By Rebecca Latta

The worst plant-damaging high heat event I’ve seen in my career happened in early July of this year, affecting plants in the San Gabriel Valley and throughout the Southern California region. Many of you have asked for help on how to manage your trees and gardens in such a severe event. I’m responding with some tips gained from my 25 years of experience promoting healthy trees and landscaping. If you have questions, please do call on me to assist you, 626-272-8444.

In the record-busting heat wave in July, mature trees dropped green leaves, buds and fruit, plants wilted, sunburned and scorched. Shoots, seedlings and potted plants wilted and died. What caused it? Climate change is promoting heat events and drought conditions here and around the world. More extreme weather events of this nature are expected, and will continue. We need to learn strategies to protect our trees and landscaping, which I outline below.

This garden, protected by a top layer of mulch, features drought-tolerant native oaks and sycamores.

Choose the right species

Climate change presents significant challenges to the future of our urban landscape. As our once-mild Mediterranean climate heats up, we have to choose species that can adapt. Species such as redwood, birch, saucer magnolia, Victorian box and avocado tend to struggle in extreme heat events, and may need to be protected from heat and intense reflected light, or replaced with local natives, such as oaks and sycamores. Heat tolerant species from South America and the Sonoran desert, such as tipu, mesquite, desert willow, velvet ash, pinyon pine, California juniper, red willow, desert apricot and cypress may also be good choices.

Plant for both drought and heat resistance

Some plants from coastal climates can take dry soil but are not genetically adapted to high temperatures. Plants that thrive in hot conditions can feature waxy leaves, smaller leaf surface area, reflective surfaces or blue-gray color to reduce heat absorption, or can be drought deciduous, dropping leaves during dry parts of the year. Some plants store water below ground in roots or above ground in stems.

Heat damage can make plants more susceptible to opportunistic diseases and wilt, chlorosis and fruit drop. Excessive heat and sunlight can speed up disease issues. Reflected heat can do damage. Plants near hardscape, artificial turf and on sunny walls with a southern or eastern exposure can be scorched or burned. High soil temperature damages seedlings and may cause root and tissue damage that shows up later in trees and shrubs.

Burlap shade cloth protects this tomato plant.

Implement cooling strategies during the event

Provide shade. Plan ahead and plant tall annuals like sunflowers to shade smaller plants. During the heat event, use shade cloth, cardboard and patio umbrellas to protect plants from heat. Protect mature shade trees that are shading other plants in the landscape, and keep them well hydrated prior to high heat events. Wrap burlap on exposed trunks, or use a foliar spray, whitewash or latex paint on trunks for sunburn protection. Shade can also be sprayed on plants and trees in the form of a clay product called Kaolin that reflects sunlight, much as we use zinc oxide as a sun block. Apply with an inexpensive sprayer. Be sure to follow package directions and wear a mask and goggles.

Water strategically

Water to establish deeper roots – less frequent deep irrigation in the spring and fall. During the summer, watering may need to be based on the weather—weekly or monthly—even for established trees. Get ahead of any high heat events and water deeply several days in advance of the event.

Soak plants to 1-2 feet deep, and allow the top one or two inches to dry out before watering again. Make sure water is reaching the roots at and beyond the drip lines of plants. Check moisture with an inexpensive moisture meter, with a small shovel or long screwdriver.

Mulching, blowing and pruning

Mulching is the best way to protect soil and plant roots from heat. It’s also key to conserving water. Mulches hold moisture, encourage soil microbial activity, suppress weeds and improve soil structure.

As you phase out heat-and drought-sensitive plants and plant new trees and shrubs that can tolerate drier and hotter environments, be sure to chip and spread them on-site as valuable mulch. Your old plants can help protect your new plants and provide food for their growth.

Apply bark chip mulch or mulch/compost mix about 4 inches deep around plant roots. Don’t use rocks or inorganic mulch—it can retain heat in the soil at night when roots need to cool down. Artificial turf should not be used—it can overheat soil, damaging roots.

Keep your mulch in place, don’t blow it away. Plants need this organic cover to protect the roots from heat and drying. Remind gardeners that blowers are for paved surfaces only. Keep dropped leaves in planter beds—leaf litter is good plant food.

Remove understory competition such as ivy, creeping fig and vinca. These plants can take water and nutrients away from trees and shrubs. Mulch these areas or plant sparsely.

Resist the urge to prune. Keep dead leaves on plants and trees until cooler fall weather. Those dead leaves will offer some sun protection for the rest of the summer.

Patio umbrellas provide protection for squash and pumpkin vines

Wait until fall to plant and fertilize

Plant new shrubs and trees in the fall, after the weather starts to cool off. Encourage new root and shoot growth by using products with potassium, that strengthens cell walls.

Don’t fertilize during a heat event. When you do fertilize, use products that strengthen cell walls. If soil is so dry it repels water, buy a wetting agent to penetrate the soil.

Warm Regards,

Rebecca

Weather forecasters will soon provide instant assessments of global warming’s influence on extreme events.

The Northern Hemisphere is sweating through another unusually hot summer. Japan has declared its record temperatures a natural disaster. Europe is baking under prolonged heat, with destructive wildfires in Greece and, unusually, the Arctic. And drought-fuelled wildfires are spreading in the western United States.

For Friederike Otto, a climate modeller at the University of Oxford, UK, the past week has been a frenzy, as journalists clamoured for her views on climate change’s role in the summer heat. “It’s been mad,” she says. The usual scientific response is that severe heatwaves will become more frequent because of global warming. But Otto and her colleagues wanted to answer a more particular question: how had climate change influenced this specific heatwave? After three days’ work with computer models, they announced on 27 July that their preliminary analysis for northern Europe suggests that climate change made the heatwave more than twice as likely to occur in many places.

Soon, journalists might be able to get this kind of quick-fire analysis routinely from weather agencies, rather than on an ad hoc basis from academics. READ MORE

One type of beetle could kill as many as 27 million trees in Los Angeles, Orange, Riverside and San Bernardino counties, including parts of the desert.

Watch Video by Mark Boster / Los Angeles Times

The trees that shade, cool and feed people from Ventura County to the Mexican border are dying so fast that within a few years it’s possible the region will look, feel, sound and smell much less pleasant than it does now.

“We’re witnessing a transition to a post-oasis landscape in Southern California,” says Greg McPherson, a supervisory research forester with the U.S. Forest Service who has been studying what he and others call an unprecedented die-off of the trees greening Southern California’s parks, campuses and yards.

Botanists in recent years have documented insect and disease infestations as they’ve hop-scotched about the region, devastating Griffith Park’s sycamores and destroying over 100,000 willows in San Diego County’s Tijuana River Valley Regional Park, for example.

It’s heartbreaking to see trees dying in such dramatic numbers in famously lush cities like Pasadena, Alhambra and Arcadia.

It’s not a pretty one.

His initial estimate is that just one particularly dangerous menace — the polyphagous shot hole borer beetle — could kill as many as 27 million trees in Los Angeles, Orange, Riverside and San Bernardino counties, including parts of the desert.

 

The polyphagous shot hole borer beetle on a sycamore tree in Craig Regional Park in Fullerton. (Mark Boster / Los Angeles Times)

 

If as many trees as projected die, the cost to remove and replace them could be about $36 billion, he said.

That’s roughly 38% of the 71 million trees in the 4,244 square mile urban region with a population of about 20 million people.

And that insect is just one of the imminent threats.

“Many of the trees we grow evolved in temperate climates and can’t tolerate the stress of drought, water restrictions, higher salinity levels in recycled water, wind and new pests that arrive almost daily via global trade and tourism, local transportation systems, nurseries and the movement of infected firewood,” he said.

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There will be no miraculous recovery
of these urban ecosystems
after the beetles are done with them.

—Mark Hoddle, director of UC Riverside’s Center for Invasive Species.

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If as many trees as projected die, the cost to remove and replace them could be about $36 billion, he said.

But Southern Californians would face many other costs.

“Catastrophic loss of our canopy,” McPherson said, “would have consequences for human health and well-being, property values, air-conditioning savings, carbon storage, the removal of pollutants from the air we breathe, and wildlife habitat.”

Jerrold Turney, plant pathologist for Los Angeles County, likened the surge in urban tree mortality to “watching a train wreck in slow motion.”

“It’s heartbreaking,” he said, “to see trees dying in such dramatic numbers in famously lush cities like Pasadena, Alhambra and Arcadia: sycamores, all the maples, olives, liquidambers, flower plums, myrtles, oleanders and oaks.”

Mark Hoddle, director of UC Riverside’s Center for Invasive Species, said that the tree loss is “starting to cascade across the urban landscape.”

“Without shade trees, water temperatures will rise and algae will bloom in riparian areas, for instance,” Hoddle said. “As a result, fish, frog and native insect populations will diminish, along with the pleasure of hiking, because there’ll be nothing to look at but dead boughs of trees.”

Source: U.S. Forest Service @latimesgraphics Trees species at most risk California live oak 4.5 million Prunus species (Almond, peach, cherry, etc.) 2.9 million Avocado 2.5 million Citrus species 1.8 million Fraxinus species (Ash) 1.4 million The polyphagous shot hole borer breeds by penetrating into tree trunks and in the process emitting a lethal fungus that prevents the transport of water and nutrients from roots to the leaves.

“And,” he added, “there will be no miraculous recovery of these urban ecosystems after the beetles are done with them.”

Among the hardest-hit native species of urban trees are California sycamores, typically found along streams and commonly used as shade and street trees in places such as Griffith Park and along downtown’s Wilshire Boulevard.

“Here’s the sad news about sycamores,” said Akif Eskalen, a plant pathologist at the University of California, Riverside. “If we cannot control the shot hole borer, it will kill all the sycamores in California. And when they’re done with sycamores, they’ll move to other trees.”

By 2012, pathologists knew that the shot hole borer was transmitting a fatal fungal disease to 19 species of trees in Southern California, he said. Since then, scientists have identified 30 additional host species.

If we cannot control the shot hole borer, it will kill all the sycamores in California.

Akif Eskalen, plant pathologist at the University of California, Riverside.

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“We expect the number of tree hosts to grow even higher over the next few years,” Eskalen said. “And at this point, there’s not much we can do about that.”

Meanwhile, the relatively recent invasion of shot hole borers is only part of the crisis that scientists are scrambling to control.

• In San Diego County, it was a cousin of the ployphagous beetle, the Kuroshio shot hole borer, that infested more than 144,000 willow trees in Tijuana River Valley Regional Park last year, officials said.
• The goldspotted oak borer has killed tens of thousands of drought-stressed oak trees while moving from San Diego County to Los Angeles County.
• The aphid-size Asian citrus psyllid is transferring an incurable bacterial disease from tree to tree as it feeds on citrus leaves.
• A flying insect known as the glassy-winged sharpshooter is spreading oleander leaf scorch, a bacterial disease that was first discovered in the Palm Springs area and has now spread throughout Southern California.
• The 1 1/2-inch-long South American palm weevil poses a serious threat to palms throughout the Southwestern United States after turning up in 2016 in the neighborhoods just north of the Mexican border.

In Orange County’s Holy Jim Canyon, “the word is plague,” said Michael Milligan, chief of the local volunteer fire department. “Thousands of dead and still-standing alders look like pick-up sticks propped up along the creek.”

At Craig Regional Park in Fullerton, circular dirt patches of dirt are all that remains of dozens of venerable sycamores that had shaded a canyon where families came to picnic and even hold weddings.

Vanessa Fields, 47, of Brea, and her friend Diane Swanson, 67, of Buena Park, were stunned by the devastation they came upon during a lunch-hour stroll on a recent weekday.

“What the heck happened here?” Fields asked. “Where’s the gorgeous trees?”

That same day, horticulture expert John Kabashima was checking up on one of the many stricken sycamores at that park.

Peering through a magnifying glass, he marveled at the persistence of polyphagous beetles fulfilling their complex life-cycles. The bugs are smaller than sesame seeds, but they bore by the thousands into the bark, then line the tunnels with a species of fungi that disrupts the transport of water and nutrients from roots to leaves. Within a few weeks, their larvae hatch, mature and mate to produce new generations in the tree.

“This pest is unusual in that it reproduces . . . within the tree,” he said. “As a result, it is extremely difficult to reach and treat.”

When it comes to invasive insects and disease, agriculture gets all the attention and money.

John Kabashima, horticulturist

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Kabashima said state and federal officials have been slow to respond with organized campaigns to eradicate the pests that are ravaging the trees because urban forests don’t support logging crews and regional economies.

“When it comes to invasive insects and disease, agriculture gets all the attention and money,” Kabashima said with a sigh. “Urban forests have been left out in the cold.”

Many of the invasive insects and diseases ravaging Southern California arrived as stowaways on trees and plants installed during the post-World War II housing boom.

Unchecked by natural predators, their numbers exploded during the most severe drought on record, which officially ended earlier this month.

Scientists say that an average of nine new insect species establish populations on the landscape each year, and three of those interlopers become significant pests.

 

Frank McDonough, botanist at the Los Angeles County Arboretum, suggested that urban forests are suffering partly because “so many of the trees we grow don’t belong here and aren’t sustainable without plentiful supplies of imported water.”

“Historic photos of the region show coastal shrubs, oaks on the foothills and sycamores along streams and rivers,” he said. “Yet, we planted way too many trees from areas that get two to three times as much rain as we do.”

Liquidambar is one example. The species evolved in the Southeastern United States, then developers and property owners planted it in Southern California after World War II. At the time, the trees seemed perfect: Not too tall, with lustrous maple-like leaves that turn into a spectacular show of yellow, pink and red hues in the fall.

Decades later, leaf scorch, shot hole borers and drought are quickly adding Liquidambar to the long list of trees whose days in Southern California appear to be numbered.

Andy Lipkis, founder of the nonprofit TreePeople, said Southern Californians are starting to pay attention to the crisis, recognizing just how indebted they are to the region’s trees.

“Losing our trees would cost us a lot,” he said, including “the accelerated loss of the California Dream in neighborhoods throughout the region.”

“Trees” he said, “reduce heat and light intensity, protect water, rid the air of pollutants and instill a sense of peace by filling the landscape with the sights, smells and sounds of nature.”

McPherson said he hopes his report wakes up the state’s leaders to the die-off. The next steps, he said, should include monitoring the unprecedented damage the urban forests are suffering, and taking steps to remove dead trees and plant new, probably different ones: “a new, diverse palette of well-adapted species that may not be currently available in nurseries.”

“It may be, for example, that trees that grow well in, say, Phoenix … are the ones that will grow well in Los Angeles in decades to come,” he said.

Louis.Sahagun@latimes.com

@LouisSahagun