Three University of Florida Institute of Food and Agricultural Sciences researchers recently authored an article explaining how to manage alternate bearing in citrus with fruit thinning. Alternate bearing is the tendency for trees to carry a heavy crop one year, followed by a season of very little fruit. The authors are Masood Jan and Muhammad Shahid with the North Florida Research and Education Center and Fernando Alferez with the Southwest Florida Research and Education Center. An edited version of their article follows.
Fruit thinning — intentionally removing some young fruit — can help balance the crop and improve fruit quality from year to year.
Only about 1% to 5% of a tree’s flowers become harvestable fruit. A heavy crop uses much of the tree’s energy, leaving less for next year’s flower buds. Seeds inside developing fruit can also release hormones that suppress nearby flower-bud formation. This helps explain why a heavy crop may be followed by a light crop.
Too much fruit on a tree also competes for water, nutrients and sugars made by the leaves.
BENEFITS
Removing some fruit early reduces competition for resources from leaves. The remaining fruit can grow larger, develop better color and reach a better balance of sweetness and tartness. Thinning can also reduce small, misshapen, split or cracked fruit and allow more light into the canopy, helping the fruit ripen more evenly.
Early thinning allows remaining fruit to grow larger and become more valuable.
Thinning can also protect the tree. Removing some fruit early reduces the load on limbs and can help prevent branch damage.
Some citrus varieties benefit more than others from thinning. UF 950 and Bingo mandarins can set very heavy crops, leaving much of the fruit too small for good market value. Owari satsuma can also benefit from thinning because heavy crop loads can increase fruit puffiness.
METHODS
Growers can thin fruit by hand, by machine or with chemicals. Hand thinning gives the most control but is slow, labor intensive and expensive.
Mechanical shaking works well in some crops but is less effective in citrus because fruit holds tightly to the tree.
Chemical thinning can treat many trees quickly and usually costs less than hand labor. In Florida, the primary chemical used for citrus fruit thinning is naphthaleneacetic acid (NAA). It is labeled for oranges, tangerines, mandarins, tangelos and tangors, but not grapefruit. NAA has shown its most consistent results on Dancy and Murcott and is also used on Sunburst.
Timing is critical. NAA is used on fruitlets about 3/16 to 3/4 inch in diameter with the strongest response around 1/2 inch.
Weather affects NAA activity. It generally performs best around 75 to 85 degrees.
NAA can be applied only once a year and must not be applied within 150 days of harvest.
Thinning must be managed carefully. Removing too much fruit can reduce the current crop and income; removing too little may not solve the problem.
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