High-Density Planting of Dwarf Citrus

By Len Wilcox

Research into high-density citrus plantings is gaining momentum at both ends of the country.

In a study funded by the California Citrus Research Board, scientists are evaluating the consumption of water and other inputs used by high-density dwarf citrus trees. The scientists want to determine costs while the trees are maintaining high production levels.

Roya Campos, University of California laboratory assistant, places a protective layer over the installed sensor to prevent overheating. (Photo courtesy of UC Riverside)

In Florida, researchers are looking at high-density citrus plantings as a way to fast-track the return of profitability to the state’s distressed citrus industry.

The Florida Department of Citrus (FDOC) recently released a study called “The Economics of Restoring Florida’s Commercial Orange Tree Inventory.” The goal of the study was to find ways to rebuild the state’s annual production of 100 million boxes of sweet oranges within a 10-year period. The study, which did not include dwarf trees, recommends using a high-density planting system with 303 trees per acre (planted 7.5 feet by 20 feet) in Florida groves.

University of Florida Institute of Food and Agricultural Sciences researchers are also examining the use of high-density citrus plantings.

Aleyda Acosta Rangel, University of California graduate student, drills holes to install sensors to record sap flow in the tree trunk. (Photo courtesy of UC Riverside)

DWARF TREE RESEARCH HISTORY
At the Lindcove Research and Extension Center (LREC) east of Visalia, California, dwarf navel orange trees were first planted in 1998. The trees held potential for future production increases and were watched carefully by researchers and growers alike. Now, 20 years after the original planting, University of California, Riverside (UCR) researchers, led by Giorgio Vidalakis, are studying whether the use of these trees in commercial orchards could increase profits while reducing expenses. Improved net return could be achieved by maximizing land use and reducing inputs such as water, pesticides and fertilizers.

Vidalakis’ team of research scientists assigned to the project include Irene Lavagi and Rock Christiano. Lavagi explained in a UCR blog post that the dwarf trees were initially planted and studied to determine their yields. She said the dwarfing agent used to keep the trees small was a tiny RNA molecule called “transmissible small nuclear ribonucleic acid” (TsnRNA). The treatment resulted in a dramatic reduction in tree size.

The original study showed that the fruit yield per canopy volume and the fruit quality (size, color and sugar/acid ratio) of TsnRNA-treated trees was not affected. However, twice as many trees could be planted in the same land surface (up to 400 trees per acre), resulting in substantially more canopy per acre, and thus more fruit, than with conventional plantings.

In the blog article, Lavagi points out that production of commercial dwarf trees is key to the successful development of high-density plantings, which will be critical to meet future citrus production challenges. Dwarf citrus grown at high density is particularly attractive if citrus under protective screen (CUPS) is being considered as a preventive method for citrus greening disease.

RECENT RESEARCH
In the current study, Vidalakis and his staff are compiling data contrasting yield with the costs associated with production. Their intent is to assess the potential savings gained by planting high-density dwarf citrus. The UC Riverside scientists are tracking nitrogen fertilizer requirements, nutrient uptake efficiency, water-use efficiency and pesticide-application efficiency. They are also looking at the savings in labor time for several horticultural operations such as hedging, spraying, fruit harvesting and tree inspections.

Last winter, fruit nutrient analysis was performed to determine fertilization efficiency. This spring, LREC staff installed several electrical power sources in the field so that equipment to monitor water uptake could be set up. UC Riverside scientists installed sap-flow sensors into the trunks of the dwarf trees and connected them to dataloggers so that information on water use-efficiency can be calculated.

Lavagi reported that this project has also fostered collaboration between the Microbiology and Plant Pathology Department and the Botany and Plant Sciences Department at UC Riverside. Vidalakis and his team are working with L. Santiago and C. Lovatt to assess the potential of high-density dwarf citrus plantings in California citrus industry groves.

The research should provide new insights into using dwarf citrus to maintain high production levels with reduced overhead costs. An additional consideration is that, if the use of CUPS becomes necessary to protect trees from psyllids, smaller trees make that option more cost-effective.

Len Wilcox is a retired scientist who ran a weekly newspaper and has written for agricultural publications since the 1980s. His commentary, “The Western View,” is a regular feature on the AgNet West Radio Network.