Issue: June 28, 1999

Does watering burn plants?

Now that hot weather has arrived, people are being urged not to water during the day. The most obvious reason is to reduce evaporation and blow off. Yet, I continue to hear comments that watering during the day will burn your plants. Reasons cited -- water droplets act to magnify solar intensity and that water is the best source for heat conduction (this from a fireman).

Does water really burn the plants? It doesn't seem likely.

I don't think the water drop can be a magnifier. The focal point of this lens shape (plano-convex) is considerably below the leaf. As the drop gets smaller, the point of focus will move, but then less light is collected. So I don't accept the magnifying glass hypothesis.

The heat conduction hypothesis is one I haven't heard before. However, I also doubt this one. The water must have a source of heat from which to conduct the heat. My question is -- conducting from where and what? It can only be conducting heat from the air which is not hot enough to burn the plant. The rest of the leaf where there is no water is in direct contact with the rest of the air. In fact, the water evaporates and cools, so the spot with the drop is cooler than the rest of the leaf (though not by much). In cool weather, this cooling may perhaps cause some problems as it lowers the temperature of a small area of the leaf while the rest of the leaf is warm and active.

Perhaps the deposition of salt could explain some burning. This is especially true if there are fertilizer salts in the water. As the water evaporates, the various mineral salts that are so prevalent in our water may concentrate to a level that may cause problems. As the solution outside the cells increases in salt concentration (as the water evaporates), the water may osmotically draw water from the leaf cells, causing injury to the cells. After all, rain doesn't wait for night-time to fall, and I don't recall seeing leaf injury from raindrops unless there was hail associated with the rain.

Evaporation and misapplication of water during the day are probably the best answers to the "why not water in the day" question.

So the best reasons for not watering in the day, and especially for not watering in the afternoon, are that the winds are strongest in the afternoon and evaporation is greatest due to heat in the afternoon. The coolest time of day, when evaporation rates are lowest, is in the early morning. The stillest time of the day in New Mexico is the morning, so water goes where it is supposed to go and less evaporates in the wind in the early morning.

A final reason for watering in the early morning is that the crown of the plant dries during the day, while the water deeper in the soil remains longer. By allowing the crown of the grass to dry, fungal problems are avoided. This is also improved by not watering every day. If you water deeply (assuming soil prepared by rototilling to a depth of eight or more inches), it is not necessary to water daily and fungi will be less able to infect your lawn.