The Sweet Spot

Moderate spacing and medium tree size could be the quickest way Florida growers can return to producing 100 million boxes of oranges per year.

By Pete Spyke, Bill Castle and Ed Stover

The Florida Department of Citrus (FDOC) recently released the results of a study originally titled “Restoring Florida’s Commercial Orange Tree Inventory: Economic Impacts of APS vs. Traditional Plantings.” The study was revised and posted as “The Economics of Restoring Florida’s Commercial Orange Tree Inventory.”

FDOC STUDY
The goal of the study was to establish industry guidance to reclaim a statewide annual production of 100 million boxes of sweet oranges within a 10-year period. It simulated three planting density scenarios across five production zones for early-mids and Valencia, all using historical averages for tree size, cost and production. The three scenarios were:

1. Advanced production system (APS) at 303 trees/acre (7.5 feet by 20 feet)
2. Traditional production at 145 trees/acre
3. A hybrid planting of 50 percent APS and 50 percent traditional

The analysis supported higher net returns from APS through 13 years after planting, primarily because the authors assumed that the per-tree production would be higher with APS and also concluded that greater nursery tree production and incentives for replanting were likely necessary to achieve the 100 million boxes/year goal. The assumptions and conclusions of this study suggest it would be useful to briefly review planting density and APS in citriculture.

A DIFFERENT VIEW
We do not disagree with the basic approach or question the value of the study as presented; planning is always a good idea. However, in full consideration of the three scenarios examined by the FDOC, we believe that rather than using higher density (HD) plantings for everything, another approach, the “sweet spot” concept, is the most realistic and fastest way to reach a statewide production goal from a certain number of trees. The concept should enable the maximum production possible from each tree in order to reach 100 million boxes as soon as possible. That view is based on these points:

• The conclusions of the FDOC study are based on assumptions that are limited by inaccuracies inherent in most simplified economic models.
• The importance of properly adjusting the per-tree yields to reflect differences in tree spacing
• Our strong belief that the industry will be better served by giving full consideration to replanting new scion and rootstock cultivars
• The crucial point that different planting densities are best for different scion/rootstock combinations, aka the sweet spot. We think that in reality, things would turn out differently than reflected in the study if informed choices are made regarding scion variety, rootstock and tree spacing for each situation when planting new groves.

We argue that moderate spacings with medium-sized trees would out-produce any other scenario. The higher densities of APS groves are designed to maximize fruit production over a limited period of time per acre. This system does not maximize the production per tree, however.

APS RETROSPECTIVE
Bill Castle of the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) became interested in APS as a production strategy before the 2005 discovery of HLB in Florida. The actual introduction of APS followed a trip to South Africa in 2006 to study strategies for maintaining a profitable industry in spite of indigenous HLB. The first such Florida planting was established at Pete Spyke’s Rock Bottom Grove in Fort Pierce, in close cooperation with Castle, Japie Kruger from South Africa and Ed Stover of the U.S. Department of Agriculture’s Agricultural Research Service (USDA/ARS). Subsequent work has been continued by Arnold Schumann of UF/IFAS and research agencies and growers in Florida.

To recap the theory, the South African approach was to use APS as an HLB management and production tool. It involved initially planting at higher densities and then removing infected trees immediately upon detection of HLB. Losses in typical South African groves in areas affected by HLB were low for the first few years due to effective treatment with neonicotinoids and aldicarb, then settled down to about 3 percent of trees lost per year. Trees were not replaced when removed because the neighbors would quickly fill the gap with bearing canopy. So, over a 10-year period, for example, it was anticipated that growers would remove about 30 percent of the original trees, which would result in a dynamic spacing appropriate for the scion/rootstock without a reduction in per-acre yields. The producing trees in their groves are not infected and, therefore, do not require any special care other than what is necessary to reach maximum production from healthy trees.

APS: A COMPREHENSIVE APPROACH
APS is about more than just planting density. The approach involves fertigation with drip systems, specialized pruning and harvesting methods, and other practices as well as grove design. Those components of APS can be applied regardless of tree spacing and are actually being utilized by most growers in Florida today.

It has since become clear that infected tree removal isn’t going to happen in Florida, so the pure South African approach won’t work here. Instead, until a “cure” is found, we likely have to rely on using the most tolerant scion and rootstock combinations available along with production practices that reduce stress and help the trees work around the effects of HLB. Given that basic conclusion, the question is whether HD plantings would still be a good idea.

TREE SIZE VS. TREE SPACING
The FDOC study compares three planting scenarios with the same number of trees in each and does a nice job of breaking apart the various questions involved. However, if the boxes produced are divided into the number of trees, the inherent assumption seems to be that trees in HD plantings produce more fruit per tree than trees in more conventional settings. In reality, with tolerant scion/rootstock combinations and proper spacing to form hedgerows by the time the trees reach about 8 years of age, it likely would be the opposite.

That’s because the single most influential determinant of the amount of fruit produced by a tree is the size of the tree. In general, bigger trees produce more fruit than smaller trees, and HD spacings limit the available space for each tree. Therefore, if the same numbers of trees are planted and then allowed more space to grow larger than if they were planted in an HD arrangement, the fruit per tree is almost always greater and the amount of fruit statewide will also be proportionately increased. This focuses on the FDOC study assumption of a fixed number of trees available for planting each year rather than a fixed number of newly planted acres.

Furthermore, using those criteria alone to maximize statewide fruit production, tree spacing would allow each tree to grow as large as possible. However, that’s not always feasible since tree size must be controlled to accommodate harvesting, spray applications, and for other reasons. And, if trees are spread too thinly across the land, the cost to perform operations would increase per tree, thereby negating the value of higher production per tree. Therefore, tree planting density considerations are influenced by other practical considerations, not just maximum production per tree.

THE SWEET SPOT – OPTIMAL SPACING
For each scion and rootstock combination interacting with site-specific factors there is a tree-spacing sweet spot. That would be the in-row and between-row spacing that allows the trees to fill the space allotted with bearing canopy volume as quickly as possible, but without “crowding” the neighboring trees in one or more dimensions. It’s the “optimum” spacing, rather than the “maximum” or “minimum” for the combination.

Research and experience have shown that squeezing the tree space in even one dimension will result in a loss of production when the trees reach full size because they continually try to regain their natural shape, enhancing vegetative production at the cost of producing fruit. Thus, if it turns out to be desirable to plant at higher density, both the in-row and between-row spacings (and tree height) should be adjusted. Scion/rootstock combinations should be chosen that fit the smaller available space. Some degree of dwarfing is likely to result from HLB infection compared to fully healthy trees, even with more tolerant scion and rootstock selections. As a result, the sweet spot is likely to be different in HLB-affected versus healthy trees of the same scion/rootstock combination.

With conventional grove care and harvesting machinery, adjusting the between-row spacing beyond a certain point is generally not feasible on a large scale. Also, if the existing irrigation pump station and mainlines and submains are retained, the cost per acre of new grove establishment is much less than if the irrigation system is completely replaced to accommodate HD plantings. Thus, there is a major financial incentive to preserving the existing between-row spacing. However, if scion/rootstock combinations that produce small trees are planted with wide between-row spacing, the trees will not adequately fill the available space in the row middles, which will limit grove production. For these reasons, all three factors (in-row spacing, between-row spacing and tree vigor) must be matched to achieve optimum production per tree.

There are exceptions to the rule of maximizing production by allowing the space necessary for the trees’ natural size, most notably a grower with a fixed amount of land. If HD is considered in this case, it will be necessary to have sufficient capital and expertise to design and execute a more intense production program. It costs more per acre for establishment and management, but can have a similar or lower operating cost per box if APS principles are fully adopted, and critically, the scion/rootstock combination is matched to the spacing.

SCION/ROOTSTOCK SELECTION IS KEY
Regardless of the production system or grove design, the goal should always be to determine the spacing for each scion/rootstock combination that allows maximum production by each tree in the space allowed. That’s what we mean by the “sweet spot.” It may be different for each scion/rootstock combination and site. There is a lot of horticultural information and some financial analyses available to help growers make these choices. Grove design has been studied for hundreds of years, and both research and grower experiences have led to certain rules that should be applied. Over time, the best designs float to the top. For citrus, it is definitely possible to productively reduce spacing below that used in past decades — but only up to a point.

Currently, there are better scion/rootstock combinations that show HLB tolerance, but they will normally produce medium- to large-sized trees rather than trees that will grow well in HD plantings. For medium-sized trees, in-row spacings of 8 to 12 feet and between-row spacings of 20 to 25 feet would likely be appropriate, again depending on the scion/rootstock. Typical APS spacings would be more like 6 feet by 15 feet, obviously too close for medium-vigor tree combinations which thereby limit the options for HLB-tolerant combinations.

The first step toward reaching 100 million boxes may be to use HLB tolerance as the primary factor for choosing a scion and rootstock along with the appropriate spacing rather than using spacing by itself for each of those combinations. Tree spacing would then be the second decision, not the first, when designing a new grove. The number of acres statewide would simply be what is required to accommodate the individual grove designs.

As a purely hypothetical example, if 50 percent more acres are planted using sweet spot spacings and all the trees grow large enough to form hedgerows, the sweet spot trees at maturity should produce around 50 percent more fruit statewide than if the same number of trees were planted in higher densities on fewer acres. That’s where the assumptions used in the FDOC study depart from our experience and expectations. We believe that the sweet spot approach is actually the fastest way to achieve 100 million boxes.

Pete Spyke is a fresh-fruit grower and owner of Arapaho Citrus Management, Inc. Bill Castle is professor emeritus at the UF/IFAS Citrus Research and Education Center in Lake Alfred. Ed Stover is a research horticulturist at the USDA/ARS in Fort Pierce.