Gardeners half-correct about midday watering

Gardeners have always maintained that watering plants in sunlight can damage the foliage, but research published in New Phytologist shows it isn’t quite that simple. The intriguing new study into sunlit water droplets provides an answer not only for gardeners, but also may have implications for the control of forest fires and preventing human sunburn.

“The problem of light focusing by water droplets adhered to plants has never been thoroughly investigated, neither theoretically, nor experimentally”, said study leader Dr Gabor Horvath, from Hungary’s Eotvos University. “However, this is far from a trivial question. The prevailing opinion is that forest fires can be sparked by intense sunlight focused by water drops on dried-out vegetation.”

Horvath’s team conducted both computational and experimental studies to determine how the contact angle between the water droplet and a leaf affects the light environment on a leaf blade. The aim was to understand the environmental conditions under which sunlit water drops can cause leaf burn, or start a fire.

The researchers found that water droplets on a smooth surface (such as maple or ginkgo leaves) cannot cause leaf burn. However, the team found that floating fern leaves, which have small wax hairs, are susceptible to leaf burn. Horvath explained that this was because the hairs can hold the water droplets in focus above the leaf’s surface, acting as a magnifying glass. This finding not only partly confirms the widely held belief of gardeners, but also opens an analogous issue of sunburn on human skin after bathing.

“In sunshine water drops residing on smooth hairless plant leaves are unlikely to damage the leaf tissue”, summarized Horvath. “However, water drops held by plant hairs can indeed cause sunburn and the same phenomenon can occur when water droplets are held above human skin by body hair.”

The same process could theoretically lead to forest fires if water droplets are caught on dried-out vegetation, but Horvath believes this to be a rare occurrence. “If the focal region of drops falls exactly on the dry plant surface intensely focused sunlight could theoretically start a fire,” he explained. “However, the likelihood is reduced as the water drops should evaporate before this.”

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Source: New Phytologist

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