Sunburnt leaves: the myth debunked Two years ago, four Hungarian scientists published a paper called "Optics of sunlit water drops on leaves: conditions under which sunburn is possible" in the journal New Phytologist. Given the near-universal belief that water drops can scorch plant leaves on a sunny day (e.g. the RHS book How To Garden: "Under a hot midday sun, water droplets on leaves will act as miniature magnifying glasses and may scorch them"), you may be surprised - or you may not - that no one had previously checked to see if this actually happens. First of all, the short answer is no. At various times on a hot, cloudless day in July (in Hungary, where such things still happen), water drops were carefully placed on the surface of horizontal ginkgo and Norway maple leaves and left in the sun until they had evaporated, which generally took an hour or two. Careful examination of the leaves revealed no trace of any damage. Which leaves the interesting question: since water drops undoubtedly can act as small lenses capable of focusing the sun's rays, why do they fail to cause any damage? To find out, the researchers carefully calculated the paths of light rays falling on water drops. The first thing to say is that water makes a less effective lens than glass, owing to its lower refractive index. The second is that the shapes of water drops on a leaf vary a lot, depending on how "wettable" the leaf surface is. On a wettable leaf, such as maple, water spreads out to form a thin, shallow drop. Although such a drop is capable of acting as a lens, it focuses light well below the leaf surface (or it would if the leaf were transparent - in practice it doesn't focus it anywhere). On a less wettable leaf, such as ginkgo or rowan, water forms drops that are more spherical, but these still focus light that comes from directly above, well below the leaf. Such drops can focus light from a low angle onto the leaf beside the drop, but only (assuming the leaf is horizontal) early or late in the day, when the light intensity is low. In any case, such drops easily roll off leaves, which are rarely still or exactly horizontal, so the likelihood of causing sunburn is very low. As the authors conclude, "water drops cannot cause sunburn, either on water-repellent or on wettable leaves." Are there any circumstances under which water drops on leaves can cause sunburn? Yes, but only if the leaf has a dense covering of water-repellent hairs, in which case drops can be held above the leaf surface, allowing them to focus light on the surface itself. The researchers were able to demonstrate such damage on the leaves of the floating fern Salvinia natans (inset above) which has such hairs, if they were kept still in a small container of water. But terrestrial plants with similar hairs shed water drops very quickly, again making sunburn extremely unlikely in practice. In any case, most plants with densely hairy leaves are drought-tolerant, so why would you feel the need to water them anyway? None of this should occasion much surprise. Since thundery summer rain is often followed by bright sunshine, if this really caused sunburn, the leaves of most plants would be full of burn holes most of the time, and they're not, are they? Given the unambiguously negative evidence, why is the belief that water drops can cause sunburn so persistent, and advice to that effect dispensed even by authoritative sources like the RHS? One possibility is that, although the researchers used plain tap water, many less-pleasant liquids can find their way onto leaves: drops of acid rain, seawater, chlorinated water, "grey" water, fertiliser or other chemicals. All of these can leave deposits on leaves that look like scorch marks, or may actually dehydrate leaf patches by osmosis, causing burn-like spots. Even though you don't need to worry about sunburn, it probably is a good idea to avoid watering during the middle of the day - especially in sunny or windy weather - since this may lead to excessive water loss by evaporation. The optimal watering time is early morning, since evening watering can mean leaves and growing points remaining sodden all night, which can encourage fungal pathogens. The Hungarian scientific paper led to a lot of press attention at the time. Here's how the news was greeted by the Daily Mail: "Sun shining? Then don't water your plants or you'll burn them. The lesson is clear: don't believe everything you read in the papers. Except this one, of course.