Digging In - Advice on Leaf Spots on a Black Gum Tree, Dividing a Hardy Hibiscus

By Scott Aker
Special to The Washington Post
Thursday, April 30, 2009

Q We have a black gum tree, now about 20 feet tall. Last summer we noticed some of the leaves had spots and holes. How do we prevent this again? We have raked all the fallen leaves away from the tree, but should we be applying a chemical, and if so, which one?

A Your black gum was infested with tupelo leafminers. The tupelo leafminer is an insect in a group known as shield bearers. They are moth species that lay eggs on new leaves in spring. When the eggs hatch, the small caterpillars tunnel into the leaf and feed between the upper and lower surfaces, where they are well protected from predators.

As each one feeds, it makes an elliptical spot in the leaf, leaving only the dead upper and lower surface. When mature, the caterpillar fastens these leaf surfaces together, and the resulting disk drops to the ground, along with the leafminer. There may be several generations of tupelo leafminers each year.

Removing all traces of the fallen leaves should control them. They are not a severe problem every year, suggesting that predators and parasites may be active in controlling them, as they are for most native insect pests.

I have a five-year-old hardy hibiscus, variety Lord Baltimore, that has become crowded and needs watering almost daily during peak season. It has as many as 100 blooms a day and is the delight of the neighborhood. Can I divide this, and when?

Hardy hibiscus, also known as marsh mallow, is one of the last perennials to sprout in the spring garden. You should dig and divide it in late April or early May before growth starts. The roots of Hibiscus moscheutos are far-reaching and brittle, so take care when digging them up. Once you have dug the clump from the ground, you can use an ax to chop it into pieces, each of which should have at least four vigorous buds. Plant the divisions in a sunny location with fertile soil, and they should bloom by late summer.

I would like to plant a fruit-bearing shrub or small tree in the sunniest part of a shady yard. It probably gets half a day of direct sunlight. Most fruit bushes need full sun. Do you have any recommendations for my situation?

Pawpaw is tolerant of shade, and although fruit production may be diminished, the fruits produced will be as tasty as those grown in full sun. Blackberries may also produce a respectable crop with a half-day shade. Serviceberry is another possibility.

Scott Aker is a horticulturist at the U.S. National Arboretum.

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