If you have a maple in Eastern Iowa, chances are you have either seen a growth like the one pictured above, or currently have these bumps on your tree…but what is it?
The small bumps on the leaves are called plant galls. They can be green, red, yellow, and even black. These abnormal growths are caused by microscopic mites, so unfortunately you would not have spotted them before the galls form.
So, you may wonder, how do the galls form? The mites feed on the leaves and the reaction caused between the mite and the hormones in the plant causes these bumps to form. The abnormal cell growth is best compared to a tumor. While that sounds really scary for a human, it is mostly a cosmetic issue in plants.
What do I need to do? When the galls have formed, treatment is not typically recommended. It is very difficult to control the mites once the galls have formed because the mite and eggs are protected within the tissues of the leaves. For future control, we recommend doing two applications of a dormant oil before the buds on the tree break This would typically mean one application in early April and the second in mid-April. If we have unseasonably warm temperatures in March, this could be earlier. We have this oil available in the garden center. It is an All Seasons Dormant/Horticultural Oil.
We have also seen galls on Linden, Ash, and Oaks this year. The same treatment would be recommended for these varieties.