Do water droplets on plants start fires and cause sunburn?

Most people will tell you to avoid watering your plants in the middle of the day. For a start evaporation will be high and you are probably ‘wasting’ some of our precious water.

The same people might also suggest that water droplets on leaves are dangerous. Dangerous to the plant because it will get sunburn. Dangerous to you and me (and the plant) because it might start a fire.

Well, some different people have done a little science to test these last two theories. Gabor Horvath from Eotvos University in Hungray led a group which just published their results in the journal New Phytologist.

Using experiments and computer models they set out to test whether the lens formed by a water drop could focus heat onto a leaf.

In theory it’s possible. Light travelling through water will be refracted, and change direction. If, because of the shape of the water blob, light from lots of different directions happens to be refracted onto a small area then lots of light, and energy, hit that area. That’s as much as I can remember of the physics.

But does it really happen? Well Horwath and his fellow researchers found that on a smooth leaf (e.g. a maple or ginkgo) a water drop won’t cause sunburn. A tiny glass lens will, but not a flattened water drop.

On a hairy leaf (they used the aquatic fern, Salvinia), however, the water drop is held away from the leaf surface and can really burn.

By analogy they suggest that water on hairy human skin will cause burning, presumably in lots of small focussed areas. Watch out for this next time you emerge from the sea or pool.

As to starting fires, they consider that unlikely. In theory a drop could sit on hairs above a dry leaf and like a lens above an ant, set it on fire. It is their considered opinion that the water will evaporate first.

Image: Water droplets on the plant of the moment, the frangipani - all this weekend, at the Tropical Centre, Royal Botanic Gardens Sydney. The leaf is hairless so no sunburn likely here.


Chris said…
So that's why it shouldn't be done! It still seems far fetched though. All the while I thought there was some chemical reason for not watering plants during high noon.