Annually, I receive several questions on nut set. Many are aware that not all of the flowers on the tree will set a nut, but how many do? This, of course, can range between 15-40%. Most orchards, however, set between the 20-30% range, with average orchards around 25%. This percentage varies year-to-year, and is dependent on flower density, temperature at bloom and post-bloom, and tree health.
Spur dynamics play a key role in fruit bud density and the ability for a flower to set. Research by Tombesi and colleagues found that a fruiting spur, if maintained in a position with ample light for photosynthesis, tends to alternate bear. These spurs may flower the year after cropping, but rarely set a nut. This is believed to be due to carbohydrate and nutrient depletion within the spur. Surprisingly, tagged spurs that double or triple crop die, regardless of light position. Therefore, orchards that have a high set percentage deplete the spur pool, leading to a reduced set in the following year (i.e. alternate bearing). Generally, however, most orchards are able to re-develop spur positions which lead to sustained yields. Farmer practices come into play in developing and maintaining spurs- they include PROPER irrigation and nutrition, as well as adequate potassium levels to reduce spur mortality.
Tagging studies have also found that set percentage is generally inversely related to flower density. This means that trees that have fewer fruit buds/flowers will set at a higher percentage than trees with a high fruit bud/flower count. This most likely is due to a greater amount of resources able to be allocated to a fewer number of buds. This compensation for the lower bud count, however, does not typically lead to a higher yield.
Temperature can impact set as well. Almond pollination and fertilization can occur over a wide rang of temperatures. The ideal for pollen tube germination and growth occurs between 50F and 70F. Temperatures below or above can slow or prevent development. Of even more importance is that the flower only remains receptive for 3-4 days. Extreme temperatures, rain, or wind can impact flower receptivity, decreasing nut set. Rain or excessive free moisture can also cause pollen grains to burst, preventing pollination. More on this can be found in chapter 15 of the Almond Production Manual.
In discussing nut set, the three periods of nut drop must also be described. The first occurs shortly after bloom in which defective flowers drop from the tree. The second occurs within a month or so after bloom and consists. At this period, flowers that are about pea sized and have not shed their jackets drop from the tree. These flowers were most likely not pollinated or fertilized. Larger developed nuts which may have been fertilized may also drop during this period (which some suggest is another drop period). Finally, the third drop occurs around 6-7 weeks after bloom and consists of nuts that have been fertilized and dropped because of resource competition. This is the “June Drop.”
The figure below provides set data from a tagging study performed in a rootstock study near Atwater, CA. The three drop periods have been highlighted. Nut drop stabilized around 2 months after bloom, and all three varieties set around the same percentage of crop (~25%). Dropping patterns, however, were slightly different.