High Density Citrus Plantings

Craig Kallsen

UC Cooperative Extension Farm Advisor, Subtropical Horticulture and Pistachio, Kern County

     Increasingly, orange and mandarin growers in the San Joaquin Valley (SJV) of California are planting trees at much higher densities than was the case even 15 years ago. Not much data is available in the literature on yield and fruit quality that results from high-density citrus plantings as opposed to the more historic open plantings. Planting density, to some extent, is affected by the choice of variety and rootstock as some combinations of these are naturally slower growing and slower to crowd. Tree density in orchards is often increased where soil texture, soil depth or irrigation quality limit growth as trees are slower to crowd each other. Regardless of the reason for slower growth, a slow-growing tree increases the length of time that the grower benefits from the initial higher density at that site in relation to the greater cost of planting, tree size maintenance and the cost of possible eventual tree removal. Tree spacing experiments are difficult to conduct and take many years to obtain complete results. It is worthwhile to look at older efforts, to see what information may be gleaned from these. In general, for most of our varieties, the results of an older experiment on tree spacing, are probably as meaningful today as when the test was conducted. A tree density trial, using Frost Nucellar on Troyer rootstock, was conducted in Kern County, beginning in 1961 through 1974 by Boswell et al. These researchers used a number of tree spacings as follows: 9' x 11' (i.e. 9' between trees in row and 11' between rows), 9 x 15, 9 x 18, 9 x 22, 11 x 11, 11 x 18, 11 x 22, 15 x 15, 15 x 18, 22 x 22 (see Figure 1).

Figure 1. Arial view of the Boswell et al. tree spacing trial in Kern County

when the trees were about 8 years old.

Some of the parameters that these researchers evaluated were as follows:

• Yield
• Net income
• Fruit size
• Rate of color development of the fruit
• Intensity of color development of the fruit
• Development of the fruit sugar/acid ratio

Results from this experiment were published at intervals in various publications as follows:

• Various research reports
• Platt, R.G. 1973. Planning and planting the orchard, p. 48-81. In W. Reuther (ed.). The Citrus Industry, Volume III. Div. of Agric. Sci., Univ. of Calif. , Berkeley, Calif.
• Boswell. S.B, C.D. McCarty, K.W. Hench and L.N. Lewis. 1975. Effect of tree density on the first ten years of growth and production of ‘Washington' navel orange trees. American Society for Horticultural Science 100:370-373

Not surprisingly most of the very close spacings had to be modified before the end of the experiment. I will quote from Volume III of the Citrus Industry, from page 68 in a chapter written by Robert Platt who describes the early results of the experiment up to 1969 as follows:

“Boswell et al., (1970), report that the two closest spacings, 9 by 11 feet and 11 by 11 feet, had to be thinned on the diagonal to one-half the original stand following the third harvest in 1966-67 because of loss of yield due to crowding and shading out of lower branches. The skirts on the remaining trees regrew, however, and increased production per acre is reflected in the 1968-69 harvest. Blocks with 15- and 18-foot spacings between rows required hedging to maintain adequate illumination and allow sufficient working space in row middles. All trees were topped to a 13-foot height in 1967 and maintained at this height with subsequent topping. They found that pruning to avoid crowding reduced yield (per tree) in proportion to the amount of foliage removed.”

TREE DENSITY AND NET INCOME

     As expected, orange yield was higher in some of the plots with higher tree density in the first five years of production (1965 – 1969), but plots with lower tree densities tended to catch up in the following five years of production (1970 - 1974) as shown in Table 1. However, the 11' x 22' and 15' x 18' spacings remained slightly ahead in terms of yield after 10 years of bearing.

Net income, the amount of money made by the grower after subtraction of costs such as tree, tree planting, tree removal with excessive crowding, fruit harvest, hedging and topping, frost control, nitrogen fertilizers and irrigation, are shown for some spacing plots in Table 2.

     In general, then, some of the more practical higher planting densities increased early and total yield and net income in the first decade of the fruit-bearing years. However, a number of plots with the very highest tree densities had to be reconfigured by thinning early due to excessive tree crowding. Over the time period of this study, the 9' x 11' spacing had a $1712 ($10,000 in today's dollars) tree removal cost and a five times higher tree and tree planting costs. Maximization of net income requires a balance between minimizing total costs and maximizing yield and fruit quality. This study was conducted with a single cultivar on Troyer rootstock. The success of a given tree spacing, partially at least, is a function of the cultivar selection and the rootstock, but growers are limited by the cultivars and rootstocks available. Most combinations produce full-sized trees, but some, such as scions on the rootstock C-35 do it more slowly. The performance of a given orchard may be a financial delight for the manager during the first years of production in terms of yield but a nightmare as the orchard grows older and decisions and expenses related to preventing tree-crowding increase.

OTHER OBSERVATIONS MADE IN THE FIRST FIVE YEARS OF BEARING IN THIS EXPERIMENT

Fruit size parameter: Average individual fruit size was larger in the 22'x 22' spacing.

Fruit color parameter: Orange color developed first in the 22' x 22' tree spacing. Fruit color was delayed as much as 45 days in closes tree spacings.

Color intensity parameter: Orange color was more intense in 22' x 22' spacing.

Sugar/acid ratio parameter: Higher sugar/acid ratio in 22' x 22' spacing. Trees spaced 9' x 11' were 11 days later in reaching legal maturity.

These observations suggest the following for growers seeking to influence fruit quality parameters. For larger fruit size plant trees further apart. For smaller fruit size (which may be desirable for marketing of some mandarins, for example), plant trees closer together. For a sweeter, earlier orange, plant trees further apart. For earlier color development and intensity, plant trees further apart. In general, to hasten fruit maturity overall, plant trees further apart. Finding a “perfect” plant spacing for meeting multiple objectives for fruit quality and yield may not be possible. For example, let's assume the objective of a new mandarin planting is to maximize profits by adjusting spacing for an early-maturing mandarin cultivar as follows:

As can be seen in the table above, maximizing earliness is at odds with producing high initial yields of smaller sized fruit. It may be possible to take advantage of higher yields of early-maturing fruit by initially planting to a higher density, which would be followed by future tree thinning once the trees began to crowd. However, experience suggests that unless the orchard is thinned in an early and timely manner, a considerable yield reduction occurs in the first couple of years post-thinning until the trees fill in the open spaces. Contrarily, if the objective was to maximize lateness with a late-maturing cultivar, high tree density may be the way to go. High density should produce smaller fruit size, further delay maturity and produce high initial yields.

     While no one plants Frost Nucellar navels anymore, there were considerable acreages of these planted in Kern County in the 1970s on an 11' x 22' spacing. This variety was an early attempt to grow a variety relatively free of viruses and viroids. By the time I saw orchards like these in 1990, as evidenced by the old stumps, most of these orchards had been thinned to a 22' x 22' spacing. So why were these orchards thinned? One obvious indication that the trees in an orchard are over-crowding is the inability of pickers to pass from one row to another in between adjacent trees in the row during harvest. A later indication is when the lowest parts of the tree canopy of the tree, called the “skirts”, start “rising”. As trees grow taller, less light reaches the lower canopy due to shading, finally reaching a level that will not support branch growth. Rising skirts means that the fruit bearing surface of the tree is getting further from ground level, which increases harvest costs and makes effective foliar pest control applications more difficult and costly. In general, if the lowest hanging branches of the skirt are five feet from the ground, five feet of the upper canopy can be topped. This action will bring the height of the bearing canopy closer to the ground without a loss of bearing volume. By carefully managing tree height, the tree canopy can be used as a tool to provide sufficient shade to control weed growth on the orchard floor without limiting the productive fruit bearing area of the orchard and without requiring unnecessarily long ladders that can reduce harvest efficiency (i.e. fruit picked per unit time) and increase risks of injury.