Gardening Help FAQs

Here are answers to some of the most common questions we receive about garden plants. You will find concise information on general gardening techniques as well as plant selection and care. For detailed information on specific plant pests and problems refer to our Common Garden Pests and Problems page.

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Horticulture Questions and Answers

What are these galls or growths on my tree?

When you look closely at the leaves of many trees, you may notice small bumps, velvety, colored patches, or even bizarre distorted growth. These growths are called galls and are most likely caused by a specific insect or mite. Although they look unsightly, damage is usually only a cosmetic problem and will not affect the health of the plant. Galls are not contagious and the insect or mite which cause the galls in one type of tree or shrub does not attack other types of plants. Galls are common in some years on some plants. It is common for gall activity to differ from year to year.

Hackberry trees have a green nipple-shaped growth on the leaves caused by a psyllid insect. Maple trees have spindle-shaped growths or velvety red patches of tissue caused by eriophyid mites. Oak trees have hundreds of tiny wasps and midges that cause all sorts of strange growths on leaves and twigs. Galls are also common on rose bushes, ash trees, hickory trees, and grape vines.

Galls form when an insect or mite lays eggs on, feeds on, or in some other way, irritates actively growing plant tissue. The plant responds by growing in a non-typical way. The enclosed insect or mite receives shelter or food inside the gall. The plant receives little benefit. Most galls are initiated shortly after the plant starts growing in the spring.

You rarely need to control galls. Galls are not a life or death matter, but a cosmetic problem for the plant. Once you see a gall growing, it is too late to treat for the season. Also, for most species, there is only a 3 to 7 day window in spring when you can successfully treat plants. This varies depending on the temperature, the plant, and the insect or mite that is attacking the plant. If chemical control is attempted you must thoroughly spray the tree or shrub. Large trees are hard to spray, even if your timing and other variables are correct.

For some galls, such as the mossy rose gall on rose and grape leaves, cutting and destroying the galls before the insect emerges is effective. Raking and destroying leaves will help for other galls if the insect or mite has not escaped the gall. You can use dormant oil sprays if the insect or mite spends the winter on the tree. If you decide to spray, determine what causes the gall, proper treatment, and what product is most effective.