As we get deeper into the hot days of summer, we start to think about the suns rays and how they may affect our plants. To the gardening world it may have always been considered a fact, but science has never proved the widely held belief that watering your garden in the midday sun can lead to the burning of plant foliage. Just recently I came across a study into water droplets collecting on the leaves of plants. This study, published in New Phytologist, provides an answer that not only will have garden clubs and plant enthusiasts thinking, but also may have implications for forest fires and human sunburn.
Can trees get sunburn from watering? The problem caused by light focusing on the water droplets adhered to plants has never been thoroughly investigated. However, this is far from a trivial question. It is a widespread belief that forest fires can be sparked by intense sunlight focused by water drops on dried-out vegetation and that plants can be burned in the same way. But is it true?
The experiments need to study how the contact angle between a water droplet and a leaf affects the interaction between light and water -- that is, to clarify the specific conditions, environmental or anatomical, under which water droplets can cause leaf burn. It has been found that water droplets on a smooth leaf surface, such as maple or ginkgo leaves, cannot cause leaf burn because these leaf surfaces lack “hairs” – technically the presence of hairs is called pubescence. However, some types of ferns which have small waxy hairs, are susceptible to leaf burn, and this is because the hairs can hold the water droplets in focus above the leaf's surface, acting as a magnifying glass.
Many people use ferns in the landscape – the best place to plant ferns is in the shade; in nature, we find ferns in the more shaded wooded environment.
Remember, right plant, right place! This information not only partly confirms the widely held belief of gardeners, but also opens a similar issue of sunburn on hairy human skin after swimming and lying out in the sun. Sorry to say, but a “hairy” person is more prone to sunburn by the same effect -– the hairs hold the water droplets in “position” to magnify the power of the suns rays. Basically, in sunshine water droplets residing on smooth hairless plant leaves (and smooth hairless humans) are unlikely to damage the leaf tissue or the human skin.
However water drops held by both plant hairs and human hairs can indeed cause sunburn. Theoretically, this same process could lead to forest fires if water droplets are caught on dried-out vegetation and lifted to the correct angle. If the exact angle of the water drops falls exactly on the dry plant surface intensely focused sunlight could, in theory, start a fire. My feeling is that this likelihood is reduced because the water droplets should evaporate before they have time to ignite, so these claims should be treated with a grain of salt.
So remember, as always – Right tree in the Right Place. Try and keep the leaves with pubescence in shadier areas and the smooth, hairless leaves in the more sun focused locations. Watering in the evening is not a better alternative as this can lead to foliar disease. The best bet is to always water in the early morning, before the greatest impact of the sun and also giving the trees enough time to dry out.