Our Master Gardener Help Line has received several calls this past month from gardeners concerned about citrus trees turning yellow. An article entitled “Yellow Citrus Leaves in Winter Usually Not a Disease,” written by Craig Kallsen, retired Citrus Farm Advisor in Kern County, does such a good job explaining the problem that I asked for and received his permission to reprint his entire article, presented below.

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 occurs during the winter in citrus. 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 trees are evergreens. New leaves are normally produced when shoots elongate.  Periods of shoot elongation are called “flushes.” Typically, an orange, mandarin or grapefruit tree in California will produce two to five flushes during the growing season from March through October. Lemons tend to be more vigorous and may produce leaves nearly continuously through the growing season. Unlike deciduous plants, citrus sheds older leaves throughout the year  Leaf yellowing and drop in the winter is probably attributable to excessive or too little water in the root zone or dry winds in combination with cool soils. 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.  Excessive leaf drop manifests itself rapidly under these conditions.  Periods of high wind can remove large numbers of old leaves tree 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.

Reduce winter water applications to citrus trees that have defoliated or that have a significantly thinning canopy. Trees like this require little, if any, supplemental irrigation.  Even a mature tree with a full leaf canopy will require less than 10% of the water that it would require during the summer.

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 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 farm or family backyard around for years to come.

Ed Perry is the emeritus Environmental Horticultural Advisor for University of California Cooperative Extension (UCCE) in Stanislaus County where he worked for over 30 years.