This summer I have seen an overabundance of fruit on many of the local fruit trees. I have seen some fruit trees with so much fruit growing, that the limbs were breaking off. So, why is this year such a good year for fruit production?
The answer to this question is not a simple one. The production of fruit yield is a complex process. Fruit yield depends on such things as plant maturity, plant structure, number of flower clusters/plant and fruit size. The processes that affect fruit yields include pruning, irrigation, fertilization and pollination. Also, environmental factors will affect fruit yields, such as, occurrence of frost, amount of sunshine, length of growing season, amount of heat, insect pests and diseases. Two critical periods during the growing season include the time surrounding flowering when cool and freezing temperatures can have significant negative effects on effective pollination and fruit set, and during July and August (the previous year) when the flower buds for the current year were being to formed.
Variation in fruit yield can be associated with the degree of effective pollination. Pollination can be influenced by a wide variety of factors. These include the temperature range before, during and after bloom, humidity, pollen source, amount of pollen, the presence and degree of activity of the required pollen carrier, light intensity (or amount of shading), supplies of carbohydrates, nitrogen and other nutrients, amount of rainfall and wind. For example, long, cold winters and cold spring temperatures can reduce the amount of viable pollen, and high spring temperatures can sterilize pollen; both can result in less effective or no pollination and possible fruit boss.
Irregular, alternate or cyclic fruit crop production (also called alternate or biennial bearing) is characteristic of many fruit crops, such as the apple, and pear trees. This phenomenon often consists of a heavy flower and fruit crop one year (the 'on' year), followed by a light one the next year (the 'off' year), although a regular biennial pattern (one year on, one year off in terms of fruit crop lad) does not necessarily exist. It is not unusual for many fruit crop and forest tree species to produce fruit or seed in an irregular, alternate or cyclic manner.
It's important to note that, in woody, perennial fruit crops like the apple, flower bud production for the following season begins while the current season's fruit are growing and ripening. The presence of fruit with seeds inhibits flower initiation. The reason for this inhibition is somewhat controversial but appears to be associated with the presence of chemical growth regulators called gibberellins, which are produced by seeds, and the depletion of the tree's carbohydrate supply below that required for flower bud production. So, last year if the fruit production on some apple trees was low, that would allow for more flowers to be produced this year, resulting in higher fruit yield.
At present, it's difficult to say how widespread irregular bearing is, or what the causes are. However, the following management practices may help even out crop production. Yearly thinning of older stems (greater than 1 in. in diameter) will help maximize leaf exposure to the sun. Thick or overly dense canopies limit the penetration of light which results in poor flower bud formation on interior branches.
The excessive application of nitrogenous fertilizers, which increase shoot growth and vigor, will decrease flower bud initiation. The excessive application of water, which also increases shoot growth and vigor, but again inhibits flower bud formation, should also be avoided. Insects and diseases that affect leaves and cause defoliation must be controlled because the loss of leaves may decrease the formation of flower buds.
Just remember, the production of fruit is a complex process. Actual yield is a result of interaction among weather, soil, insect pest and disease problems, and orchard management. Some of these factors we can control and many we cannot.
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