- Author: Joanna Solins
Front yard with water-wise landscaping. Photo Credit: Cadenasso Lab, UC Davis
California's urban water agencies have long relied on conservation measures to promote reliability and manage costs. Since landscapes are a large portion of water demand in California cities, many water agencies encourage water conservation by incentivizing the replacement of water-intensive turf lawns with water-efficient or “water-wise” landscaping.
In California's Mediterranean climate, traditional turfgrass lawns require irrigation to maintain their appearance through the hot summer months. Inefficient, poorly managed spray irrigation systems can also result in considerable water loss from runoff and evaporation. Replacing turf and overhead sprinklers with low-water-use plants, alternative ground covers like mulch and kurapia, and more efficient drip irrigation systems can potentially save hundreds of gallons of water per square meter every year.
Although the water savings from turf replacement can be substantial, generating widespread buy-in is a challenge. Landscape renovation can be expensive and intimidating, requiring residents to learn new landscape management practices or pay for professional assistance. In addition, many people appreciate their lawns as spaces for children, pets, or entertainment. Some prefer the look of turfgrass, especially where neighborhood norms promote a unified lawn aesthetic.
To help overcome cost barriers and encourage water-wise landscaping, government agencies and water suppliers commonly offer “cash for grass” programs, in which residents are offered a rebate for landscaping expenses based on the amount of turf they replace. These programs also aim to promote neighborhood adoption of water-wise landscaping by providing attractive examples. Turf replacement incentive programs are likely to expand under proposed new water use efficiency regulations, Making Conservation a California Way of Life (AB1668 - SB606).
While large-scale, regional turf replacement programs in Southern California have been evaluated in peer-reviewed studies, questions remain about the extent and impact of these programs in other parts of the state. How widely are turf replacement programs utilized, and by whom? Are these programs responsible for a substantial amount of water-wise landscaping?
Water-wise landscaping in Sacramento
With these questions in mind, we wanted to understand how both rebate recipients and houses with independently installed water-wise landscaping were distributed across an entire city in inland Northern California. The study was recently published in the Journal of Urban Ecology.
We visually surveyed the front yards of all single-family homes in 100 census tracts across the city of Sacramento—109,062 homes in total—and compared the prevalence of water-wise landscaping with the city's turf replacement rebate data. We especially wanted to understand how this landscape water conservation measure was being adopted by communities of varying socioeconomic and demographic characteristics, and which communities were benefitting from the rebate program.
Front yard with traditional turf. Photo Credit: Cadenasso Lab, UC Davis
In Sacramento, the City's Department of Utilities has offered a rebate of (typically) $1.50 per square foot of turf removed since the summer of 2014. To qualify for the rebate, replacement landscapes must meet several criteria, including efficient irrigation systems and approved plants that cover more than half of the area at maturity. The planting requirement addresses an important concern with turf replacement—the potential for an increase in urban heat. Lawns have a cooling effect through evapotranspiration, and if they are replaced primarily with non-living groundcovers, temperatures could increase. However, such program requirements can make some residents reluctant to participate.
When we conducted our visual survey of front yards in 2018, we found that approximately 10% of Sacramento's single-family-homes had water-wise yards, while 88% had conventional lawns. Only around 3% of water-wise yards were rebate recipients, though, meaning that the vast majority—97%—of water-wise yards were landscaped independently from the city's rebate program. We also found support for neighborhood adoption effects, with clustering of water-wise yards at the city block scale.
The prevalence of independent water-wise yards should be good news for water conservation, since it would not be feasible for the city to fund many thousands of turf replacement projects. However, independent turf replacement projects don't need to comply with the city's requirements for plant coverage or irrigation system efficiency, meaning that they could be contributing to increased urban temperatures or failing to save water.
Our study also showed that water-wise landscaping was more common in census tracts with less diverse, more highly educated, and more affluent populations, as well as fewer households with children. Rebate recipients followed the same trends, suggesting that rebates weren't overcoming barriers to adoption. The uneven distribution of resources necessary for turf replacement—money, time, and information—is likely an important driver of this pattern, along with cultural or functional preferences for lawns. Lawns may play a particularly important role for families as spaces for children to play.
Photo Credit: Cadenasso Lab, UC Davis
A Tool in the Toolbox
Turf replacement is one important tool in the water conservation toolbox, and our research suggests that many residents voluntarily install water-wise landscapes even without a rebate. However, adoption is not equal across communities. The roots of this disparity likely reflect uneven distributions of resources and preferences.
With thoughtful program design and outreach, incentive programs could help overcome some of these barriers, although funding and staffing limitations in water agencies pose a challenge for implementation. For instance, carefully crafted direct installation programs could reach lower-income residents who find it difficult to pay up-front costs associated with rebates. Tailoring efforts to the needs of lower-income and marginalized communities will be important to ensure that they do not end up with unirrigated, high-heat landscapes instead of attractive, water-wise yards.
In the Sacramento region, focus groups conducted for the Regional Water Authority revealed that some residents have a very strong preference for lawns. In these cases, water suppliers may have greater impact by promoting efficient irrigation practices, turfgrass varieties that use less water, and climate-appropriate shade trees. Partial lawn conversions are also a viable water-saving strategy, and one that Sacramento's rebate program supports.
As California's cities grapple with climate change, water reliability, cost effectiveness, and meeting new water use efficiency standards, reducing high-water-use turf lawns will continue to be an important adaptation strategy. Figuring out how to support effective and equitable landscape water conservation programs must be a focus of future research and evaluation studies to promote a California where everyone benefits from climate adaptation measures.
Joanna Solins is an Environmental Horticulture Advisor with UC Cooperative Extension, serving Sacramento, Solano, and Yolo Counties, and is an Affiliate Researcher with the California Institute for Water Resources and the UC Davis Department of Plant Sciences.
Acknowledgments: Many thanks to Erik Porse, Amy Talbot, Jasmin Green, and Mary Cadenasso for their input and assistance with this post.
Additional Reading
Green, J. C., Solins, J. P., Brissette, L. E. G., Benning, T. L., Gould, K., Bell, E. M., & Cadenasso, M. L. (2024). Patterns of water-wise residential landscaping in a drought-prone city. Journal of Urban Ecology, 10(1), juae003. https://doi.org/10.1093/jue/juae003
Alliance for Water Efficiency. (2019). Landscape transformation: Assessment of water utility programs and market readiness evaluation. https://www.allianceforwaterefficiency.org/impact/our-work/landscape-transformation-assessment-water-utility-programs-and-market-readiness
Matlock, M., Whipple, R., & Shaw, R. (2019). Just for the turf of it: Turf replacement as a water conservation tool. Journal of Soil and Water Conservation, 74(5), 449–455. https://doi.org/10.2489/jswc.74.5.449
Pincetl, S., Gillespie, T. W., Pataki, D. E., Porse, E., Jia, S., Kidera, E., Nobles, N., Rodriguez, J., & Choi, D. (2019). Evaluating the effects of turf-replacement programs in Los Angeles. Landscape and Urban Planning, 185, 210–221. https://doi.org/10.1016/j.landurbplan.2019.01.011
Simpson, S.-A., Altschuld, C., Ortiz, A., & Aravena, M. (2023). Green to gold mile: An environmental justice analysis of drought and mitigation policy impacts on home landscapes in Sacramento California. Landscape and Urban Planning, 234, 104729. https://doi.org/10.1016/j.landurbplan.2023.104729
- Author: Janet Hartin
1. Select drought-resistant plants that grow well in your climate zone and microclimate (sun, shade, etc.). Try these search engines:
SelecTree: A Tree Selection Guide (calpoly.edu) Plant Search Database
Plant Search Database - Water Use Classification of Landscape Species (WUCOLS IV) (ucanr.edu)
For inland San Bernardino County: Home - Inland Valley Garden Planner
2. ‘Hydrozone': Place plants with similar water needs (very low, low, medium, high) together and water the hydrozones on different valves (or, if hand watering, water plants requiring the most water longer but not necessarily more often than other plants).
3. Make sure your irrigation system is operating properly (pressure, spacing, no weeds around heads, no broken parts, etc.).
4. Irrigate based on species and seasonal water needs (highest in summer) and soil type (sandy loam, clay loam, etc.).
5. Irrigate slightly below the current root zone depth of your plants to encourage deep rooting into cooler soil: - 6”-8” for annuals, perennials, and turf - 8”- 1' for shrubs - 1' or deeper for trees
6. Water early in the morning when soil evaporation is minimal.
7. Control weeds. They compete with your garden plants for water.
8. Spread and maintain 2-4” of mulch around garden plants and trees (3-4” for wood chips, 2” for pebbles, decomposed gravel, etc.) keeping it a few inches away from tree trunks.
9. Avoid over-fertilizing. Too much nitrogen creates weak growth and the need for more water.
10. If you have a lawn and still want to keep it, water based on the UC ANR 'Lawn Watering Guide': http://ucanr.org/freepubs/docs/8044.pdf
Contact a trained University of California Cooperative Extension (UCCE) Master Gardener in San Bernardino County for more help! mgsanbern@ucanr.edu (fastest!) or: (909)387-2182 (leave a message for a return phone call)
Redlands trees
- Author: Janet Hartin
ANR Environmental Horticulture (EH) academics develop and extend research-based information to producers and end-users of landscape, nursery, turfgrass, and floriculture plants. Our work focuses on optimizing the environmental, social, and economic benefits plants provide while conserving and protecting natural resources required for their production, use, and care.
Research and education we provide is aligned with the following ANR public value statements:
- Protecting California's natural resources
- Building climate-resilient communities and ecosystems
- Promoting healthy people and communities
- Developing a qualified workforce
- Developing an inclusive and equitable society
Our applied research and education is relevant to all five ANR Strategic Initiatives (Endemic and Invasive Pests and Diseases; Sustainable Natural Ecosystems; Sustainable Food Systems; Healthy Families and Communities; and Water Quantity and Quality).
Crucially, it impacts all urban Californians (95% of our population) by increasing the sustainability of our living urban environments and enhancing physical and mental health. We help remedy the historical inequality in the distribution of environmental benefits through research and education we extend to plant producers, arborists, park superintendents, landscapers, planners, community-greening non-profit organizations, and others supporting urban sustainability and environmental justice.
While our work is beneficial to all, it is especially critical in resource-limited cities, neighborhoods and communities most negatively impacted by climate-change.
Issues Adressed by ANR EH Academics:
Reducing Impacts of Climate Change in Our Cities. Properly selected and maintained urban trees, shrubs, and turf mitigate the impacts of climate change by cooling urban heat islands, reducing energy use, providing shade, and sequestering carbon dioxide. Trees reduce surface temperature of urban heat islands in inland and desert cities in California up to 20°C (68°F) and air temperatures up to 2.0°C (35°F). Due to climate change and urbanization, the rate, intensity, and duration of heatwaves in these urbanized areas are increasing, as is the number of heat-related deaths. Members of underserved and disadvantaged communities are at high-risk of experiencing health-related consequences of climate change, further exacerbated by living in neighborhoods with low tree canopy cover and hotter conditions. The poorest Californians are often the most impacted by climate change, leading to a form of endemic and prolonged social injustice.
Our applied research and education focuses on identifying heat, drought, and pest-resistant trees that withstand impacts posed by a warming climate and urban heat islands. Managers of trees in disadvantaged neighborhoods, parks, schools, and green spaces often lack resources to hire outside experts to assist them with proper tree selection and care, relying heavily (and sometimes exclusively) on our trusted, objective expertise. We provide location-specific, relevant and scientifically-supported information on complex technical issues related to horticulture, arboriculture, water management, and policy. Our technical expertise helps the green industry produce and properly maintain landscape trees and shrubs best equipped to perform under the pressures of climate change and mitigate its impacts. Our work directly leads to cooler, more habitable communities and neighborhoods, demonstrating our important role supporting environmental justice and climate change mitigation.
Reducing Water Use in Commercial, Public, and Residential Landscapes. Our involvement in research and education measuring evapotranspiration (ET) rates and determining the minimum irrigation requirements of landscape species spans over 30 years. Early work included the design and implementation of the California Irrigation Management Information System (CIMIS) weather station network and determining crop coefficients for warm and cool season turfgrass based on historical ET and CIMIS data.
Our work identifies the minimum irrigation requirements of established landscape trees, shrubs, and groundcovers in diverse climate zones throughout the state, supporting urban water conservation. Through a partnership with the California Department of Water Resources, we developed the Water Use Classification of Landscape Species (WUCOLS) system in the early 1990s which, to date, has classified more than 3,500 landscape plant species into very low, low, moderate, and high water-use categories based on observation and personal experience by UC and industry experts. WUCOLS continues to be updated as we obtain more data from replicated trials. Our work in precision irrigation using smart controllers, remote sensing, and geospatial analysis under controlled conditions improves irrigation efficiency. Irrigation training and certification for public and private landscape managers also remains a priority because, even with advanced smart controller technologies, water savings cannot occur with poorly designed and malfunctioning irrigation systems.
We also develop and extend critical information leading to the formation of statewide water budgets and policy for commercial and residential users through participation on legislator-appointed boards and committees. Our work reduces the obstacles that were inhibiting widespread landscape water conservation including: a lack of credible information regarding landscape water requirements, inadequate training across a large segment of the landscape industry, lagging irrigation system technology, and an inadequate supply of locally available drought-resistant landscape plants.
Reducing Overuse of Pesticides and Fertilizers in Commercial, Public, and Residential Landscapes. Urban landscapes are now the single greatest source of non-point pesticide pollution of California waterways. Home use of pesticides is unregulated, and residential users are often unaware of proper application practices leading to runoff polluted with fertilizers and pesticides. Inefficient irrigation practices by residents and professional landscape managers contribute significantly to runoff leading to waterway pollution. Applied research and education that we develop and extend through the Green Gardener and other educational programs directed at the commercial sector and through the ANR Master Gardener Program (MGP) to non-commercial gardeners reduces reliance on pesticides and fertilizers in urban environments, further reducing water pollution.
Preventing and Controlling Pests and Diseases in Commercial Nurseries/ Greenhouses/ Controlled Environments and Landscapes. Global trade and illegal imports of plants from unlicensed facilities have introduced devastating pests and diseases that irreversibly impact production, end-user sectors of the green industry, and California's natural landscapes, often requiring expensive, area-wide control programs. As global trade and movement of pests into California continue to increase populations of exotic pests and diseases, the need for applied research in detecting and managing them in production nurseries and landscapes has also increased. We help protect California's green infrastructure from the threat of pests and diseases by developing, updating, and implementing science-based best management practices that contain and control them.
We regularly train arborists, growers, land managers, and other green industry personnel to identify and manage a wide variety of pests and diseases, including new invasive species. We also provide education to ANR MGP volunteers and the general public on ways to reduce the spread of invasive species and, in some cases, we involve citizen scientists in our work detecting invasive tree pests.
Addressing Water Quality and Quantity Issues in Nurseries/Greenhouses/Controlled Environments. Federal, state, and local governments mandate elimination and/or reduction of wastewater discharges that exceed established water quality criteria. The passage of the Sustainable Groundwater Management Act in 2014 to protect groundwater resources brought us and ANR colleagues in related disciplines to the forefront working with regulatory agencies and industries to develop long-term groundwater sustainability plans and address fertilizer management to mitigate groundwater pollution.
In addition, due to water scarcity and imposed restrictions due to drought, we develop and extend research-based information to enable water conservation while supplying adequate amounts of water to produce high-quality, functional plants. We also develop technologies and systems to improve the quality and delivery of recycled water which reduces reliance on potable water but can impair irrigation water quality, leading to plugging of nozzles, high substrate salt levels, and plant damage. Our expertise helps nursery and floriculture producers obtain and retain sustainable alternative water sources that minimize dependence on potable water without diminishing long-term economic viability.
Enhancing Human Health and Well-being/Quality of Life. Our work improves the health and quality of life of all Californians benefiting from products and services provided by the green industry. Numerous studies document the myriad ways that landscapes enhance our physical, psychological, sociological, and emotional well-being. Besides providing sources of exercise and recreational opportunities (parks, schoolyards, sports fields, etc.), research affirms that gardening and exposure to nature enhances work productivity, mood, creativity, social bonding, cognition, mental acuity, and a sense of belonging. Studies have also shown that indoor flowering plants reduce stress and improve concentration, productivity, and happiness. In addition, many people engaged in horticulture therapy activities with trained professionals recover more quickly and fully from physical injuries with improved mental health. It is now widely recognized that separation from peaceful, landscaped environments causes stress, increased crime rates, decreased life-spans, and increased incidence of chronic disease in densely populated and sparsely vegetated urban areas, even when controlled for education, race, and income.
Providing ANR MGP Oversight/Research and Extension. We provide vital program oversight and research-based information to over 6,000 ANR MGP volunteers who, in turn, use this information to teach the public how to landscape more sustainably and grow food. Our work provides the foundation to ensure that accurate information is conveyed to the public, ultimately helping protect California's natural resources, reducing overuse and misuse of pesticides, supporting locally grown food, and enhancing health and the quality of life for all Californians.
ANR MGP volunteers are a trusted source of objective research-based information by the gardening public, recording over 395,000 volunteer and continuing education hours in 2019/2020. Surveys completed by Californians after participating in ANR MGP volunteer-led events and classes during this same period document the impact of the program: 74% monitor for insects, weeds, and diseases, which reduces the use of pesticides and waterway pollution; 73% use improved practices to grow food, which reduces food deserts and improves health; 69% spend more time outdoors, which enhances individual and community health; and 67% make use of more plants that attract and support pollinators, which enhances biodiversity and a healthy ecosystem. Our role in providing oversight and research-based information through the MGP will become even more vital due to increased interest in landscaping and gardening by the public. More than four in ten U.S. households (42%) report participating in food gardening in 2020, significantly higher than in 2019 (33%). Importantly, participation by lower income populations, people of color and younger people has outpaced many other demographics.
Developing a Qualified “Green Collar” Workforce for California. Our outreach is critical to familiarize and train interested Californians for green industry careers (arboriculture, landscape pest management, urban water management, nursery, floriculture, controlled environments, etc.) at all educational levels, from those entering the skilled trade to mid-level managers and seasoned professionals and educators seeking to advance their careers and stay abreast of scientific developments. We are front-line teachers for many adult education programs focused on green industry training, often through trade groups and professional associations. Because we are familiar with the local workforce and issues pertinent to clientele in counties we serve, we play a particularly vital role educating the green industry. Often, we are the only source of research-based information locally for early-career individuals with limited resources unable to attend conferences miles away. In addition, we often provide subject-matter material in Spanish through the Green Gardener and other valued programs. As noted below, not only is professional development critical to the individual workers, but a knowledgeable workforce is essential for the proper care and maintenance of California's urban green infrastructure.
Size and Scope of California's Environmental Horticulture Industries
Nursery & Floriculture. The California nursery and floriculture industries have a farm gate value of $3.73 billion (annual average from 2013-2017) and $3.74 billion in 2019, the largest in the United States, accounting for over 20% of U.S. production from 2,609 operations. In fact, only dairy and milk, almonds, and grapes have higher farm gate economic values in California. In addition, California producers accounted for 14% of national bedding and garden plant sales, 34% of potted flowering plant sales, and 78% of total wholesale cut flowers. In 2017, San Diego County ($1.19 billion) dominated California with 43.3% of the state's nursery and floriculture production. (San Diego County and 8 other counties each realized over $100 million and produced $2.25 billion of production). Stanislaus, Monterey, Ventura, Santa Barbara, and Riverside combined were the next largest producers, accounting for 38.7% of total nursery and floriculture production in California. Nursery and floricultural crops ranked in the top 10 agricultural commodities in 36 of California's 58 counties that reported agricultural production.
Landscape Horticulture. The value of the landscape industry in California has been steadily rising as well, totaling over $11 billion of products and related services in 2017. There are more arborists in California (9,560) than any other U.S. state and this number is expected to grow by 11.6% (1,300 jobs) between by 2028. Unfortunately, this increase coincides in many cases with the prevalence of poorly trained, non-certified tree care workers unaware of best practices necessary for ensuring the health and longevity of our urban trees. Less than 10% of individuals in California providing tree care service are certified through the International Society of Arboriculture, a professional development process that increases the competency of an individual's abilities to provide proper tree selection and care and ensures ethical practices.
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As we enter another summer of drought, conserving water is essential. Luckily, there are a number of ways you can significantly reduce water use in your vegetable garden.
Mulch, mulch, mulch! Mulching is probably the single most important thing a gardener can do to reduce water consumption in a vegetable garden, or the home landscape in general. Mulching reduces evaporation from the soil by up to 35% and can significantly cut irrigation needs. Mulches also smother weeds, which compete with vegetables for water.
Mulching with black plastic sheeting is the most effective way to conserve soil moisture, but it also raises soil temperatures, so its use should be limited to heat-loving vegetables such as tomatoes, peppers, eggplants, cucumbers, squash and melons. Plan to lay down black plastic early in the season, so that by the time extreme summer temperatures arrive, plants will have matured and their leaves will shade the plastic.
Grass clippings make excellent mulch because they release nitrogen into the soil as they break down. To avoid matting, clippings should applied in thin layers between one-quarter and one-half an inch deep and be allowed to dry out between layers. Do not use grass clippings from lawns that have been treated with herbicides. Other organic mulches include wood shavings, compost, bark, straw, or alfalfa. Organic mulches should be one to three inches thick in vegetable gardens; the larger the mulch particles, the thicker the mulch needs to be. Also, thicker layers of shavings, straw, or bark may lead to a carbon-nitrogen imbalance in the soil. A layer of newspaper two sheets thick can be placed under organic mulches to enhance their effectiveness. (Do not use glossy printed material for this purpose, as the ink may be toxic to plants.)
When deciding which vegetables to plant, be aware that the amount of water needed by different crops varies. Corn and most beans require the greatest water use of commonly-planted garden vegetables. Vegetables that are more drought tolerant include chard, black-eyed peas (cowpeas), heat-tolerant tomatoes (including many cherry varieties), some varieties of okra, mustard greens, eggplant, jalapeno and poblano peppers, Black Knight zucchini, and New Zealand and Malabar spinach. Shallow-rooted crops, including potatoes, onions (and most other bulb, root or tuber crops), celery, and plants in the cabbage family, require more frequent irrigation. For this reason, mulch is especially beneficial for shallow-rooted crops. Tomatoes, sweet potatoes, melons and asparagus are more deep-rooted and can tolerate longer periods without irrigation. Beans, carrots, peppers, summer squash and cucumbers are intermediate in root depth. Additionally, low-growing plants and plants with smaller leaves tend to lose less water through their leaves than taller, leafier varieties. Bush beans, for example, use less water than pole beans. Also, miniature varieties of vegetables, such as mini bell peppers and eggplants, need less water for fruit development than standard varieties.
How a garden is irrigated can affect how much water it uses. Water early in the morning, so that water has a better chance to soak into the soil before it evaporates. A drip system (under mulch) is an efficient way to deliver water to crops with minimal evaporation loss. To avoid overwatering, monitor soil moisture. Generally, if garden soil has dried out to a depth of two to four inches, it's time to water. Including a timer as part of an irrigation system will reduce waste due to human error. Overhead sprinklers and hose-end sprayers are inefficient, result in high evaporation losses and are more likely to trigger diseases such as mildew. Also, the amount of water needed by plants varies over their life cycle. As a rule of thumb, water is most critical for plants during the first few weeks of development, immediately following transplanting, and during flowering and fruit production. Target the timing and amount of water with these plant needs in mind. It is also important to check and maintain your irrigation system frequently to keep it performing at top efficiency.
For more information, see “Water Conservation Suggestions for Your Home Vegetable Garden." UCCE Master Gardeners of Trinity County.
The Master Gardeners Spring 2021 Workshop Series is underway. It is not too late to sign up for our workshop on Honeybees, and an upcoming 4-part series on Firewise Landscaping. Check our website to read about these free workshops and register for them.
UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area, visit our website. If you have a gardening question or problem, email the Hotline at mgbutte@ucanr.edu (preferred) or call (530) 538-7201.
- Author: Janet Hartin
I hope all of you and your loved ones are staying well and healthy as the pandemic continues. Involving yourself in outdoor activities is not only a great way to escape 'cabin fever' and improve your physical health but it is also good for you psychologically, as well.
A silver lining for me over the past few months has been the opportunity to summarize results of some research studies including an update from a mulch trial that I'll report on virtually rather than live at the American Society for Horticultural Science conference (that was to be held in Orlando). I thought you might be interested in the results.
The objective of the research project was to measure the impacts of organic mulch treatments on the growth and health of four species of low maintenance, drought-tolerant landscape trees under deficit irrigation. Species selected were Parkinsonia x ‘Desert Museum' (it's thornless!); Chilopsis linearis ‘Bubba' (Desert Willow) (beautiful magenta flowers!), Pistacia ‘Red Push' (hybrid from P. atlantica and P. integerrima with brilliant orange fall foliage), and Prosopis glandulosa ‘Maverick' (thornless). Trees were planted in a randomized complete block experimental design in at the Chino Basin Water Conservation District (CBWCD) in Montclair in October, 2016. Half of the trees received 4” of organic mulch and half did not. Trees were transplanted from 15-gallon containers and irrigated with recycled water at 80 percent of reference evapotranspiration (ETo) the first 12 months.
Irrigation was reduced to 50% ETo in November, 2017 which was maintained until May 2020, when irrigation ceased. While the study continues through October 2020, there are some interesting early results. Organic mulch applications enhanced growth with no loss in quality in the Pistacia, Prosopis glandulosa, and Chilopsis linearis trees while growth was actually better in the Parkinsonia trees that did not receive mulch. This may be due to the fact that its trunk and branches actively photosynthesize as well as its leaves. Parkinsonia leaves also feature sunken guard cells, providing another form of drought avoidance. Furthermore, they develop deep root systems and may not need the added benefit of reduced soil evaporation in the top few inches of soil provided by the mulch. While all of the species selected have wonderful attributes, the Parkinsonia 'Desert Museum' has all the best traits of its palo verde heritage including having no thorns and a continues bloom throughout summer.
A huge thanks to our UCCE San Bernardino County Master Gardener team (led by Irene) who take data quarterly on the trees, recording trunk circumference at 6" and 3'. This is another great example of the breadth and depth of the contributions of our wonderful volunteers!
The four species of landscape trees in this study are part of a larger study at University of California, Riverside to determine the ability of 12 species of landscape trees to mitigate impacts of climate change. While I'll be long retired, growth and health data of all species will continue to be evaluated through at least 2035. The study at CBWCD provided the opportunity for a mulch/no mulch treatment for four of the most promising species which was space-limited at UCR.
Below are plot photos from October 2016 (right after planting), July 2020 (no irrigation for 3 months), a mature 'Desert Museum' tree in prolific bloom (photo credit to Dr. Bob Perry, Emeritus Professor, Cal Poly Pomona), and a mature Chilopsis linearis 'Bubba' from our UCR plot.
Save the date! We will be hosting a virtual UCR Field Day on September 3, 2020 featuring our drought-tolerant landscape plots at UC Riverside that was to be held live last May. More information is forthcoming but it will provide at least four hours of continuing education hours and a chance for you to ask questions of Dr's Amir Haghverdi, Don Merhaut and myself.
Have a wonderful August in your garden!
Janet