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Garden Q&A: What is gall?

Q: What on Earth is going on with this maple leaf? I saw it on a wild tree while taking a walk down a neighborhood path, but I wonder if it’s something that can spread to nearby gardens.

A: This is a great example of a gall, which is a tissue deformity on a plant caused by either insects, mites, fungi, bacteria or nematodes. Usually galls cause swelling or weird projections on leaves or plant stems, but sometimes the more obvious feature is a color change like this.

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The activities of the organism responsible creates chemical changes in the leaf tissue, redirecting tissue formation to suit its needs. For instance, insect-made galls give the larvae their own little house to feed in while being protected from most predators or harsh weather. (Impressively, tiny parasitoid wasps, little bigger than a dash on this page, still find their prey inside these structures and interrupt their life cycle. Isn’t that amazing?)

Despite how drastic galls may look to us, they don’t cause much harm to their host plants, which can be trees, shrubs, or perennials. Oak trees are renowned for harboring many kinds of eye-catching galls, some of which become most noticeable when they fall out of the canopy onto our lawns or gardens. See if you can find anything living inside those swollen red or brown lumps or balloon-like pockets on leaves. A wise bird or other insect may have beaten you to it, though, or the culprit is long gone and already flew away as an adult before the plant jettisoned the injured leaf.

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If an eyesore, you can clip off heaviest infestations of leaf galls on witch hazel (caused by insects), azaleas (fungus), oak saplings (usually insects) and any other easy-to-reach plant. Keep in mind that the unaffected portions of those leaves are still functioning to feed the plant, so don’t remove too much growth. Otherwise, I suggest you leave them alone and just marvel at the intricacies of the natural world. Gall-forming insects can feed songbirds and don’t risk the health of the plant. As with any organism, populations wax and wane over time and galls might be prevalent one year and nearly absent the next.

We have several webpages with more gall information, including Shade Tree Galls and Eyespot Galls on Trees.

Q: My evergreen hollies are shedding lots of leaves, though all the remaining growth looks good. What’s causing that, and is it preventable?

A: The hollies are probably just fine and doing what is natural for many evergreens in spring. Although we think of evergreens as being ever-leafy, they do still shed leaves. Instead of a single, complete leaf-drop event (like deciduous trees do before winter), they retain leaves for two or more years before discarding the oldest.

How old the leaves get before being shed largely depends on the species, but environmental factors can influence the process and cause a plant to drop more leaves than average. One potential cause is stress, such as from oversaturated soil or drought. Ironically, the opposite can also produce the same result: an exceptionally good growing season one year that produces lots of new leaves can cause prolific shedding a few years later because that bumper crop of foliage has aged-out.

American hollies and yews, for instance, shed old leaves in spring; Eastern white pines shed old needles in autumn. One indicator that shedding is probably unconcerning is that it’s not happening to the youngest growth. Older leaves are located further back on a stem (more in the interior growth) and on lower branches receiving more shade as a shrub or tree matures. Look at the leaf undersides on your holly and make sure you don’t see lots of scale insects, but usually high levels of a pest or disease that would contribute to shedding will be easily detectable. As we move further into the growing season, the shedding will wane and stop.

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


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