Some trees produce heavy crops each year. Other trees go back and forth between heavy bearing years and light bearing years. This is called alternate bearing.
Also known as biennial bearing, this tendency can explain why your fruit or nut tree doesn’t produce the same amount each year. It is not caused by pests or disease. It is simply the way some trees grow, mostly.
Generally speaking, fruit and nuts are produced on long stems or on short, stubby stems called spurs. The spurs of alternate bearing trees tend to produce blossoms one year and leaves the next. This allows the tree to gather more of the sun’s energy, carrying it through its two year cycle of production. Therefore, it is not the tree, as a whole, that alternates production each year. Instead, it is the individual stems and spurs.
Other causes of alternate bearing include weather conditions, such as a heavy frost that damages the blossoms, can result in a low production year. Since resources were not used to produce as much fruit that year, the following year will tend to be a heavy production year. Some trees may even take two years off after a particularly heavy year. In other cases, mineral deficiencies or excessive fertilization, especially nitrogen, can result in a cycle of alternate bearing to begin.
Most perennial fruit crops start making fruiting buds during the previous season. If conditions are especially good or bad, those developing buds will become stronger or weaker, respectively. There is also some research that suggests the developing ovules of a heavy production year produce hormones that inhibit next year’s flower production. Scientists are still learning the details about what causes alternate bearing.
Some commercial growers reduce the impact of alternate bearing trees by thinning blossoms and fruit on heavy production year. This is done to counteract the fact that fruit produced during heavy years tends to be smaller, while fruit produced during low production years tends to be of poor quality and more susceptible to pest and disease damage. Evening things out improves crop quality and tree health.
Citrus, apple, pear, avocado, apricot, plum, mango, cranberry, blueberry, olives, pecan, and pistachio are common plants that exhibit alternate bearing behavior in the home garden. Grapes almost never do.
You can reduce the negative impact of alternate bearing on your fruit and nut trees with proper pruning, irrigation, and feeding, along with regular fruit thinning, soil tests, and protective treatments against pests and diseases.
Now you know.
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