What is that?! Peering into the dense woodlands surrounding my house, I see a large number of oddly shaped growths on the leaves of my oaks, hickories, elms, and maples. And I see them on the perennials too. What is going on here? One simple answer, these are galls.
What is a gall? A gall is by definition, an abnormal growth on a plant or plant tissue, produced as a reaction to chemicals injects by insects or mites in preparation to egg laying. In a general landscape, you can expect to find galls on three or four species of plants. Some galls are more abundant, such as Hackberry nipple gall or Ash flower gall.
Galls are very interesting for their looks and what they may do to trees or shrubs. Some galls resemble tiny hedgehogs, while others look like balloons. Most of them do not affect the health of a plant, just the appearance. Ash flower gall, for example, is produced when the ash flower gall mite injects a chemical into the individual flowers of an ash tree. The galls that form are unsightly and greatly harm the appearance of the tree, while not actually hurting the health of the tree. While it is possible to prevent gall formation with insecticides/miticides, it may be expensive or harmful to beneficial insects on the tree. As with diseases and insects, each year is different as to the severity of gall formation.
Hackberry nipple gall is a very common gall throughout the range of common hackberry (Celtis occidentalis). In really bad years, the gall production may be heavy enough to defoliate a tree through the heat of summer, when trees are most stressed, however, we rarely recommend treating hackberries for these galls.
Any tree or shrub can be affected with the formation of galls, but some are more prone to attack than others. Oaks, for example, have been shown to have a large number of gall producing insects using them as hosts for their eggs. The majority of galls are produced by tiny wasps, which are great pollinators for flowering plants. These wasps are so small that most people would not recognize them as wasps.
Galls can also form on perennials too. I often find large stem galls on goldenrod in my meadow garden. They do not seem to affect the overall health of the plant, nor the flowering capability, just the look. On the goldenrod, I think they rather improve the look!
Next time you are out for a walk or checking out your landscape, see how many galls you can find. Photograph them and show them to your friends to impress.