Common advice: you should not water the garden at midday because the water droplets act like magnifying glasses and will burn leaves. Is this true?
There are other similar beliefs that have nothing to do with plants. Many dermatology websites say that water droplets on skin will burn it. In the forest industry, it is believed that a rain drop can heat dry leaves to the point of ignition and cause forest fires.
Common sense and science will reveal the truth.
A Magnifying Glass Can Cause Fires
If you played with a magnifying glass as a kid you know that they can be used to focus the light beam from the sun onto a spot and make it very hot. You can use this to burn your skin, burn holes in paper and even fry ants. I’ve tried them all and it works.
But if you think back to how this happened, you will remember that you had to hold the magnifying glass at a specific height so that the light going through the glass was focused on a small spot. At the wrong height, nothing happened.
This should be the first clue to help debunk this myth. Rain droplets sit right on the leaf. There is nothing holding them at a specific height to focus the light. Granted rain droplets form a lens that is a different shape than what you find in a magnifying glass so rain drops may work better sitting on an object.
The Science of Rain Drops
This issue was studied as early as 1892 and they came to the same conclusion as some more recent studies.
After testing various shaped droplets, it was concluded that droplets on smooth surfaces like most leaves and human skin would not burn the surface. The shape of these droplets is such that they focus the light at a point below the leaf, so in effect they don’t focus the light, and therefore no damage is done.
However, if the droplet is suspended above the surface by either human hairs or plant hairs, it can cause damage. Such a raised droplet forms a different shape and now focuses heat onto the plant where there is a lack of water for cooling. For this to cause damage, the droplet needs to be the right height for the shaped droplet – not a common event.
In the case of droplets sitting on dry vegetation, any heating that might occur would be cooled by the water until it evaporates, at which point it stops focusing the light. It is unlikely droplets cause forest fires.
I find it hard to believe that any gardener would believe this myth. All you have to do is go outside after a midday shower and you don’t even need a scientist to help with this. Look at the leaves a few hours after such a rain. They don’t show any damage. If water droplets at midday damage leaves, we would have almost no viable plants.
Should You Water at Midday?
Some people suggest that watering at night is a problem because the slow evaporation of water on leaves will lead to fungal infections. I had a close look at this in Best Time to Water – Morning, Noon or Evening?
It is warmer at midday and so water evaporates quicker. More water will get into the soil if you water when it is cooler, but I doubt this makes a lot of difference.
When should you water? The best time is when the plant needs it and when you have time to do the watering. Morning, noon or at night makes little difference.
Tips and Tricks for Watering Plants
Here are some more posts about watering plants.