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 growths or gall on my oak tree?

If you look very closely at oak trees, you will see various abnormal bumps, growths, and distortions on the leaves and twigs. These growth come in all shapes and sizes, and are called galls. They may be woolly, velvety, or spiny. They may also look like small buttons, spindles, or blisters on the leaves. Oak galls may also be thick swellings on the twigs or leaf stems, round apple-like growths or fuzzy cotton balls on the leaves. These growths can be tan, black, brown, red, white, or pink.

Some galls are caused by mites, or small flies called gall midges, but tiny gall wasps cause most problems. There are over 700 different species of gall wasps associated with oaks. Most galls start in the spring when new tissue begins to actively grow. The female deposits an egg inside the tissue and the egg or larvae irritates or stimulates the plant to grow in a way that is characteristic for a given species. The insect or mite receives both food and shelter from the plant. The tree receives no benefit.

Galls are difficult to stop, but trees rarely need treatment. Galls on oaks rarely cause more than cosmetic injury to the plant. They can look very unsightly, but do not affect the long term health of the tree. The one exception is the "gouty oak twig gall". This gall causes swollen twigs that can result in branch dieback and even tree death.

Individual trees vary in their susceptibility to certain galls and the gall activity varies widely from year to year. Once you see the gall, it is too late to treat. For most species, there is only a three to seven day period during the year that you can spray the trees successfully. Additional problems include poor spray coverage on large trees and spray drift. It is hard to justify treatment for cosmetic reasons. To control the gall insects, and mites that over winter on fallen leaves, rake and destroy the leaves. You can also study and enjoy the many oak galls with names such as Woolsower, Hedgehog, Oak Apple, Oak Potato, Spangles, Bullet gall - they are intriguing examples of the diversity of nature.