What happens if you water in the middle of the day?
Is it true that watering plants in the middle of the day can cause leaf damage or even a fire? TIM ENTWISLE looks at the way sun and water interact
Iwas a country kid. When I wasn’t trapping sparrows under propped-up tin cans, or building Lego prisons for grasshoppers, I was irritating ants with a sunbeam focused through my magnifying glass. That magnifying glass was also good for scorching, and sometimes burning, paper and dry leaves.
Of course, all fires were contained and all animals escaped unharmed, but I did learn a bit of basic physics during these escapades. From my experiments I learnt that a convex-shaped, transparent object could focus the sun’s radiant heat into a tiny dot, and that tiny dot could start a fire.
Stories abound of water droplets doing just that, should they remain too long on a leaf or on our skin. The fear is mostly of discolouration on leaf or skin, but there is also the idea that bushfires might start due to a freakish drop of water on a dry plant.
All very plausible to my childhood self. Indeed, the ‘scorching water droplet’ hypothesis, if I can call it that, is repeated on about three-quarters of horticultural websites. But is it true?
FLAT VERSUS FAT
Thanks to clever research in Hungary, followed up by neat school experiments in the same country, I can confidently say that most leaves (and skin) are extremely unlikely to be burnt by water droplets. The tests showed that glass beads, simulating water drops, have to remain on the leaf surface for more than half an hour before there is even a chance of focused sunlight burning the leaf. On a day with enough sunlight to burn leaves, real water droplets will usually evaporate in less than 25 minutes. While in contact with the leaf, they keep it cool as well.
In any case, water droplets on most leaves are quite flattened. I remember from my childhood that the shape of the lens and its distance from the ant was very important. Only when sunlight was focused on a small point did the ant heat up, causing it to jump away. For a plump, roundish lens, that point was close by, and for a lens shaped like two dinner plates glued together, the burning point was much further away. Our flattened water droplets are usually too close to the leaf to cause any damage.
Late or early in the day, when the light shines through the side of the droplet, it could conceivably focus light closer to the surface, but then the intensity of the sunlight is too weak to cause damage.
More spherical water droplets might form on the water-repelling surfaces of ginkgos and water lotus, but they generally roll off. Only by raising the droplet above the leaf surface do you shape a better lens, focusing the light directly onto the plant tissue and keeping the cooling water away from the target site.
Hairy plants can do this, particularly if the hairs are waxy and repel water. It has been seen in the floating fern Salvinia molesta, but I expect things like lamb’s ears (Stachys byzantina) or the native woolly cloak-fern (Cheilanthes lasiophylla) might suffer the same fate. On such long-haired plants, a water droplet could conceivably sit aloft for long enough and far enough above the leaf to cause burning.
For hairless or mildly hairy plants, watering in the middle of the day won’t burn their leaves. Brown spots are more likely due to impurities in the water such as chlorine, salt or a high concentration of fertiliser. There could also be some physiological stress associated with spraying very cold water onto a very hot plant surface, but that’s even less likely.
For a hirsute plant, I’d recommend you avoid watering onto foliage in full sunlight. Whether the leaves are damaged or not will depend on where the droplets end up in relation to the leaf surface and the angle of the sun, and how quickly they evaporate. Still, if your plant is wilting and you can’t avoid spraying the leaves, I’d risk it.
As to fear of rain igniting dry vegetation, I think you should get over that one. Even for hairy plants, the moisture would most likely dampen the fuel load for as long as it took strategically suspended droplets to evaporate. Like sunburn on plants, there are far more likely causes of bushfire.