AS A botanist, gardening has – unsurprisingly – fascinated me since I can remember. However, it isn’t only the mysteries of plants that I find so full of wonder, but also the colourful, and often puzzling, human behaviour we see in the world of horticulture.
Steeped in centuries of received wisdom, gardening’s many “rules” have been repeated so often they can seem like incontrovertible truths. This is despite scientific trials demonstrating that many have little basis in fact, so following them may be unnecessary at best and could give you inferior results at worst.
Advertisement
A classic example is the long-held idea that watering plants around noon on a sunny day should be avoided, since it might harm their leaves. The explanation is that tiny water droplets can act like lenses, focusing the sun’s rays onto sections of leaves just like a magnifying glass would, resulting in scorched foliage and reduced plant health.
This belief has even been cited by professional foresters as the cause of wildfires. Given the devastating impact these can have, it is pretty astonishing that on the topic wasn’t published until 2010.
Four researchers – most of whom were at Eötvös Loránd University in Hungary – set out to learn more, running experiments on living plants and carrying out computer modelling. They found that spreading small glass spheres over the surface of smooth-leaved plants could indeed have this “magnifying glass” effect, causing damage right across the leaf surface. But when this was repeated with actual water droplets, such damage didn’t occur.
This is because water behaves rather differently to glass. Firstly, the shape of a water droplet on a leaf is more elliptical than spherical. The computer modelling showed that the maximum damage through a lens of this shape would occur when the sun was at a low angle in the sky, so in the morning or in the afternoon.
However, the sun’s intensity at these times is too low to cause any harm. Even if the intense light of the midday sun did somehow come at the most potent angle, the heat at this time of day would invariably cause the water droplets to evaporate before they had an effect.
The moral of the story? If your plants are in need of a good watering, give them some water. Not watering thirsty plants on a hot and sunny day for fear of leaf scorch will almost certainly lead to more damage from drought stress than could be caused by the magnifying glass effect. While it remains generally true that the ideal time to water a plant is in the morning or evening – to lessen the amount of water that evaporates before reaching the plant’s roots – the evidence doesn’t support the idea that watering at midday will cause burning.
For other projects visit newscientist.com/maker.
