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Posts Tagged: trees

Arnon Dag: How to Improve Cross-Pollination in Almond, Pear, Apple and Cherry Trees

How can you improve cross-pollination in such tree crops as almond, pear, apple and cherry? Senior scientist Arnon Dag of the...

A honey bee pollinating an almond blossom. (Photo by Kathy Keatley Garvey)
A honey bee pollinating an almond blossom. (Photo by Kathy Keatley Garvey)

A honey bee pollinating an almond blossom. (Photo by Kathy Keatley Garvey)

A honey bee pollinating an apple blossom. (Photo by Kathy Keatley Garvey)
A honey bee pollinating an apple blossom. (Photo by Kathy Keatley Garvey)

A honey bee pollinating an apple blossom. (Photo by Kathy Keatley Garvey)

A honey bee pollinating a cherry blossom. (Photo by Kathy Keatley Garvey)
A honey bee pollinating a cherry blossom. (Photo by Kathy Keatley Garvey)

A honey bee pollinating a cherry blossom. (Photo by Kathy Keatley Garvey)

Posted on Wednesday, November 22, 2023 at 12:00 AM
Focus Area Tags: Agriculture, Economic Development, Environment, Innovation, Natural Resources, Yard & Garden

Undergrad Thesis Scores the Cover of Environmental Entomology

It's not every day that an undergraduate thesis makes the cover story of a professional journal. But that's the case with UC Davis doctoral student...

A screen shot of Grace Horne's work that appears in the current edition of the journal Environmental Entomology.
A screen shot of Grace Horne's work that appears in the current edition of the journal Environmental Entomology.

A screen shot of Grace Horne's work that appears in the current edition of the journal Environmental Entomology.

Posted on Wednesday, February 22, 2023 at 4:31 PM
Focus Area Tags: Agriculture, Economic Development, Environment, Innovation, Natural Resources, Pest Management

Now is the time to plant climate-ready trees, says UCCE researcher

Climate-ready trees from the U.S. Forest Service and UC ANR research project at UC Riverside as of November 2022 with no watering since March 2020.

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.

Janet Hartin presents her research at the Lead With Trees conference in Palm Springs.
“We've collaborated extensively with her over the years, and we knew Janet had been developing a regionally scaled concept for connecting community members to climate-appropriate trees, alongside access to technical assistance from regional UCCE Master Gardeners to ensure long-term tree health and survival,” said IERCD District Manager Mandy Parkes.

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. 

David Lahti, Oswit Land Trust board member and UCCE Master Gardener, Tamara Hedges and Janet Hartin at the Prescott Preserve in Palm Springs.

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.”


Posted on Tuesday, December 6, 2022 at 11:03 AM
Tags: climate (14), drought (171), Hartin (1), heat island (1), Janet (1), resilient (1), shade (6), trees (16), urban (3)

Where do fall colors come from?

"Green, yellow and orange colors are always present in leaves, but chlorophyll – the green pigment responsible for photosynthesis – is so dominant during most of the year that it masks out the other colors," says Igor Lacan, UC Cooperative Extension urban forestry advisor.

Did you ever wonder why trees "turn" color in the fall? The short answer: It's primarily a function of long, cool fall nights and short, sunny days.

The longer answer? Chlorophyll is responsible for the basic green color of leaves we see in spring and summer and is a necessary component of photosynthesis, which uses sunlight to manufacture sugar (food) that is stored during the dormant period of the year. Carotenoids produce yellow, orange and even brown pigments in crops such as carrots, squash, bananas and many ornamental plants such as daffodils and poppies. Anthocyanins are red and orange in color and are most linked to lavish displays of brilliant fall foliage. They also give rise to coloring of strawberries, plums and cherries.

Here's the kicker: While chlorophyll and carotenoids are present in leaf cell chloroplasts throughout the entire growing season, most anthocyanins are only produced in fall due to bright light and excess plant sugars within leaf cells.

So, in reality foliage doesn't "turn" orange or red at all. Carotenoids and anthocyanins are always in the leaves; they are simply unmasked once the active growing season is finished and chlorophyll is no longer produced. This happens when nights lengthen in fall.

Interestingly, the actual timing of color change varies across species and appears to be genetically inherited. The same species will exhibit a similar color scheme in cool temperatures in higher elevations at nearly the same time as it does in warmer lower elevation climates. The intensity can vary quite a bit however.

Where do temperatures enter the picture? Both the amount of color and the overall intensity of fall color is very linked to weather conditions that occur prior to and during the actual time the chlorophyll in leaves winds down. The most brilliant displays occur after several warm, sunny days and cool, crisp (above freezing) nights. This is because although lots of sugars are made in leaves during sunny daytime hours, the corresponding cool nights prevent the sugars from moving out. The amount of soil moisture also helps ensure that from year to year fall colors vary even in the same trees. So, either a late spring or a prolonged drought can both delay the display of fall color by a few days or even a few weeks.

What's the recipe for the most brilliant fall display? Most likely a warm, moist spring followed by a warm summer and sunny fall with cool autumn nights. 

Although fall color is not nearly as spectacular in lower elevations of Southern California compared to other colder areas of the nation, the liquidambar or American sweet gum (Liquidambar styraciflua) offers some pretty impressive fall color and an impressive 300 to 400-year life span. (Did you know that liquidambar got its name because it at one time was a sought-after chewing gum for Native Americans?)

To help guarantee vivid colors for years to come, growers carefully propagate trees by cuttings to yield identical clones that will produce just as brilliant fall foliage as their parents. You may have come across the popular cultivar named Festival or even have one in your own yard. I like it because it stays more compact and columnar than most liquidambars. The downside? It is less cold tolerant than other liquidambars but does well in most warmer areas of Southern California.

The cultivar Moraine is broader and a better choice if one has adequate space for a shade tree; it is also adapted to cooler temperatures and it rewards homeowners and people passing by alike with beautiful, red fall foliage.

Posted on Saturday, November 26, 2022 at 2:13 PM
  • Author: Janet Hartin, UC Cooperative Extension Environmental Horticulture Advisor
Tags: Fall colors (1), trees (16)
Focus Area Tags: Environment, Yard & Garden

Save trees first: Tips to keep them alive during drought

UC ANR and U.S. Forest Service are doing research on "climate-ready trees" at UC Riverside. Photo by Janet Hartin

Trees essential to lowering temperatures, cooling ‘heat islands'

Water restrictions prompted by the drought are driving Californians to prioritize how they will use their limited water. Because landscape irrigation is a major water use for many households, residents are looking outdoors to conserve water.

When choosing which landscape plants to save, “trees come first,” said Janet Hartin, UC Cooperative Extension area environmental horticulture advisor for San Bernardino, Los Angeles and Riverside counties. “Healthy communities need trees. Fortunately new California water restrictions allow for provisions to ensure trees receive adequate water to stay alive and healthy.”

“Mature trees are instrumental in cooling urban heat islands and we can't afford to lose them and start all over,” Hartin said. “Shade from mature trees can reduce surface temperatures by as much as 65 degrees in asphalt-covered parking lots. Shade from a single tree can reduce these surface temperatures from 165 to less than 100 degrees when air temperatures reach 110 degrees. Even with air temperatures in the 90s, surface temperatures can reach 140 degrees.”

In addition to providing shade, trees absorb and store carbon dioxide, release oxygen, enhance pollinators and wildlife habitat, filter pollutants from air and water and can reduce energy use, according to Hartin. Because trees take years to grow, they aren't as easily replaced as other plants.

As residents let lawns go brown, she recommends watering trees that are near or surrounded by lawn.

“If a tree is in the middle of a lawn, it is almost certainly watered by lawn irrigation,” Hartin said. “If it's not on a separate drip system, drag out a hose and allow the water to slowly trickle into the soil early in the morning or in the evening. Deep watering for two hours once every couple of weeks will keep most established trees alive."

In most jurisdictions, watering restrictions do not apply to hand watering and hand-held watering devices such as hoses, which may be used for longer periods of time than the restrictions permit otherwise. However, watering may be restricted in all cases to prescribed times of day. 

“Check to see if your jurisdiction also requires a hose shutoff valve,” Hartin said. 

“For fruit trees, we may have to forgo fruit production for a year or so. There may not be enough water to support fruit production, but the goal is to keep the trees alive during the drought,” she said.

Trees can be watered by drip irrigation systems or soaker hoses as well as hand-held devices. Photo by Janet Hartin

She recommends watering trees away from the trunk, halfway between the trunk and the dripline – where the foliage ends and rain drips off the leaves – because “roots grow outward quite a distance as well as downward. Leave the hose on so the water is just trickling out,” she said. “You want water to seep into the soil and encourage the roots to grow deeper. The slow water flow will seep down a foot or so and the roots will follow, which will help anchor the tree. Move the hose around every half hour to hour in quadrants around the tree for more even watering.”  

Don't have time to move the hose? Hartin suggests getting a soaker hose and wrapping it in concentric circles 2 to 3 feet apart. 

“Soaker hoses are made from recycled tire rubber and allow water to slowly ooze out of the pores along the hose, distributing the water fairly evenly throughout the hose length. Avoid using soaker hoses longer than 75 feet due to pressure issues.”   

To reduce evaporation around the tree, spreading mulch a few inches from trunk can help.

“Dark mulches can heat the environment so it's best not to use them,”Hartin said. “If you are in a fire-prone area, don't use organic wood-based mulches because they are flammable. Use decomposed gravel or pebbles, rock-based products instead. To keep sunlight out and discourage weeds, large wood chip mulches should be maintained 3-4-inches deep and smaller inorganic mulches at 1-2 inches.”

Residents may want to maintain some grass for children and pets because bare feet and paws can sustain serious burns on surfaces hotter than 120 degrees.

“People don't realize how hot fake grass can get,” Hartin said. “Research I conducted last summer in the Coachella Valley and Redlands found that surface temperatures of synthetic lawns can be more than 65 degrees higher than living turf and groundcover surfaces on several dates in between May and August.”

For California lawns, there are drought-tolerant grasses that can thrive on 30% less water than bluegrass and other cool season varieties. Examples are buffalograss and bermudagrass. They still require maintenance, such as mowing, but are great for play and recreational surfaces for people and pets.

Jim Baird, UC Cooperative Extension turf specialist based at UC Riverside, said, “Turfgrasses offer numerous recreational, aesthetic, and environmental benefits including player safety, property value, mental health, erosion control, groundwater recharge and surface water quality, organic chemical decomposition, carbon sequestration and environmental cooling.” 

UC Cooperative Extension specialists Amir Haghverdi and Don Merhaut are studying groundcover water use. UC Riverside Ph.D. candidate Anish Sapota monitors the test plot.

There are also non-turf groundcovers that are drought resistant. 

“As they transpire, plants cool the environment. We have more and more drought-resistant alternatives to high-water-requiring plants on the market now, and that's where we should be going,” Hartin said.   

For people considering replacing their lawns and adding new landscape plants, she recommends planting low-water using groundcovers in the fall.

“It's too hot to plant in summer and even native and drought-resistant plants require water several times week until they get established,” she said.

Most counties have a UC Master Gardener Program with a helpline staffed by well-trained volunteers dispensing advice to help keep plants alive and recommend plants that are well-suited for the local environment. Find a local UC Master Gardener Program at

University of California Cooperative Extension
Drought and Landscape Tree Care Resources 

Keeping Plants Alive Under Drought and Water Restrictions (English) 

Mantener las plantas del exterior vivas con poca agua (Spanish) 

Prioritizing Trees Under Drought and Water Restrictions (5-minute video) 

Tips to Keep Your Landscape Trees Alive During Drought 

Landscape Tree Irrigation to Maximize Tree Health, Benefits, and Beauty 

Landscape Tree Irrigation 101 

Top 10 Ways to Conserve Water in Your Landscape and Garden 

Use of Graywater in Urban Landscapes in California  


Posted on Thursday, June 9, 2022 at 10:34 AM
Focus Area Tags: Yard & Garden

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