Why are my rhododendrons spitting water? On hot sunny days this summer, we’ve been sitting in their shade and noticing occasional tiny needle-fine droplets landing on us. Is this caused by insects (I don’t see obvious signs of them on the leaves), or is it normal?
My hunch is that the leaves are transpiring. I found an article by Bill Letcher in the Journal of the American Rhododendron Society (vol. 41, no. 2) entitled “Rhododendrons with a Drinking Problem.”
The article is about the importance of water in successful rhododendron cultivation, but also does a good job of explaining transpiration:
“The water absorbed by the root hairs, or rootlets, is drawn up in the stem of the plant and into the leaves, where it exits via small holes called stomata into the air as vapor. This process is called transpiration […] Transpiration requires a lot of energy, and this comes, as we might expect, from the sun. If all else is equal, transpiration amounts are directly proportional to the amount of radiant energy striking the leaf surface of a plant: the more energy, the greater the amount transpired. […] One factor of paramount importance to rhododendron growers which effects transpiration is the relative humidity of the air surrounding the leaves. Dry air causes appreciably more transpiration than humid air, and the presence of wind increases this dramatically by replacing the layer of air next to the leaf surfaces before it can become saturated. Ambient air temperatures will also markedly increase transpiration as they rise, sometimes with disastrous results. Our family of rhododendrons has less ability to extract moisture from the soil than, say, manzanita (which can pull all of the available water from the soil down as far as six feet!).”
If we have more hot weather, you may want to give your shrubs an occasional supplemental drink of water, since they are shallow-rooted, and our summer-dry climate (depending on how hot and dry it is) can cause stress to the plants. Keep an eye on how they are looking and do what you can to maintain their health (and preserve your shady refuge!). To quote Letcher again:
“The family of rhododendrons has either never been forced to adapt to very low available moisture conditions, or has not adapted well to them. Their roots tend to be quite shallow. The foliage is lush, dense, and of a dark, absorptive color instead of being light and reflective of the radiation. They display their displeasure with your attempts to wean them from the conditions they most enjoy, by doing exactly what a great many other plants will do: they shorten their internodal growth, reduce the number and size of their new leaves, drop some of their older leaves, and just plain sulk! Ultimately, they languish into a pitiful state of scrawnyness and pallor which is not a pleasant sight in your garden.”