Q&A: Watering in the Sun

Question: When is the best time of day to water? Is it true that water droplets will scorch leaves in the sun?

Answer: It is widely believed that the best time of day to water the garden is very early in the morning—for the reason that water droplets + hot midday sun = burnt leaves. But only recently was the idea put under scientific scrutiny, with mixed results.

“The problem of light focusing by water droplets adhered to plants has never been thoroughly investigated, neither theoretically, nor experimentally," says lead researcher Dr. Gabor Horvath, an award-winning biophysicist at Hungary’s Eotvos University. His team of researchers conducted experiments to pinpoint the conditions under which sunlit water drops can cause leaf burn. The study and its results were published in New Phytologist, a journal of research in plant science.

They concluded that water droplets on smooth leaves, such as maples, cannot cause leaf burn, regardless of the time of day. However, they found that water droplets on leaves with small hairs, such as ferns, can cause burns. The hairs can hold the water droplets above the leaf surface and act as a magnifying glass to the light beaming through them. Dr. Horvath points out that a similar phenomenon is how people can easily get a sunburn after swimming if water droplets are allowed to sit suspended on body hair.

The implications for gardeners? You would be safe, in terms of sun-scorch risk, to water smooth leaves any time of day. But because most gardens contain a mix of plant species, some with smooth leaves, some with hairy leaves, it would be simpler to water them all in the early morning, for the safety of the hairy plants. Also, the hairs can be quite small, so unless you want to inspect each plant with a magnifying glass, err on the side of caution and water in the morning.

Additionally, watering in the morning has other advantages. The water soaks deep into the ground without the risk of evaporation. It bolsters plants for the day. Plant roots are more receptive to water in the morning and the evening than they are while the plants are under the stress of the midday sun. Finally, watering at night increases the risk for foliar diseases like mildew.

So the bottom line is indeed to water your garden and lawn early in the morning.

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2 thoughts on “Q&A: Watering in the Sun

  1. Checking an article about this in the Guardian*, I find “[o]nly 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.”

    This would contradict even that admonition in your article. In fact, “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.”

    The reasons for early watering are not at all based upon this folklore, even for hairy-leaved plants. Rather, the other reasons you list are the valid considerations.

    * – http://www.telegraph.co.uk/science/science-news/7823032/Sunburnt-plants-myth-is-debunked.html

  2. When is the best time to water? I concur that it is best to water in the morning but if you see that your plant is wilted then its time to water. As soon as you notice it would be better to water to avoid more sun damage than maybe a few sun burn spots.

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