bloomington Tree Service Pros offers free on-site evaluations throughout the Bloomington area. Our ISA-certified arborists can assess your trees and give you practical, honest guidance on what they need. Call us at (812) 432-2013 to schedule a visit.