Is it bad to water the leaves of plants in the sun?

Q:  I’m told not to get water on the leaves of my plants when the sun is out because the droplets act like a magnifying glass and burn the leaves.  Is that true?

A: The short answer is no. It's one of gardening's most persistent myths, but research has shown that water droplets on leaves do not act like tiny magnifying glasses that burn plant tissue.  Here in Colorado we get short rain showers followed by bright sun.  If this were true there wouldn’t be a leaf left on a tree.

Here's why the myth doesn't hold up. 

People assume a round water droplet focuses sunlight the way a magnifying glass does. While a droplet can bend light, it isn't in the right position or shape to concentrate enough heat on the leaf surface to cause damage.  It actually cools the leaf surface as it evaporates.  

On a related topic, if you're overhead watering tomatoes and other disease prone veggies like squash and cucumbers, do it in the early morning after sunrise so the foliage dries quickly.  As long as they dry quickly after getting wet, there should be no problem. It's wet in evening through the night that causes trouble. The cooler temps, higher humidity and wet foliage through the night is an invitation to fungi and bacteria. 

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Read our Q & As with Keith Funk answering common Colorado gardening questions here. See his website: gardenwiseguy.com and hear him on the Garden Wise Radio Show