Key takeaways
- Two to three inches is the target depth for most beds.
- Never pile mulch against a trunk; you should see the root flare.
- A wide, shallow ring beats a tall, narrow one.
- Check depth and rake old mulch before adding new.
Two to three inches is the target
For most beds, a layer of two to three inches of shredded hardwood or bark mulch is enough. At that depth it blocks light so weed seeds struggle to sprout, slows evaporation in a hot Ohio July and keeps soil temperature steadier through the swings between spring thaws and hard freezes.
Less than two inches and weeds push through. More than three and you start causing trouble. A thick layer holds water, and the soil underneath can stay wet and short of oxygen. That matters in the clay soils common across Southwest Ohio, where drainage is already slow. A deep layer can also give mice and voles a place to nest near your shrubs.
Finer, shredded mulches mat down and shed water if they are too thick, so keep them toward the thinner end. Coarser bark can go a little deeper. If the mulch smells sour, like vinegar or ammonia, spread it out and let it air for a few days before using it.
What a mulch volcano is and why it hurts trees
A mulch volcano is a cone of mulch piled against a tree trunk, sometimes a foot high. It looks tidy, which is why it is so common. It is also one of the most reliable ways to shorten a tree's life.
Bark on a trunk is meant to stay relatively dry. Wet mulch held against it encourages rot and gives insects an easy way in. Roots can also grow up into the mulch and wrap around the trunk, which can squeeze the tree years later when it is large. Many trees that decline slowly in older neighborhoods have been mulched this way for decades.
The test is easy. At the base of a healthy tree, the trunk flares out where it meets the roots. If you cannot see that flare, the tree is buried, either under mulch or because it was planted too deep. Pull the mulch back and look.
How to mulch a tree or shrub correctly
Follow these steps for any tree or shrub in a bed or a lawn:
- Pull old mulch back until you can see the root flare.
- Remove grass and weeds from the area you plan to mulch.
- Spread two to three inches of mulch, tapering thinner toward the trunk.
- Leave a few inches of bare soil right around the trunk or stems.
- Extend the ring outward as far as is practical. A wide ring helps the roots more than a tall one.
The same rule applies at the house. Keep mulch several inches below wood siding, trim, vents and sills. Mulch holds moisture, and wet material against a wall invites rot and pests. That is especially worth watching on older homes with low wood trim.
Refreshing mulch without piling on
Mulch breaks down and fades, most noticeably in the first year or two. Before you order more, measure what you have. If two inches are still in place, you probably do not need to add anything. Rake it to break up the crust and mix the faded top layer with the darker material beneath. That restores the look and lets water through.
If it has dropped below two inches, top up only enough to bring it back to two or three inches total. If years of yearly topping have built a thick layer, remove some before you add more. Adding a fresh inch or two on top of a deep layer every spring is what leads to mulch that stays soggy and sheds water.
Spring, after the soil has warmed and you have pulled weeds, is the usual time. A fall refresh also works and gives some winter insulation to new plantings. For a new bed, skip the landscape fabric and put down a layer of plain cardboard before mulching if you are covering grass; it breaks down in a season.
If a lot of beds need attention, you can have it done through mulch installation. For the bigger picture on bed design and planting, start with the landscape beds guide. Trees that look thin or unhealthy despite correct mulching may need a look from someone trained in tree and shrub care.
Frequently asked questions
Can I put new mulch over old mulch?
Only if the total stays at two to three inches. Rake the old layer first and add just enough to make up the difference. Stacking fresh layers on top of thick old ones traps water.
Does mulch near the house attract termites?
Mulch holds moisture and can hide pests, so keep it several inches away from siding and wood trim. If you suspect pest activity in or near the house, a pest control professional should inspect it.
What kind of mulch is best?
Shredded hardwood and bark mulch both work well in Southwest Ohio beds. Choose whichever you like the look of and use the right depth. Avoid mulch that smells sour until it has aired out.
3 min read.