- Author: Saoimanu Sope
In a drought-prone region like Southern California, working with Mother Nature is not only wise but necessary, according to Janet Hartin, UC Cooperative Extension horticulture advisor for Los Angeles, San Bernardino and Riverside counties, who studies climate-ready trees.
In 2020, Governor Newsom launched the California Climate Action Corps, empowering Californians to protect their communities from the impacts of climate change. Newsom's call to action emphasizes the need for long-term and sustainable solutions like Hartin's research, which urges Southern California to care for existing trees and plant new ones.
In collaboration with the U.S. Forest Service and other UC Cooperative Extension scientists, Hartin is amid a 20-year research study identifying trees suitable for California's different climate zones. Her work provides a comprehensive understanding of trees and their benefits related to human and environmental health, particularly as Californians navigate climate change's evolving challenges.
One of these concerns is urban heat islands. UHIs are areas in which heat is reradiated from paved concrete or asphalt surfaces. In cities covered in asphalt, like Los Angeles, average temperatures can become six degrees hotter than surrounding areas.
To reduce urban heat islands, she has been working with community organizations to plant trees. In March, for example, Hartin teamed up with the Inland Empire Resource Conservation District to increase tree canopy in the Inland Empire.
Trees keep cities cool
To keep the city cool, some Los Angeles neighborhoods are repainting pavements with reflective coating. According to a 2020 study published in Environmental Research Letters, reflective coating can decrease pavement temperatures up to 10 degrees. As helpful as this is, augmenting urban landscapes to include heat-, drought- and pest-resistant tree species, whether native or not, can significantly reduce the impacts of urban heat islands too.
“Trees can cool impervious surfaces by 40 to 65 degrees,” Hartin said. During a 2021 study, in May and June Hartin discovered that unshaded asphalt could be more than 60 degrees hotter than shaded asphalt during late spring and early summer in inland and desert cities.
Other than providing shade, trees are effective at deflecting the sun's radiation and cooling the atmosphere through evapotranspiration. Given that they absorb and store carbon as well, trees lessen the impacts of pollution from fossil fuels.
“A well-tended mature landscape tree can absorb 40 tons of carbon over its lifespan,” said Hartin.
In a 2021 blog post, Hartin suggests trees be selected based on their adaptation to the “micro-climate” in each particular landscape, noting factors to consider like shade, proximity to buildings, space needs below and above ground, soil type and water source. She also recommends the Sunset Western Climate Zone maps for reference, noting that they are “more precise than USDA zones for our warmer climates.”
Based on the study with the U.S. Forest Service examining the performance of 12 species of underplanted but promising landscape trees at UC Riverside, favorable candidates include bubba desert willow and maverick thornless honey mesquite for their drought resistance, and red push pistache for its drought and heat resistance.
Tamara Hedges, executive director of UC Riverside Palm Desert Center and member of the Board of Directors for the Oswit Land Trust, agrees that trees are important in our fight against climate change:
“Through our partnerships with the UC California Naturalist and the Master Gardener Programs and many other nonprofits in the Coachella Valley, natural ecosystems are being protected and expanded and built environments cooled through the planting of appropriate tree species. These UC/USFS studies go a long way in identifying new underrepresented tree species."
General tips for planting
For California, planting in early fall through late winter provides ample time for trees to establish a strong root system before enduring the summer heat. Doing so also means that natural rainfall can fulfill water needs, as opposed to solely relying on irrigation systems.
Unlike newly planted trees, mature trees should be watered infrequently but deeply. Watering too often can reduce the level of oxygen in the rootzone and result in waterlogged soils prone to crown and root rots. 
During the fall, trees only need about 15% of the water they would require in the summer. When watering, keep the tree trunk dry. Because the roots of the tree grow outward and are usually a foot deep into the ground, Hartin recommends watering the area around the trunk rather than the trunk itself. This will also help avoid water waste.
“Trees not adapted to the climate they're planted in and not receiving proper care are much more susceptible to invasive pests like shothole borers and diseases,” said Hartin. “Even the loss of one front yard tree can significantly reduce shade, increase the surrounding temperature, and diminish energy savings.”
- Author: Belinda J. Messenger-Sikes
- Contributor: Karey Windbiel-Rojas
With ongoing drought and local water restrictions in place in many areas of California, you may start to see landscape plants suffering from stress caused by a lack of water, calledwater stress. Water-stressed plants can have symptoms that resemble diseases caused by plant pathogens. In addition, plants affected by drought can and be predisposed to disease, opening the way for pathogens to infect the plant when it's not at their healthiest state.
Plants suffering from a lack of water will often show symptoms in their leaves first, losing their luster, then shriveling and drooping.If water stress continues, other symptoms can appear like defoliation, split bark, brown branch tips, stunted growth, and in the worst scenario, plant death. These symptoms could be mistaken for diseases that attack the plant's root and vascular systems like Armillaria root rot and Verticillium wilt.
Diseases promoted by drought
Prolonged drought stress weakens a plant's natural defenses. Water-stressed plants are less able to fight off diseases because they can't make chemicals that protect them from pathogens. Drought-stressed plants have both slower defense reactions and slower wound healing. Split bark from water stress can provide an opening for pathogens to infect. Root systems injured by water stress are more susceptible to root rots like Armillaria.
Canker diseases are often stress related and become particularly damaging during droughts. Common canker diseases like Botryosphaeria canker, Cytospora canker, and Nectria canker can damage or even kill a water-stressed plant. Botryosphaeria canker attacks a wide range of woody plants like oak, alder, redwood, avocado and even California natives like ceanothus and manzanita. This disease is typically more serious in unirrigated landscapes.
Common plants susceptible to water stress
Plants from temperate zones require more water to grow and stay healthy under drought conditions than plants from drier climates. Common ornamental plants that are more susceptible to water stress include azaleas, rhododendrons, tulip poplar, English ivy, sweet gum (Liquidambar), ornamental plums, magnolia, and willow. They lack adaptations to control water loss like extensive root systems and small, leathery leaves. During droughts, they must be carefully tended since they can be injured bywater deficit.
What can you do during a drought andunder water restrictions?
Reduce water waste. Repair anyleaks in the irrigation system and adjust sprinklers to water plants, not pavement. Apply only the amount of waterthat the plants need. Plants need less water in the cooler seasonsso be sure to adjust automatic timers to match water needs.
Switch from sprinklers, which lose a lot of water to evaporation, to a drip irrigation system. Drip irrigation directs the water to the plant's root zoneso less water is wasted. If changing your system isn't possible, water early in the morning when it's cool and less windy.
Trees should be your top water priority. Established trees can survive with one deep irrigation in the spring and one or two deep irrigations during the summer. Continue to water trees growing in a lawn, even if the grass is allowed to die. Mature trees can suffer damage if under drought stress for more than a couple of seasons. If there is no significant rainfall, continue with periodic deep irrigation throughout the year.
Create a water-wise landscape. Replace nonessential grass with ground cover, mulches, decks, or walkways. Some lawn areas can be useful for recreation, cooling, and reducing dust. Drought-tolerant ground covercan be used in place of grass to save water. Choose plants that grow well in your climate. No plant is drought-tolerant until it is fully established. Wait until the fall to introduce new plants so they have winter rains to help them grow.
Vegetables are not drought tolerant so are difficult to keep alive during water restriction. Limit the size of the vegetable garden to only the amount your family needs. Plant shorter season crops. Know the critical times for watering, such as after transplanting and during fruit development. Use compost to add nutrients to the soil and cover with mulch or landscape cloth.
Manage your landscape to be water efficient. Apply a 3” to 4” layer of mulch around plants and across the ground. Control weeds, which compete for water with desired plants. Pull weeds when they are small, making sure to remove the roots.
While water restrictions may seem daunting, you can have a healthy, flourishing garden by following these tips and your local municipal water district guidelines.
Further Reading
Hartin J, Geisel P, Harivandi A, Elkin R. 2014. Sustainable Landscaping in California. UC ANR Publication 8504. Oakland, CA. https://anrcatalog.ucanr.edu/pdf/8504.pdf
Hartin J, Oki L, Fujino D, Faber B. 2015. Keeping Plants Alive During Drought Restrictions. UC ANR Publication 8553. Oakland, CA. https://anrcatalog.ucanr.edu/pdf/8553.pdf
[Originally published in the UC IPM Home & Garden Pest Newsletter, Summer 2022 issue]
- Author: Anne E Schellman
The Benefits of Trees
Trees provide so much more than shade. Here are a few reasons trees are important, courtesy of the International Society of Arboriculture (ISA):
- Air-conditioning costs are lower in tree-shaded homes & businesses; heating costs are reduced.
- In workplaces with trees, people report decreased workplace stress and fatigue.
- Cleaner air: leaves filter the air we breathe by removing dust and other particles.
- Cooler environment: trees reduce heat in and around parking lots and paved areas.
Trees and the Drought
California residents are being asked to lower their water use by 15%. One easy way to do this is to turn off your sprinklers. But wait! What happens to the trees? Although they may look okay for now, the stress of going without water will take a toll. Lawns can be easily replanted and replaced, but trees take many years to become established.
What do Trees Need?
How Should I Water My Tree?
There is no “silver bullet” on how often or how much to water. This depends on many factors. Instead, after you water, take a shovel (away from tree roots) and check your soil. How deep did the water penetrate? Make sure water penetrates to a depth of 18” by digging with a shovel. Once that area is dry, water again.
Prioritize Watering Trees with the TRIC
For more information about the value of trees, visit the following resources.
Resources
California Center for Urban Horticulture. UC Davis. Tree Ring Irrigation Contraption. https://ccuh.ucdavis.edu/tric
International Society of Arboriculture (ISA). Trees are Good. https://www.treesaregood.org/Portals/0/TreesAreGood_Benefits%20of%20Trees_0321_1.pdf
Water Talk. Janet Hartin. Podcast Episode 21. https://water-talk.squarespace.com/episodes/episode-21
This article was originally published on July 26, 2021.
- Author: Janet Hartin
[From the Spring issue of the UC IPM Retail Nursery & Garden Center News]
Most disorders impacting landscape trees result from abiotic (non-living) disorders rather than attacks from biotic (living) pests like plant pathogens, insects, and vertebrates. Damage caused by abiotic and biotic disorders can appear similar, making diagnosis difficult at times. For example, discolored leaves on a Ficus nitida tree could be due to drought stress, a fungus, or a nutrient toxicity or deficiency.
In some cases, biotic injury may be obvious and abiotic disorders can be ruled out. For instance, many insects and diseases are often restricted to a single plant species and will not affect multiple plant species in the area.
To determine if damage is the result of an abiotic disorder, look at the landscape as a whole. Are symptoms exhibited by a single plant species or by a wide array of species? Usually, uniform damage to multiple species within a limited area of the landscape signifies one or more abiotic factors are to blame.
Below are several common landscape disorders encountered around home landscapes and ways to prevent them or remedy problems once they occur.
Too Little or Too Much Water
Providing inadequate water can adversely impact plants until they are established, even drought resistant species. Outbreaks of insect pests, such as certain bark beetles, can quickly infest stressed, water-deficient trees. Because drought-stressed trees cannot recover from wounds as quickly as healthy trees, pathogens causing several canker diseases are often common during and just after drought.
However, many established landscape plants suffer from too much water rather than not enough water (Figure 1). As woody plants age, they prefer to be irrigated more deeply and less often than when initially planted.
Heat Stress
You can access the American Horticultural Society (AHS) heat zone map information (based on the number of days with temperatures above 86°F) to find the heat zone in your area. Many nurseries now include this information on plant tags.
Mechanical Injury
Mechanical injury, like damage from tight, unremoved staking ties (Figure 2), can extend into the vascular system of plants, cutting off the flow of water and nutrients. Early symptoms include wilting, stunting, and general decline. Bark can also be damaged from mowers and weed trimmers. Advise customers to avoid trunk injury since damaged bark cannot be repaired.
Salt Build-up from Overfertilization or Recycled Water
With landscapes increasingly being irrigated with recycled water, leaching needs to be a routine component of maintenance. Saline soils can be remedied by leaching salts below the root zone with large amounts of fresh water (the original source of saline water cannot be used for this). In some cases, when the irrigation water is more saline than most plants will tolerate, less sensitive species should be selected.
Prevent abiotic disorders by selecting healthy plants well suited to the climate and microclimate, incorporating recommended planting techniques, providing a soil environment that optimizes healthy root systems, and implementing sound cultural management practices (such as irrigation, fertilization, pruning, and aeration).
This article was modified from a previously published article from the Winter 2017 issue of UC IPM's Green Bulletin (Vol. 7, No. 2). For more comprehensive information about abiotic issues, refer to the UC ANR publication, Abiotic Disorders of Landscape Plants: A Diagnostic Guide.
