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Garden Q&A: Caterpillar droppings and reestablishing a new shrub

These are the droppings, known as frass, of a caterpillar that becomes a Polyphemus moth, the largest silk moth.
These are the droppings, known as frass, of a caterpillar that becomes a Polyphemus moth, the largest silk moth. (Ellen Nibali/For The Baltimore Sun)

I saw these on the driveway daily. They look like grenades or pine cones, but they crumble when you touch them and they are under maples. What are they for?

These delicate little grenades are frass, the term for caterpillar droppings, but they are a wonderful find. These are the calling cards of a huge moth known as the Polyphemus moth. It is named for a cyclops in Greek mythology because of its beautiful eyespots — one on each wing. It is the biggest of the silk moths.

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Often frass is the only way you are alerted to them, because the caterpillars are a bright leafy green color and feed on leaves high overhead. They don’t do significant harm to plants. The cocoon is a non-descript brown, like a deflated fragile football attached to twigs. Adult moths have up to a 6″ wingspan, in painterly shades of chestnut to fawn brown, and are short-lived since they have only a vestigial mouthpart. Sighting one is an event.

Our two-year-old shrub has always had random discolored or almost dying branches, even though we water it when the weather is dry. It got fall color and started dropping leaves in August!

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It’s not intuitive, but we’d recommend digging it up and replanting. Your shrub has not established.

Many shrubs are vigorous growers that easily become root-bound in their pots prior to purchase/planting, so loosening-up and untangling the roots prior to planting is essential to get them to establish quickly. Plants will show stress and even nutrient deficiency not from nutrients lacking in the soil, but rather from roots that can’t function properly.

While your shrub is still a manageable size, dig it up, check the roots and loosen what remains root bound. Direct roots away from the trunk.

Another good reason to “rough up” the root balls of a new plant is that when root balls are left intact at planting, the two different soil types (inside the root ball and the ground soil) don’t mix and they develop water permeability problems — one will tend to stay too wet and the other too dry. This impairs the root system’s ability to grow and take up enough water and nutrients to keep the foliage in good condition.

Your shrub’s premature fall coloration suggests this is happening. Stressed plants shed leaves as a way to conserve resources until root-leaf balance is restored. Replant so the root flare is visible and level with the surrounding soil. Mulch no deeper than three inches, never letting mulch touch the main stems.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Maryland’s Gardening Experts” to send questions and photos.

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