- Author: Michelle Davis
It's raining, but it's not water from the sky. It's fruit drop from my Satsuma mandarin tree. Small, immature fruit is littering my patio and my garden bed. The official name for this event is “June Drop”, but it can happen May though July. For some reason, it seems worse this year.
Did you know that, according to the California Master Gardener Handbook, more than 95% of citrus blossoms will NOT produce fruit and that once the flowers have dropped, the tiny, immature fruit is next to fall? Why does this occur?
The primary reason is natural thinning. Natural thinning is when the tree drops blossoms and then its fruit to protect itself from the stress of bearing too much fruit. Flowers will be dropped because they were not pollinated (possibly due to not enough bees) or because they were wind-blasted. Every fruit that makes it beyond the flowering stage then goes into direct competition with its neighbor on that same tree for the same water and nutrients. This is Darwin's Theory in action. Once growing conditions stabilize, the tree will selectively thin its fruit to provide water and nutrients to only the larger, hardier fruit. If the fruit has seeds, the tree will selectively thin those with a lesser amount of seeds. That is because, from the tree's perspective, the purpose of fruit in the first place is to provide for species continuation. Any tree that doesn't selectively thin will produce much smaller, lesser-quality fruit, unless, the caregiver thins it out. Once fruit is able to grow to about an inch in diameter, it has a better chance of becoming a mature fruit.
Another cause of fruit drop from citrus trees can be nitrogen deficiency. An adequate amount of nitrogen is needed for the fruit to set. Recognize nitrogen deficiency by the citrus tree's older leaves becoming a mottled green and yellow and then progressing to all yellow before dropping. Later the younger leaves are affected. On the other extreme, too much nitrogen stimulates lush leaf growth and causes delayed and/or decreased fruit production.
Sudden high temperatures can also cause fruit drop as can underwatering. I can't do anything about our recent wild weather for my mature in-ground tree, but I can increase the amount of water the tree is receiving. Per UCANR IPM, citrus trees need 4-6 inches of water per month during the summer. It is recommended to water every 7- 10 days at the tree's canopy line. Check the drip line, and when the soil is dry 6 inches down, water. The University of Arizona Cooperative Extension recommends building a 4-inch-tall dike around the tree one foot beyond the tree's canopy to create a water basin and contain the water. Fruit on the tree can also act as a water source to the tree to keep it alive when the tree is not receiving enough water from nature or the tree's caregiver. Recognize underwatering as the problem when the tree's fruit shrivels up and drops. Better yet, catch the first signs of dehydration, when the tree's leaves are curling.
Silva, D., C. Lovatt, and M.L. Arpaia. 2002. Chapter 18, Citrus. Master Gardener Handbook, University of California Agriculture and Natural Resources Publication 3382.
Wright, G.C. 2021. Irrigating Citrus Trees. The University of Arizona Cooperative Extension Publication AZ1151-2021, May 2021
UC IPM University of California Agriculture & Natural Resources. 2017. Watering Avocados and Citrus