Hi y’all,
From what I have seen so far this year, it looks like we are in for a low year on many of our satsumas. Jake recently posted some data from his satsuma block showing evidence of alternate bearing. I wanted to share a few notes on how alternate bearing occurs and how we can correct and avoid that pattern in the coming years.
First and foremost, alternate bearing is defined as the tendency of a tree to produce a heavy crop one year followed by a light crop the subsequent year. This phenomena is a common problem in many mandarin varieties and has made their marketing difficult in the past. However, alternate bearing is not always a perfect on-off-on-off pattern. Very heavy years can be followed by one or more low years. Likewise, the degree of alternate bearing can vary greatly based on a number of factors. For example, a normal alternate bearing pattern for a given variety may look like 150 lb/tree one year followed by 250 lb/tree the next. For another variety it may look like 50 lb/tree one year followed by 400 lb/tree the next. I’ll go over a few of the factors that affect this pattern and discuss a few control methods we can implement in the future.
Causes of Alternate Bearing
Carbohydrates
During photosynthesis, leaves produce sugars than are either exported and utilized for growth or stored as starch in the leaves for later utilization. For this reason, leaves are considered a source of carbohydrates while the organs that import carbohydrates for utilization (new flush, roots, fruit) are considered carbohydrate sinks. The movement of carbohydrates from source to sink is critical for all new growth. However, carbohydrates are a limited resource so there is a lot of competition among the different sink organs for those carbohydrates. Prioritization of one sink type comes at the expense of another. So, for example, when we have a heavy crop load, carbohydrates are being invested in fruit production rather than into vegetative flush. While this will result in higher yields at harvest, it also means there’s less new wood available for flowering in the subsequent season. In addition, when growth slows during the winter, carbohydrates will continue to accumulate during the winter to support flowering in the subsequent season. Altogether, when carbohydrate reserves have been severely depleted during a heavy year and fruit production was prioritized over vegetative flush, this limits the number of flowers that can be produced the following year. This ultimately results in a ‘low’ year. Therefore, at its core, alternate bearing patterns can be explained by fluctuating carbohydrate availability.
Hormonal Balance
Gibberellic acid (GA) is the major hormone that controls flowering. GA promotes vegetative growth over reproductive growth. GA is produced by mature fruit and exported to the surrounding buds to inhibit floral induction. This is why you rarely see flowers on branches that produced a lot of fruit the year before. GA levels have to drop in order for flowering to proceed. Harvesting too late or having fruit still on the tree during flowering can greatly reduce the number of flowers produced.
Natural Pattern
As I mentioned above, alternate bearing is a natural phenomena in many citrus species. The difference in production between on and off years can vary from variety to variety. For example, sweet oranges rarely see strong alternate bearing patterns while mandarins are more prone to them.
Environmental Stress
Extreme environmental conditions can cause damage to trees and trigger a low production year. This can trigger a strong alternate bearing pattern if the following year is unusually high. The alternate bearing pattern we are currently observing is likely the result of this. Many trees sustained significant damage during the December 2022 freeze. As a result, production was low during the 2023 season as trees prioritized vegetative growth over fruit production. The 2024 season was then abnormally large for a lot of folks and likely depleted carbohydrate reserves. The fruit set for the current (2025) season is low as a result.
Control Methods
Unfortunately, there is very little that can be done to increase the crop load in an off-year. While certain plant growth regulators (PGRs) can be sprayed during flowering to increase fruit set, those fruit may still be subject to fruitlet and June Drop in the weeks after they’ve set. In particularly heavy years, cultural practices can be optimized to alleviate any deficiencies that may arise as a result of the heavy crop load. Likewise, in the off-year, cultural practices can be used to prep the trees for the heavy on-year that is likely to follow. However, this does little to prevent the alternate bearing pattern.
Reducing the crop load during a heavy year is the easiest way to alleviate the alternate bearing pattern. Removing flowers or fruit also removes the carbohydrates they’ve already imported and utilized. Therefore, the earlier flower or fruit thinning can occur, the better as this cuts down on the amount of ‘wasted’ carbohydrates. Thinning can be done by hand or through chemical thinners. Hand thinning is more labor-intensive, but ultimately provides better control over the quality and quantity of fruit remaining on the tree. Chemical thinners thin indiscriminately so you are more likely to run into issues of over- or under-thinning. We also haven’t developed a recommended protocol for chemical thinning in Georgia citrus, so it is not recommended as of yet. Pruning, hedging, and topping in on-years is also a great way of thinning out the crop load; cutting back wood also removes all the fruitlets on that wood. While it seems counterintuitive, reserving these canopy management practices for heavy crop load years can help alleviate alternate bearing patterns.
Conclusion
Unfortunately, alternate bearing patterns seems like its going to be a common issue in Georgia citrus. The odd hard freezes we sometimes see during our winters creates an opportunity for alternate bearing. We are currently working on both pruning and fruit thinning recommendations that we hope can help alleviate the severity of this pattern in the future.
Additional Resources:
A more in-depth article on alternate bearing
The Physiology of Citrus Flowering. This is a flowering short course I made while at the University of Florida. It covers the basics of citrus flowering and looks at the factors affecting the degree of flowering more deeply.
