June 5, 2023, 3:25 am | Read time: 3 minutes
Many avoid watering their plants at midday, fearing it might harm them. Water droplets are thought to concentrate the midday sun’s rays–resulting in sunburned leaves. At least, that’s the assumption. What’s the truth behind this?
Watering plants at midday is a definite no-go for many hobby gardeners. It is believed to harm the leaves, as water droplets would act like a magnifying glass, concentrating the sun’s rays. Many prefer to water in the morning or evening hours. But is there any truth to the claim that you should leave the watering can aside at midday? Experts have delved into this very question.
How Water Droplets Affect Leaves
When light rays hit a lens, they are concentrated and converge at a point behind the lens, known as the focal point. It can get very hot there–which is why you can use a magnifying glass under sunlight to set paper and other things on fire.
Can water droplets on leaves in the midday sun have a similar effect? Not really. The water droplets almost never have the right shape needed to act like a magnifying glass, explains physicist Gernot Münster on a University of Münster website, where he addresses popular misconceptions in physics. The focal point would also need to be on the underside of the droplet to lie on the leaf. And the sunlight would need to hit the droplet at the right angle.
A droplet would need to ideally focus incoming light on the leaf’s surface–something even a perfectly hemispherical droplet doesn’t quite achieve. When water evaporates in the sun on the leaf, it even creates a cooling effect. However, if the water contains salts that remain after evaporation, these can damage the leaf’s cells.
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Researchers Have Been Studying This for Years
Researchers at the University of Budapest addressed the question of the magnifying glass effect more than ten years ago: They exposed various leaves to sunlight, placing small glass beads or water droplets of different shapes and sizes on them. They also simulated these conditions on a computer. Their conclusion: It is unlikely that water droplets on smooth, hairless leaves would damage the leaf tissue in sunlight. However, on leaves with fine hairs, droplets held by these hairs could indeed cause sunburn–if their focal points fell on the flat part of the leaf.
An Australian biologist responded to their article, published in the journal “New Phytologist” in 2010, with a scientific commentary. He concluded that further research is needed–such as on the temperature threshold that must be exceeded for leaf burns to occur. There are also initial indications in research that increased radiation intensity could damage part of the system involved in photosynthesis, the biologist writes.
More on this: 3 common watering mistakes that harm plants
Why You Should Still Avoid Watering Plants at Midday
However, it is generally better to water in the morning or evening. If you water plants in the blazing midday sun, the water evaporates relatively quickly, so it is partially wasted. Some experts also caution that large temperature differences can cause a kind of cold shock in plants.
with material from dpa