Answer: For many in California, the backyard orange or grapefruit tree is almost a member of the family and any negative change in its appearance elicits concern. One such change in appearance is leaf yellowing and drop that often occur during the winter. Citrus leaves can remain on the tree for as long as three years depending on tree vigor, but disease, inadequate or excessive nitrogen fertility, excessive salt or born in the soil, poor irrigation practices, freezing temperatures, pest pressures and low light levels significantly reduce leaf longevity. Excessive leaf drop during the growing season is more likely to indicate a serious problem than leaf drop during the winter. Winter leaf drop normally reflects nothing more than a momentary swing in the natural balance between the natural elimination of old senescing leaves and their replacement.
Citrus is an evergreen tree. New leaves are normally produced when shoots elongate. Periods of shoot elongation are called "flushes." Typically, an orange tree in California will produce two to five flushes during the growing season from March through October. Unlike deciduous plants, citrus sheds older leaves throughout the year. Leaf yellowing and drop in the winter is probably attributable to cooler temperatures, too much or too little water in the root zone, or winter semidormancy. Citrus is a subtropical plant and does not have a true dormancy period like the stone fruits. Generally, winter temperatures are too low in many citrus-growing regions in California for vegetative growth. Leaves continue to fall but none grow to replace them. Periods of high wind can remove large numbers of old tree leaves in a day, leaves which would have fallen naturally over a longer period of time, giving the impression that the tree is rapidly succumbing to a disease. The best indicator of tree health for a defoliating citrus tree during the winter will be how well it produces the first flush of new growth in the spring. A decision to keep or remove a citrus tree based on tree health should not be made during the winter. Even trees that lose most of their leaves during the winter, are capable of replacing leaf canopies with the spring flush of growth, usually with little loss in fruit production. Trees that do not produce a vigorous flush in early spring may have a more serious problem. March is an excellent month to begin applying fertilizer to encourage new leaf growth and fruit production and to help keep an old friend of the family around for years to come.
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