Mulch What Is Making Holes in My Mulch?
Short answer: Holes in mulch are usually made by earthworms (pencil-size), ground bees (¼–½ inch), cicada killer wasps (about ¾ inch), voles (1–1½ inches, no soil mound), squirrels and chipmunks (shallow digs), or skunks and raccoons hunting grubs (3–4 inch funnels). Measure the opening and note whether there is loose soil beside it to tell them apart.
Key takeaways
- Measure the hole: diameter, depth and any soil mound or castings identify most culprits.
- Timing narrows it down: cicada killers dig in late July to mid-August, skunks and raccoons dig at night, voles are active all year.
- Worm holes, beetle exits and most ground bees are harmless; just rake the mulch level.
- Keep mulch thin and away from trunks to discourage voles, and deal with lawn grubs to stop skunk and raccoon digging.
- Large burrows (groundhogs, rats) call for your local extension office or a licensed wildlife professional.
Holes in a mulch bed are rarely a mystery once you measure them. The size of the opening, whether there is loose soil around it and the time of year narrow it down quickly. This guide follows N.C. Cooperative Extension's identification notes.
Identify the hole by size
| Hole | Likely culprit | Other clues |
|---|---|---|
| Pencil-size | Earthworms (night crawlers) | Small piles of granular soil pellets (castings) about 1 inch high; spring and fall |
| ¼–½ in, many together | Solitary ground bees | Thin vegetation; 100 or more nests can be close together |
| About ¾ in, 18–24 in deep | Cicada killer wasps | All excavated dirt mounded to one side; dry, bare spots; summer |
| Nickel-size | Emerging cicadas or June beetles | Loose soil and pellets around the exit; late spring and early summer, often after rain |
| About 1 in with a mud chimney | Crayfish | Wet sites or high water table; chimneys 2–4 in tall |
| 1–1½ in, no soil mound | Voles | Active all year; damage to hostas, roses and other plants |
| Shallow (about 3 in deep), no dirt on top | Squirrels and chipmunks | Digging for or burying nuts in lawns and mulched beds |
| Up to 3 in, hidden near shrubs or woodpiles | Rats | Entrances disguised near sheds and dense planting |
| 3–4 in funnel-shaped | Skunks and raccoons | Dug at night while hunting grubs and insects |
| 10–12 in with a large mound | Groundhogs | Main entrance has a big mound; active by day |
What to do about holes in mulch
- Earthworms, beetles and most bees: leave them. Worm holes and castings are a sign of healthy soil under the mulch.
- Squirrels and chipmunks: shallow digging is cosmetic; rake the mulch level again.
- Voles: keep mulch thin and pulled back from trunks and plant crowns, since deep, fluffy mulch against stems gives them cover. The 3-3-3 rule in our tree ring guide helps.
- Skunks and raccoons: they are after grubs; repeated night digging is worth checking for a grub problem in nearby lawn.
- Large burrows (groundhogs, rats): contact your local extension office or a licensed wildlife professional for rules on trapping in your state.
After repairing a bed, check the depth: most beds need 2–4 inches (see how deep should mulch be). Topping up 1 inch over 100 sq ft takes about 8.3 cubic feet, or roughly 4 bags of 2 cu ft — the mulch calculator does the rest.
When do the holes appear? Clues by season and time of day
If you can’t measure the hole cleanly, the calendar and the clock are the next best clues. All timings below come from N.C. Cooperative Extension.
| When you notice them | Most likely | Confirm by |
|---|---|---|
| Spring and fall | Earthworms | Small piles of granular castings about 1 inch high |
| Late spring to early summer | Emerging cicadas or June beetles | Nickel-size exits with loose soil and pellets |
| Late July to mid-August | Cicada killer wasps | ¾-inch holes with all the dirt piled to one side; large wasps very noticeable by day for a few weeks |
| New holes every morning | Skunks or raccoons | Funnel-shaped holes 3–4 inches wide; grubs in nearby lawn |
| Any month, including winter | Voles | 1–1½-inch openings with no mound, linked by surface runways 1–2 inches wide |
| Summer, during the day | Groundhogs | 10–12-inch entrance with a large mound of dirt |
How to diagnose and fix holes in mulch, step by step
- Measure the opening. Use a ruler or a coin: a nickel is a little under 1 inch across. Compare with the size table above.
- Look beside the hole. Castings point to worms, a one-sided dirt pile to cicada killers, a mud chimney to crayfish, and no soil at all to voles or squirrels.
- Check the depth. Push a thin stick in gently. Squirrel and chipmunk digs are only about 3 inches deep; cicada killer tunnels run 18–24 inches.
- For ground bees and cicada killers, water the area. NC Extension says both can be controlled with a sprinkler, since they don’t like wet sites. Both tend to pick thin, bare spots.
- For voles, take away cover. University of Maryland Extension advises: “Do not pile mulch up against tree trunks or underneath shrubs.” Protect young trunks with 18-inch-high cylinders of hardware cloth set 2–3 inches below the soil line, and set snap-type mouse traps baited with apple in the runways.
- For skunks and raccoons, remove the food. NC Extension suggests controlling the grub population so they have less reason to dig.
- Repair the bed. Rake the mulch level and top up thin spots to 2–3 inches. Patching 20 sq ft at 1 inch takes 20 × 1 ÷ 12 = 1.7 cu ft, or one 2 cu ft bag.
Frequently asked questions
What makes small round holes in mulch?
Pencil-size holes with castings are usually earthworms; clusters of ¼–½-inch holes are solitary ground bees; ¾-inch holes with dirt piled to one side are cicada killer wasps. Holes 1–1½ inches wide with no soil mound point to voles.
What animal digs holes in mulch at night?
Skunks and raccoons. N.C. Cooperative Extension says they dig funnel-shaped holes with a 3–4 inch opening at night while hunting grubs and other insects.
How do I get rid of ground bees or cicada killers in my mulch?
N.C. Cooperative Extension says both can be controlled with a sprinkler, because they don’t like wet sites.
How can I tell if voles are making holes in my mulch?
Vole entrances are 1–1½ inches wide with no soil mound, and University of Maryland Extension says a network of surface runways 1–2 inches wide with many burrows is an easy sign. Voles are active all year.
Are holes in mulch a bad sign?
Not usually. Earthworm holes and castings indicate healthy soil, and squirrel digs are cosmetic; only voles, rats and groundhogs tend to need action.