- Author: Janet Hartin
Landscape trees provide shade, cool urban heat islands, reduce interior energy use and related costs, provide habitat for pollinators and wildlife, and beautify our communities. They also help clean our environment by absorbing carbon dioxide emitted by vehicles and other producers of fossil fuels.
Taking care of our urban trees is an important way to maximize these benefits. Unfortunately, their average lifespan in our cities is less than 1/4 of their potential due to poor selection and care. You can help enact change and increase the health, longevity and canopy coverage of our urban forests by encouraging your city leaders, local non-profit organizations interested in ‘green cities' arborists, landscape architects, nursery growers, HOAs, and concerned citizens to register for our free ‘Trees for Tomorrow Start Today' workshop (via Zoom) on Tuesday, March 9, 2021 (8:15am – 3pm).
Please reach out to your city leaders and local ‘green industry' professionals to encourage their attendance and participation. While the workshop is free, registration is required through this secure link: http://ucanr.edu/u.cfm?id=264
A bevy of speakers including certified arborists, horticulturists, planners, water district personnel and other non-profit and green industry representatives will discuss the benefits of urban trees; recommended practices for their selection and care; how to avoid hazardous trees that damage property and structures; and share success stories resulting from partnerships through cities, non-profit organizations, the green industry, HOAs, and street tree committees.
A highlight of the workshop will be the opportunity for attendees to participate in breakout sessions that cut across professions and interest groups (city planners, community service directors, arborists, landscape architects, landscapers, water district managers, HOA managers, golf course superintendents, nursery growers, UC master gardeners, concerned citizens, etc.) to enact positive change regarding tree selection and care.
Thank you for sharing this opportunity to help ensure a healthy future for our children's children with your city leaders and decision-makers.
In the meantime, here are some tree care tips to ‘start the conversation' between now and the workshop:
• Select trees that perform well in your climate. The Sunset Western Climate Zone maps are more precise than USDA zones for our warmer climates. Trees should also be selected based on their adaptation to the ‘micro-climate' in each particular landscape, as well (shade, proximity to buildings, space needs below as well as above ground, soil type, water source, etc.)
• Plant trees the same depth they were in their container in holes at least 2-1/2 times wider. Do not add compost or organic matter to the hole. This practice can result in circled roots that never grow laterally out of the confines of the dug hole.
• Remove any tree ties that are cutting into the trunk or branches of your trees. If trees must be staked due to windy conditions, make sure that the ties are loose enough to allow trees to gently flex in the wind. This helps trees develop the necessary lower trunk strength and stability to support the tree as it matures. Over time, you may be able to completely remove the ties and stakes once the lower trunk becomes stronger and self-supporting.
• Keep all plants and mulch several inches away from tree trunks.
• Keep tree trunks dry. They should not come into contact with water from sprinklers or hoses.
• Regularly water newly planted trees but water mature trees infrequently and deeply. Watering too often reduces the level of oxygen in the rootzone and can lead to waterlogged soils prone to crown and root rots. During fall, trees require only about 15% of the water they require in the summer.
• Prune trees only as needed and avoid topping them. Invest in the services of a credentialed and knowledgeable professional to correctly care for your valued trees. Find a list of International Society of Arboriculture Certified Arborists in your area here: https://www.treesaregood.org/findanarborist.
- Author: Janet Hartin
Many of the landscape trees adorning urban cityscapes in Southern California are at or close to the end of their lifespans.
While several species of oak, maple, crape myrtle, ficus, magnolia and other common shade trees have a life expectancy of 50-80 years or longer in unstressed environments, few reach their full potential in cities and urban areas.
Why? In order to accommodate growing populations, cities have large areas of paved concrete and asphalt surfaces that create ‘urban heat islands (UHI)'.These hard surfaces absorb large amounts of heat that builds up during the day and is released at night, leading to much higher night temperatures in cities than in surrounding areas.
The good news is that trees offer many benefits that offset the impacts of UHIs. Cities with larger tree canopies are a testament to this fact and have fewer adverse impacts from UHIs than do cities with low tree canopies.
Trees reduce the impact of UHIs by releasing heat back into the atmosphere faster than do concrete and asphalt surfaces. In addition, well-placed trees produce shade that cools the surrounding environment and reduces air conditioning needs. They also cool the air through transpiration and absorb and store carbon which moderates the impacts of pollution from fossil fuels.
Planting trees that withstand UHIs today is crucial for tomorrow. University of California Division of Natural Resources researchers are identifying landscape tree species that can remain healthy under adverse urban conditions. In one study, 12 species of underplanted but promising landscape trees in the greater Los Angeles basin that currently grow well in desert climates are being evaluated at UC Riverside. Similarly, the health and longevity of trees currently performing well in Riverside and San Bernardino are being assessed in coastal areas. The idea is that trees growing well now in warmer climate zones will be good choices for cooler zones that are becoming warmer over time.
Reducing impacts of UHIs and maximizing life expectancies of currently planted urban trees that are not so resilient is just as important as looking into alternative species. In fact, research and education on sustaining these trees is critical to maintain and expand tree canopies as trees age. Even the loss of one front yard shade tree can significantly reduce shade, increase the surrounding temperature, and diminish energy savings.
Furthermore, trees subjected to UHIs can easily become heat and drought stressed leading to a downward spiral. Trees already declining due to one stress often fall prey to other stressors such as disease-forming pathogens or insect outbreaks. Identifying the cause of the tree's decline is crucial. Applying a pesticide to a tree that has no biotic disorder but is unhealthy due to lack of water does not solve the problem and can kill beneficial organisms important for keeping actual pests at bay.
Fortunately, there are many free online search engines that allow consumers to select trees based on multiple criteria such as climate zone, pest resistance, drought tolerance, ability to withstand high temperatures. These engines also provide ‘fact sheets' for each species that provide information on the species' ultimate size and space requirements. One reliable and reputable searchable index is Urban Forest Ecosystem's found here: https://selectree.calpoly.edu/.
Since many long-lived species (such as magnolia and ficus) become large at maturity, they offer excellent shade potential and can mitigate UHIs better than smaller growing species. However, they require up to 2,500 to 3,000 cubic feet of root space and should not be planted close to sidewalks and structures. Doing so can result in significant damage.
For more information on selecting and caring for urban trees, contact your local UC Cooperative Extension Master Gardener helplines.
Here's how to contact the University of California Cooperative Extension Master Gardener helplines in your area.
Los Angeles County: 626-586-1988; http://celosangeles.ucanr.edu/UC_Master_Gardener_Program/
Orange County: 949-809-9760; http://mgorange.ucanr.edu/
Riverside County: 951-683-6491 ext. 231; https://ucanr.edu/sites/RiversideMG/
San Bernardino County: 909-387-2182; http://mgsb.ucanr.edu/