The Myth of Hot Weather Watering

When summer heat hits, many worry about watering their lawns when the sun is shining. The idea behind this is that water droplets act as magnifying glasses, focusing the sun’s energy and burning the blades of grass or leaves underneath. However, water cannot cause leaf scorch — meaning it’s time to put the magnifying glass theory to bed.

Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University wrote:

The Myth of Hot-Weather Watering “Watering plants on a hot sunny day will scorch their leaves”

The myth is as follows: if sod (or plants) are watered while the sun is shining, water drops that accumulate on the leaf surfaces act as tiny magnifying glasses, focusing the sun’s energy into intense beams that burn leaves. Furthermore, we’re told that since water efficiently conducts heat, wet leaf surfaces are more likely to burn than dry ones. This all sounds very plausible (it has the patina of physics, after all) and there is anecdotal evidence that seems to support a cause-and-effect relationship between midday watering and leaf dieback.

The Reality

This is one of those myths that refuses to die. Although most of the .edu web sites dispel this myth, hundreds of other domains on the web keep the misinformation alive. If your plants are showing signs of water stress in the middle of the day, by all means you should water them! Postponing irrigation until the evening  or the following morning could damage your sod and in extreme heat, it may not bounce back.  There are many causes of leaf scorch, but irrigation with fresh water is certainly not one of them. Hundreds of scientific publications on crop plants, turf, woody shrubs and trees have examined foliar scorch, and not one of them has implicated midday irrigation as a causal agent.

It’s crucial to watch foliage for signs of wilt. Once leaf tissues have passed the terminal wilt stage, no amount of water will save them. People that don’t recognize the signs of terminal wilt and add water anyway might then associate their midday watering with the marginal and tip leaf burn that follows. Again, consider the plant’s needs in terms of sun/shade requirements; a shadeloving plant in an area with high light exposure, reflected heat, wind, or temperature extremes is going to show leaf burn on a continuing basis.

The Bottom Line

• Wet foliage is not susceptible to sunburn

• Analyze site conditions to ensure optimal root and shoot health and prevent drought problems

• Any time plants exhibit drought stress symptoms is the time to water them

• Optimal watering time is in the early morning; watering during the day increases evaporative losses, and evening watering regimes can encourage establishment of some fungal pathogens

• Do not overuse fertilizers and pesticides, especially those containing sodium or chloride salts