The Dry Garden: Helping plants survive a heat wave

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When it’s hot and plants wilt, the solution is to water, right?

Not necessarily. As Southern California cools down from record heat this week, four horticulturists offered their thoughts on how to help your garden survive the kind of temperatures that on Monday broke the National Weather Service thermometer in downtown Los Angeles.

It’s wisdom gleaned from Ellen Zagory, horticultural director of the UC Davis Arboretum; Bart O’Brien, native plant expert at the Rancho Santa Ana Botanic Garden in Claremont; Wynne Wilson of Terra Design in Altadena; and Cal Poly Pomona professor emeritus Bob Perry.

But first, a personal note: If you are a reader who lost a hedge, a fruit tree or an entire balcony of potted plants in the latest heat wave, please don’t bristle. You are not being patronized, at least not by me. The tender top of a newly pruned Carolina cherry hedge, right, fried in my garden on Monday while my lemon tree furled its leaves in indignation, stopping just short of dropping them.

A second note: Irrigation is a highly localized science. Know your soil. If it’s clay and holds water well, you are lucky. If it’s sandy loam and fast draining, watch your moisture levels and watch what you plant. Your garden will dry out easily, and you will need to water more often.

Three of the four experts tapped for advice about heat and water led with the same caution: It’s Santa Ana wind season. Follow the weather forecast. The art of coping with the crackling heat of the Santa Anas is taking action before the furnace-hot winds arrive out of the desert. O’Brien and Wilson turn on low-flow sprinklers on their largely native gardens as much as a week before a predicted Santa Ana, allowing a gentle rain to percolate into the soil for as much as four hours. Watering is best done after midnight or early in the morning.

If you have drip irrigation instead of sprinklers, use that. Water should reach not only top soil but also the earth 18 inches down, Perry said. This is the layer where the roots are. He, O’Brien and Wilson all emphasized that recharging dry soil cannot be rushed. Opening a hose full blast on a wilted plant will create more runoff than saturation.

This investment in slow, advance watering ensures that your plants will be well hydrated when the heat hits. Native plants will use what you give them carefully, Perry added. Their pores often close in high heat, meaning that the extreme water loss through the leaves of, say, turf, just doesn’t happen with natives.

A surprising note that came up during the conversations was that when a garden plant wilts, it’s not necessarily because of water. After a particularly fiery day, many imported plants will not cope no matter how well irrigated the soil. As often happens with tomatoes, many exotics wilt in California heat purely because they can’t take up water as fast as they are losing it to evapotranspiration — the loss of moisture through plant leaves. If, after sunset, a wilted plant perks up without water, that is what happened.

Whatever the cause, if your plants wilt, O’Brien urges gardeners to get out in the evening and shower the foliage before watering the beds.

The downside of natives and other plants with sturdy foliage such as guavas, conifers and hollies is that they offer few hints of trouble. They rarely wilt.

“In many cases, they can’t,” O’Brien said. “They’re physically too tough.”

Warning signs instead will be when the leaves curl up, are warm to the touch and their color becomes dull. By the time you notice a dulling of leaf color, the plant is seriously stressed. Many gardeners unfamiliar with natives may have noticed this loss of luster in a common Mediterranean cousin: rosemary.

So water it, slowly and deeply. Because natives rely on winter rain, O’Brien recommended watering at three-week intervals “unless it rains.” If hard-core dry gardeners think this seems a lot for a state in a water crisis, consider what most homeowners apply to their lawns three times a week. O’Brien puts one of those waterings on his native garden once every three weeks.

Deciduous fruit trees also can be hard to read after Santa Ana winds blow through, but for a different reason. This time of year, heat shock can be easy to mistake for natural shedding of autumn leaves. While it’s hot, err on the side of watering them twice a week, slowly and deeply, for roughly 20 minutes, then maybe once a month through the winter so they are hydrated as they prepare for bud break.

Professional gardeners water in the morning. The soil is cool, and it is the time least likely to foment deadly fungi. However, if you can’t do mornings, water at night — as late as possible. Be sure to soak the entire root zone and not just one spot near the trunk.

All of the experts consulted for this article touched on various methods of how delicate plants might be protected from heat, but none were enthusiastic about applying reflective sprays and draping plants with burlap or even cold compresses.

“I used to fool around with that stuff, but I don’t any more,” Wilson said. “The right plants survive.”

Wilson, whose garden was featured in the Home section last spring, favors natives. So did evolution, stressed UC Davis’ Zagory. That’s why many native plants come equipped with their own air-conditioning, including the aforementioned pores that shut down to prevent heat loss. Some have fuzz to cool leaves, silver coloring to deflect hot sun, even cleverly louvered stems.

Reviewing the Monday damage in my own garden, the upshot is plain. Most of it survived because most of it consists of native and Mediterranean climate zone plants. The plants that suffered were exotic and ill equipped to hack it when the going got tough.

I wouldn’t have asked for the heat damage, but I’m glad it happened. The garden has edited itself. I’m willing to fuss over plum, apricot and lemon trees. I eat the fruit. But if the Carolina cherry hedge doesn’t make it, so be it. I don’t have time to worry about a living wall getting sunburned, and the hot West doesn’t have the water to slake unfit plants.

-- Emily Green

Green’s column on drought-tolerant gardening appears here on Fridays.

Top photo credit: Los Angeles Times

Lower photo credits: Emily Green