Why do my pecan trees have a large crop followed by a small crop?

Charles J. Graham, Bollich, Patricia A.  |  8/8/2006 1:48:36 AM

The tendency of pecan trees to follow a high production year with one or more years of relatively low production is generally referred to as alternate bearing or biennial bearing. Alternate bearing has been known as one of the major problems of pecan since the sixteenth century. Pecan tree traits associated with alternate bearing include late season fruit maturation (trees defoliate only weeks after the nuts mature), late season dry matter accumulation (over 2/3 of the nut dry matter accumulates in the weeks prior to shucksplit), and the high lipid content of pecan kernels (lipids require more energy to produce than soluble carbohydrates and starch). These traits combined with a heavy crop will deplete the tree of carbohydrate and nutrient reserves at the end of the season and contribute to low fruit set the following year.

The exact cause of alternate bearing continues to elude researchers, but is generally attributed to the high energy requirement to produce high quality pecan kernels. Flower initiation and development in pecan is controlled by an interaction of several factors including carbohydrate reserves and a balance of plant hormones. Flowers that produce next year’s crop are initiated during the time the current season’s crop is maturing. Therefore, stress during flower initiation affects next year’s crop. Several popular cultivars suffer from this type of over-production including Cape Fear, Creek, and Pawnee. Pecan cultivars vary in their tendency toward alternate bearing, but all demonstrate a cyclic increase and decrease in their production as they increase in age. Alternate bearing is a strongly expressed genetic trait of pecan trees, but a grower can minimize this cyclic behavior by implementing intensive management techniques including a pesticide spray program, herbicide strips, irrigation, fertilization, and adequate tree spacing.

For more information please see the Pecan Crop Load Management handout on the Horticulture or General Orchard Maintenance pages of the Pecan Station website.

Question answered by Dr. Charles Graham, Pecan Research-Extension Station horticulturist. 

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