After hauling home bags of potting soil and planning and planting a container garden worthy of a two-page spread in a gardening magazine, waking up to a series of 4-inch holes in the potted soil can be maddening. Several events can explain these mini-excavations, which can either threaten the health of the plants or simply pose a nuisance.
Holes that begin appearing as soon as the container is planted may signal a hungry animal. Browsing mammals cause holes all season long. Holes that appear in summer where soil that has been in a container for more than a year may be tunnels dug by hatching beetles as they emerge from pupal stages. Spring and early summer holes may result from the metamorphosis of toads, particularly in pots placed near backyard ponds.
As urban and suburban development consumes more wildlife habitat, small creatures search for places to forage and preserve the fruits of their labors. Many are able to excavate to a depth of 4 inches. Several small mammals adopt easy-to-excavate flowerpot soil as a substitute for habitat soil. Chipmunks actually eat shallow plant roots. Squirrels use flower pots as storage bins for seeds and nuts that they harvest and rats or voles may dig the harvest up. Only a large crow would have a large enough beak to dig out stored seeds.
A few large insects pupate in soil and a few are large enough to leave 4-inch holes when they emerge in late spring or summer. Cicada killer wasps tunnel as deep as 6 inches and then tunnel 6 inches horizontally to lay eggs. They need planters or large containers for nests because they not only lay eggs, they drag their cicada prey in to feed the offspring. Adults measure up to 1 1/2 inch and emerge throughout the summer. June beetles lay their eggs 2 to 5 inches deep and the pupal grubs work down 3 to 6 inches into the soil and emerge the following spring.
Toads live the first part of their lives as tadpoles in ponds and streams but crawl up on land to complete their metamorphisis into toads. They look for cool, moist places to burrow into the earth during hot afternoons and, occasionally, a container or planter full of moist soil and shady plants fills the bill. As night falls, they pop out, leaving a hole that can be 4 inches deep if the toad is fully mature.