Lorence Oki

University of California, Davis | UCD · Department of Plant Sciences and Department of Human Ecology

Many arid lands across the globe are experiencing more frequent and extreme droughts due to warmer temperatures resulting from climate change, less predictable precipitation patterns, and decreased soil moisture. Approximately 60–90% of household water is used for urban landscape irrigation in the western United States, necessitating the establishm...

To reduce nitrate contamination of groundwater in intensive agricultural production areas, crop producers should identify nitrogen (N) inputs and outputs to calculate potential N available for leaching into groundwater. However, poor understanding of N fate in container plant production may result in inaccurate estimation of potentially leachable N...

There is an increasing interest in transitioning conventionally soil-grown specialty crops (i.e. fruit, vegetables, ornamentals) to soilless culture. However, growers are often not fully aware of the opportunities, challenges, or measures to define success in adapting specialty crops to soilless systems. A North American needs assessment for soille...

Carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) flux from a fir bark substrate was estimated at an ornamental plant nursery in California during a typical 81-day production cycle. The soilless substrate consisted of 4:1 (v:v) fir bark:sand, incorporated with 3.47 kg m‑3 Apex 9-2-0 sulfur-coated urea and 4.51 kg m‑3 Osmocote Plus 15-9-1...

This research aimed to estimate methane (CH4) and nitrous oxide (N2O) fluxes and subsequent global warming potential (GWP) for a Douglas fir (Pseudotsuga menziesii) bark-based substrate production system. The fir bark-based substrate had controlled release fertilizer (CRF) incorporated with differing amounts of surface-applied fertilizer. In a nurs...

Potted poinsettia (Euphorbia pulcherrima) is an important commercial commodity for the U.S. floriculture industry. The production of poinsettia demands intensively managed light control, heat, fertilizer, and water; inhibiting elongation with plant growth regulators, and protecting plants from diseases and pests with pesticide applications. Excessi...

Recapture and recycling of irrigation water is often required to meet enormous water demands at horticultural nurseries. We tested four water types associated with a recycled irrigation system at a commercial container nursery in southern California for presence of oomycete plant pathogens from July 2015 to December 2017. These water types included...

Nursery, floriculture, and propagation production accounted for 79% ($13.3 Billion) of 2017 ornamental specialty crop production in the United States. Access to high quality water sources is increasingly limited for irrigating these economically significant crops. Given the production, environmental, and economic issues associated with the use of w...

Irrigation scheduling determines both the frequency and volume of irrigation to apply to ensure that container-grown crops do not suffer from water stress or high salinity. This publication will first define the variables that affect irrigation scheduling and then describe two methods to measure leaching fraction. Both offer a low-cost means to sch...

Water conservation has become a critical issue in urban landscapes in summer-dry climate regions where irrigation must be applied to keep plants healthy. Part of the strategy for reducing landscape water use is incorporating plants with low water needs into the design. To implement this, landscape professionals need information on which available p...

UC has been heavily involved in research and extension efforts impacting landscape water conservation legislation for over 30 years. In 1981, UC implemented the California Irrigation Management Information System, a network of weather stations that provides data for local estimates of plant water needs. Those estimates led to UC being able to advis...

In the U.S., container plant growers use high-quality water sources which can be expensive. The use of recycled irrigation runoff water could save growers money. The objective of this study was to compare the cost of recycled irrigation water with the cost of untreated municipal water at a nursery in Southern California over multiple years. Water c...

University of California (UC) researchers have been involved in research and extension pertaining to measuring evapotranspiration (ET) rates and determining the minimum irrigation requirements of landscape plants for more than 30 years. Early work included the design and implementation of the California Irrigation Management Information System (CIM...

Insecticides, such as pyrethroids, have frequently been detected in runoff from urban areas, and their offsite transport can cause aquatic toxicity in urban streams and estuaries. To better understand the wash-off process of pesticide residues in urban runoff, the association of pyrethroids with sediment in runoff from residential surfaces was inve...

Mantener las plantas del exterior vivas con poca agua. Esta publicación le ofrece información fiable sobre cómo mantener las plantas del exterior vivas durante la sequía o racionamiento de agua. Entre los temas a tratar se incluyen los síntomas que causa el estrés por agua, consejos para ahorrar o conservar agua en su jardín o paisaje, métodos para...

This paper presents a comparison of pollutant load estimations for runoff from two geographically distinct residential suburban neighborhoods in northern and southern California. The two neighborhoods represent a single urban land use type: low-density residential in small catchments (<0.3 km²) under differing regional climates and irrigation pract...

Sources and mechanisms of nutrient transport in lawn irrigation driven surface runoff are largely unknown. We investigated the transport of nitrogen (N) and phosphorus (P) in lawn irrigation driven surface runoff from a residential neighborhood (28 ha) of 56% impervious and 44% pervious areas. Pervious areas encompassing turfgrass (lawns) in the ne...

Runoff flow, pH, electrical conductivity (EC), total organic carbon (TOC), and total suspended solids (TSS) concentrations in individual runoff samples collected at 3-hour intervals in June 2008. (PDF)

Loads of nitrogen, phosphorus and total suspended solids in runoff collected at the outflow pipe draining a southern California residential neighborhood during one-week of intensive sampling in June 2008. (PDF)

Concentrations and proportions of phosphorus forms in individual runoff samples collected at 3-hour intervals in June 2008. (PDF)

Comparisons of estimated nutrient and sediment export rates from this study and other residential runoff studies. (PDF)

Concentrations of total suspended solids and total organic carbon, and electrical conductively in residential runoff collected from the outflow pipe draining a residential catchment during an intensive sampling period of 1-week (June 16 to 23, 2008). (TIF)

Mean recorded flow at the outflow pipe draining a southern California residential neighborhood during 2007–2008 wet and dry seasons. (PDF)

Concentrations and proportions of nitrogen forms in individual runoff samples collected at 3-hour intervals in June 2008. (PDF)

Comparison of water quality parameters in residential runoff from this study, treated wastewater (reclaimed water), and potable water from various sources. (PDF)

References. (PDF)

Concentration distributions of nitrogen and phosphorus in residential runoff (n = 56) collected at 3-hour intervals from outflow pipe draining a residential catchment during an intensive sampling period of 1-week (June 16 to 23, 2008). The line represents daily mean concentration and the different letters indicate significant difference (p<0.05). (...

Materials and methods. (PDF)

While governments and individuals strive to maintain the availability of high-quality water resources, many factors can “change the landscape” of water availability and quality, including drought, climate change, saltwater intrusion, aquifer depletion, population increases, and policy changes. Specialty crop producers, including nursery and greenho...

Slow sand filters (SSF) are effective in removing pythiacious organisms from captured runoff to prepare the water for reuse in irrigation. Plant viruses can also be present in this runoff, but until this study, it hadn’t been clearly shown that SSFs can remove these pathogens. In this study, purified preparations of Tobacco mosaic virus (TMV) were...

Slow sand filters (SSF) are an effective technology, capable of developing high quality water from untreated sources including irrigation runoff. The sand serves as a substrate on which a microorganism community grows. This microbial community can breakdown a wide range of pollutants including plant pathogens. This report reviews results on the rem...

Garlic (Allium sativum) is a commercially and culturally important crop worldwide. Despite the importance of garlic, there have been few studies investigating how garlic growth and development will be affected by the atmospheric enrichment of carbon dioxide (CO2). A split-plot experiment with CO2 concentrations as main plot and nitrogen (N) fertili...

This publication provides credible information on how to keep outdoor plantings alive during drought and under water restrictions. Topics covered include symptoms of water stress, tips to conserve water in your current landscape, methods to help specific plants survive drought, and considerations and tips regarding planting a drought-resistant land...

Researchers received funding for the Specialty Crops Research Initiative (SCRI) Coordinated Agricultural Project “Clean WateR3 —Reduce, Remediate, Recycle—Enhancing Alternative Water Resources Availability and Use to Increase Profitability in Specialty Crops” in September 2014. This project stemmed from a coordinated effort by scientists through a...

Recycled wastewater is a popular alternative water resource. Recycled water typically has higher salinity than potable water and therefore may not be an appropriate water source for landscapes planted with salt intolerant plant species. Coast redwoods (Sequoia sempervirens) are an important agricultural, horticultural, and ecological species assume...

Investigations into the microbiological impacts of urban runoff on receiving water bodies, especially during storm conditions, have yielded general paradigms that influence runoff abatement and control management strategies. To determine whether these trends are present in other runoff sources, the physical, chemical, and microbiological components...

Site-specific fertigation can limit fertilizer waste and loss to the environment, but is not easily implemented when there are many small hydrozones, each demanding a unique rate of fertilizer application. With a simple and inexpensive fertilizer injection system, a separate injector could be installed at each zone to provide a unique fertilizer de...

Nursery, floriculture, and propagation production accounted for 81% ($9.48 billion) of the 2009 specialty crop production in the United States. Access to high quality water sources is increasingly limited for irrigating these economically significant crops. Given the production, environmental, and economic issues associated with the use of water —i...

Slow sand filtration has been shown to effectively reduce Phytophthora zoospores in irrigation water. This experiment tested the reduction of Phytophthora colony forming units (CFUs) by slow sand filtration systems after switching the pathogen contaminating plant leachate from Fusarium to Phytophthora and the resilience of the system to a short per...

Unstable water supplies in the Western U.S. have become more uncertain due to increasingly drier winters and rising urban populations, and chemical laden runoff from over-watered urban landscapes threatens the health of waters that receive the runoff. In California, this has led to regulations on landscape planning and installation that require kno...

Grower priorities in water research need to be regularly identified. The most recent strategic outlook on water management was reported by Beeson et al. (1). While many previous concerns remain, their priority has changed with the inclusion of emerging issues. This publication provides a synopsis of a nationwide survey and grower listening sessions...

Urban water availability is of primary concern to the summer-dry Western U.S. Unstable supplies have become more uncertain due to increasingly drier winters, and chemical laden runoff from over-watered landscapes threatens the health of waters that receive urban runoff. This has led to regulations on landscape planning and water use. To address the...

Insecticides are commonly used around homes for controlling insects such as ants, termites, and spiders. Such uses have been linked to pesticide contamination and toxicity in urban aquatic ecosystems. Fipronil is a relatively new and popular urban-use insecticide that has acute toxicity to arthropods at low-ppb levels. In this study, we collected r...

Wireless sensor networks for crop monitoring have become more common, but typically support sensing only and not control. Much of the work on wireless sensor networks with integrated control has been conducted in academic research. To promote the accessibility of commercially-available wireless sensing and control networks, valve control hardware a...

We investigated genetic differences in salinity tolerance among 20 saltgrass (Distichlis spicata (L.) Greene) genotypes, including constitutive, gender-based and phenotypic plasticity traits, to better understand the basis of adaptation and acclimation by saltgrass in diverse environments. On average, the plants survived NaCl treatments up to ~1 M,...

To reserve potable water for appropriate beneficial uses, reclaimed water is provided for irrigation of parks, golf courses, and other public landscapes within the Santa Clara Valley Water District in California. Unfortunately, the use of this water to irrigate landscapes appears to have led to the decline or death of coast redwoods within those si...

Fipronil, an increasingly popular insecticide used for urban pest control, is known to readily transform into several degradates that generally have similar or greater toxicity to aquatic organisms than the parent compound. However, knowledge on the fate of these degradates in the environment is obscure. In the present study, degradation kinetics a...

A web-based Irrigation Research and Information Center is currently being developed at the University of California. The goal is to develop an educational outreach and extension service to facilitate and coordinate the dissemination of research-based information, educational material and activities related to irrigated agriculture.

Irrigation is the process of delivering water to plants so as to meet their needs for several important resources. There are several approaches to making irrigation decisions in the nursery: look-and-feel, gravimetric, timer-based, sensor-based, and model-based methods. While providing water is certainly a major facet of irrigation, various other n...

Fipronil is an urban-use insecticide, and the increased use has led to its frequent detections in urban streams. Most studies on the environmental fate of fipronil so far have focused on soils, and little is known about its behavior in sediment-water systems. In this study, we investigated the transformation and sorption of fipronil in urban stream...

There is a growing need in the state of California for landscape plants that require fewer inputs of water and chemicals. To address this issue, a program was initiated at UC Davis to test the landscape potential of California native plants not currently in widespread horticultural use. Ten unused or underused California native plants were screened...

Variations in plant water and nutrient demand and environmental regulations to protect water quality provide significant justification for development of site-specific irrigation/fertigation systems. But to be accepted by growers, a system must be easy to install and operate. We have developed wireless valve controllers that self-assemble into a me...

Stem length is a major determinant of rose productivity because cut flower roses are graded and marketed by stem length. The ability to predict stem length at harvest could be an advantage for growers for forecasting crop pricing and potential cash flow. Management decisions that optimize stem length could be made by monitoring stem elongation duri...

Changes in stem elongation rates of Rosa hybrida L. ‘Kardinal’ stems were measured using linear displacement position sensors (LDPS) while plants were exposed to salinized nutrient solutions for 2 or 12 h. Greenhouse-grown plants were acclimated for at least 24 h in a growth chamber before treatment. While in the growth chamber under constant 25 °C...

A electrical conductivity (EC) cell introduced into a porous ceramic cup was developed to continuously sample the solution and measure EC from different growing media. Application of pressure head creates a continuous flow of solution from the growing media tested, into the ceramic cup, and through the EC cell. Continuous recording of the EC was ac...