It is a piece of advice that has been followed by generations of gardeners – never water your plants in the full glare of the midday sun.
But new research has contradicted the widely-held belief that watering in direct sunlight can cause leaves to suffer from unsightly “leaf burn”.
A team of physicists, troubled by the lack of scientific evidence for the phenomenon, set out to test the theory that water droplets on leaves can act like mini magnifying lenses, focusing the sun’s rays and leaving a leaf’s surface covered in scorch marks.
Using computer modelling as well as tests on real leaves, the researchers claim to have disproved the theory.
They found that water droplets on a leaf surface were not able to focus the sun’s energy sufficiently to damage the leaves before the water evaporated.
Only on some tropical plants with hairy leaves were the water droplets held sufficiently far from the surface to cause burning. But hairy leaves tend to shed water, so droplets would be unlikely to stay on them long enough to do damage.
Dr Gabor Horvath, who led the research at Eötvös University in Budapest, Hungary, said: “This problem has been dealt with only by amateurs, gardeners and laymen, who could only speculate about this subject. The consequence is that myths rule.
“We believe that completely unrelated types of leaf damage might be partly responsible for the widespread belief about sunburn caused by water drops.
“For example, drops of acid rain, salty sea or tap water, chlorinated water and concentrated solutions of fertiliser or other chemicals can all cause sunburn-like brown patches.
“Plants could also suffer some kinds of physiological stress from putting cold water onto hot leaves.”
Advice against watering plants in direct sunlight is widespread. The Royal Horticultural Society states: “Water on leaves can act as a lens that concentrates the light leading to burnt patches,” while experts at the Royal Botanic Gardens, Kew, say they have followed the advice for years.
However, the researchers’ computer simulations of how water droplets refract and focus sunlight through the course of the day found that the most likely times for sunburn to occur were actually early morning or late afternoon, when the sun is lower, rather than at midday when it is at its highest and hottest.
When water droplets were placed on leaves taken from maple and Ginkgo biloba trees, which were then left out in the sun at various times of the day until the water had evaporated, they found no evidence of leaf burn.
As a control, other leaves which had glass beads placed on them suffered severe burning. A glass sphere acts as a stronger lens than a water droplet.
In a third test the researchers used leaves of a floating fern with water-repelling hairs on their surface. The droplets formed spheres in the hairs above the leaf surface which focused the sunlight and caused burning after around two hours.
Dr Horvath believes that there are other, more compelling reasons for not watering plants during the full heat of a sunny day. He said: “Water evaporates quickly from the soil surface, and so it cannot be used efficiently by plants.”
Stewart Henchie, a senior horticulturalist at Kew, said it was common for gardeners and horticulturalists not to water plants on hot sunny days, but he was not aware of any scientific evidence to back up such advice.
He added, however, that young plants could still be vulnerable to burning as their leaves were not hardened against the weather.
He said: “If we are watering these young plants we tend to use a fine mist to stop water droplets from forming. I’d be certainly be keen to try out a few tests of my own now to see if what the effect might be on different leaf types.”
Simon Thornton-Wood, director of science and learning at the Royal Horticultural Society, said: “This research is challenging the traditional notion about why you shouldn’t water plants on bright sunny days.
“There are other reasons for not doing so, the main one being water conservation. It is possible that it may be chemicals – feed and pesticides – that many gardeners put in their water that is causing the damage instead.”