The Dry Garden: Predictions of La Niña winter
Autumn and early winter are traditionally considered planting season in Southern California because nature can be expected to cooperate. As days shorten and rains come, seeds germinate, newly transplanted saplings deepen their roots and established plants awaken from dormancy.
Yet not all years are created equal, and this coming planting season has all the hallmarks of a tricky one.
National Weather Service predictions for a La Niña cycle are becoming less tentative and more ominous. That means ocean temperature trends in the equatorial Pacific have shifted to the opposite of last winter -- a way that augurs drought.
How dry our rainy season might be is unknowable; this brooding La Niña might even produce a wet year, but the odds are stacked sharply against that. According to Jet Propulsion Laboratory climatologist Bill Patzert, 82% of the La Niñas since 1949 have had below-average rainfall. “Some are way below average,” he said. “This is a strong La Niña. It really tilts the scale. It’s an 80% to 90% probability of a dry winter.”
How dry is dry? Referring to the most recent satellite image of the Pacific, Patzert said, “See the blue purple thing at the equator? That’s it, and as we go into fall and winter, it will become even more impressive.” At present, it reminds him of what the equator looked like in the advent of the 1988-89 La Niña, although he suspects that we may see less rainfall than that year’s 9.08 inches.
Last winter, during a moderate El Niño, downtown Los Angeles received more than 16 inches.
For Southland gardeners, the prospective loss of a third or more of already scant rainfall requires quick adaptation. Although we still have municipal reserves of imported water for irrigation, we may need to postpone autumn planting to accommodate our traditional Santa Ana winds.
Why? While, to quote Patzert, La Niña “delays and diminishes the rain.” We have no guarantee that Santa Anas won’t breathe their traditional October-December gusts of hot desert air over the coastal plain. If this happens and we plant, we will irrigate to replace rainfall. That could kill a lot of our best-loved native plants, warned Susanne Jett of the Santa Monica firm Jettscapes.
If Santa Anas do beset us, her advice is to proceed planting, but carefully.
Keep reading to find out how ...
“Do not throw caution to the wind just because you want to get that project in,” she said. “If fall and early winter are too hot, it might be better to wait. Things might cool down in January.” At an extreme push, planting can even be postponed into spring months.
The thing to avoid, Jett stressed, is watering while the soil is hot, which gives rise to the conditions and pathogens that swiftly rot the roots of such garden staples as our native lilac, Cleveland sage and fremontodendron.
First, when planting, soak the planting hole well, even repeatedly, and allow the water to percolate into the surrounding soil. This will create a store of moisture around the root ball while also teaching you about the drainage.
Second, while leaving the root crown clear, mulch heavily around the plants, as deep as three inches. This will help keep the soil cool.
Third, water in the morning. The soil will have a chance to cool down overnight. Watering at night is trickier because while the air will have cooled, but the soil may still be warm.
Rancho Santa Ana Botanic Garden's Bart O’Brien adds this advice: Watch your forecast and stay in front of any heat waves. “You want to make sure that your plants are well hydrated before the Santa Ana event and check them after the Santa Ana is over. If it’s a bad Santa Ana, they may need water again right after. That goes for potted plants other than succulents. Anything in a container dries out that much faster.”
O’Brien also cautions to remember established plants.
“The established plants are used to getting rain," he said. "And if we’re entering a markedly subnormal year, then you have to make up for it during the winter.”
Even if the Santa Anas do not bring heat, he said, you may need to water in November and December. “Don’t forget about it because it’s cool outside and you think things might be wetter outside than they are. Sometimes they’re not.”
According to the National Oceanic and Atmospheric Administration, “the jury is still out” on whether or not there is a link between the frequency and severity of La Niñas and El Niños and climate change.
A spokesman at the National Weather Service at Oxnard said there is no known connection between the frequency and intensity of Santa Ana winds and duration and depth of dry spells caused by a La Niña.
If you’re a gardener in the Southland, though, the ramifications are clear. We should observe and adapt accordingly.
-- Emily Green
Green's column on low-water gardening appears here on Fridays.
Photo credits, from top: AFP/Getty Images, Jet Propulsion Laboratory, Francine Orr / Los Angeles Times