When to water tomatoes and myth of hot weather watering | Sacramento Bee

Our very warm summer continues. That’s helped fuel a red-hot debate over when to water tomatoes.

Recent advice on not watering during the heat of the late afternoon left several readers – and other plant experts – steamed.

Kate Karam of Monrovia Nurseries, the wholesale giant, warned that, “In extreme heat, you don’t want to give a plant water, even if it’s wilted. The plant needs a chance to recover first and for the soil to cool down.

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“When temperatures reach over 100, water late at night or early in the morning,” she said. “Don’t water in the late afternoon or you’ll boil your plants’ roots alive. They’ll literally steam; the soil is just too hot.”

Our very literal readers pointed out that water boils at 212 degrees Fahrenheit, significantly hotter than the water warmed in a hose sitting in the sun all day. But steam will arise at lower temperatures as cooler water hits hot surfaces.

“It doesn’t really make sense from the standpoint of basic soil science or physics,” wrote Don Shor, longtime owner of Redwood Barn Nursery in Davis. “Steam arising from the surface doesn’t reflect the temperature of the soil below, which is not at 210 degrees F. Water from the hose may be hot at first, then will cool rapidly. You will not ever ‘boil the roots’ unless you pour boiling water onto the roots for a sustained period of time.”

Pouring boiling water on most any plant will indeed kill it. (Try it on weeds in sidewalk cracks.) But water at just 120 degrees can scald your skin; that can’t be good for plant roots (which was Karam’s point).

If the soil is dry, plants must have water – warm or cold, Shor said.

“Don’t judge a plant’s water needs by their appearance in the middle of the day; leaves will wilt even if the soil has moisture,” Shor said. “But there is no sound reason for not watering at any time of day if the soil indicates the plant is stressed. And there is nothing wrong with getting water on the leaves. The only reason for not watering midday is to avoid evaporation and conserve water.”

Advice against afternoon watering is “a variation on an old garden myth,” said reader Ed Bass. “People used to believe that watering the plants would scorch the leaves.”

Bass cited a study at Washington State University, “The Myth of Hot Weather Watering.”

“If your plants are showing signs of water stress in the middle of the day, by all means you should water them!” concluded WSU horticulturist Linda Chalker-Scott, the study’s author. “Postponing irrigation until the evening – not a good time to water anyway, as this can encourage fungal pathogens – or the following morning could damage your plants and open them up to opportunistic diseases.”

Shor noted that drip irrigation systems routinely run during the day with no ill effect.

“We water our nursery any time the plants need water,” he added. “That certainly includes when it is over 100 degrees. The plants appreciate it, even if we aren’t too thrilled about being out there doing it. I’m more worried about the gardener in that heat than about the plants.”

This story was originally published August 11, 2017 2:00 PM.