Regional Gardening Reports :: National Gardening Association

In the Garden:
Western Mountains and High Plains
January, 2006
Regional Report

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These galls -- common on oak trees -- add interest to the winter landscape scene.

Galls: Marvels of Nature

When I walk along nature pathways, or hike in the woods, I'm always on the lookout for interesting plant materials or other aspects of the winter landscape. Just recently I came across some swamp white oak trees and native cottonwoods that had strange growths on the stems. These mysterious formations on plants -- termed hypertrophies or, more simply, "galls" -- have intrigued naturalists for centuries. From my perspective, these outgrowths do add interest, but they may be of concern to homeowners and gardeners.

Plant/Insect Partnership
Galls can be smooth, rough, spiny, or sometimes fuzzy, and they come in a array of colors, depending on the growing season. They are generally not harmful to the host plant, but rather provide the food and brooding home for various insect species or mites. They represent an interesting relationship between plants and their special symbiotic insects. The insects may provide a service to their host plant by helping with pollination of the flowers. This symbiotic relationship may also serve to protect the plant species in a competitive environment in which these plants would not otherwise survive.

Although galls are most visible in fall and winter when deciduous trees have lost their leaves, spring is the time when gall formation occurs -- during the unfolding of the buds or accelerated growth period of new leaves, shoots, and flower buds. The gall-forming organism (mite or insect) develops within the gall, and the gall continues to grow as the organism feeds and matures. Once gall formation is initiated, galls continue to form for a year or longer.

Some of the most unusual and colorful galls are caused by fungi that permeate the plant host tissues. If you have junipers or cedars in your landscape, be on the lookout for reddish orange balls called "cedar apples" early this spring. These galls are caused by rust fungi of the genus Gymnosporangium. The galls are covered with glistening gelatinous spikes that bear thousands of spores called teleutospores. When mature, the spores infect nearby apple trees, crab apples, and hawthorn trees, where they complete their life cycle. This is one important reason not to plant junipers close to your home apple orchard.

Next time you're on a nature hike or just strolling through the park or open space trail, see what kinds of galls you can spot. They are truly marvels of nature.

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