Citrus fruits are subtropical in origin, but can grow well into temperate zones, with kumquats the hardiest of all. Unfortunately, citrus can sometimes drop quantities of fruit for a variety of reasons.
Spring Fruit Drop
It is common for citrus trees to drop large amounts of tiny fruit at blossom, then again three weeks later when the fruit are pea-sized, but this is almost inevitable because the tree cannot sustain the number of tiny fruit that grow from the blossom. There is likely to be another drop of fruit in May.
Summer Fruit Drop
Excessive summer heat and periods of drought can cause a lot of stress to citrus trees, and a natural response is to drop growing fruit. Young navel orange trees appear to be particularly vulnerable, and insufficient water when the fruit is in its infancy can make the trees more likely to drop fruit. Poor health because of root disease or nutrient deficiencies can also cause a tree to drop its fruit prematurely.
Pre-Harvest Fruit Drop
Some citrus, such as navel oranges and grapefruit, are more likely to drop fruit than others, and a certain loss late in the growing season is natural, but disease and insects can cause late fruit drop as well. Brown spot in mandarins, spiny citrus bug in lemons, citrus fruit borer and fruit fly can all take their toll.