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.
Dale Rush
March 7, 2016Thanks David and the rest of the folks who did the research. Very good to keep all this in mind, especially when we have had various long-term stresses within many orchards over the last several years. Clearly, there are many contributing factors acting upon bud development, set, and pollination, both within and out of the control of growers and PCAs.
David Doll
March 7, 2016Agree, Dale. Thanks for the feedback.
Tom Ferrari
March 8, 2016The “June drop” is nearly always caused by insect damage or disease infection. Symptoms can be detected by careful examination of the aborted nut. For example, a small needle like exudation on the hull is indicative of aphid damage. A brown spot at the base of the developing nut is indicative of an infection during bloom. These problems are common and occur over a period of several months – usually April, May and June.
David Doll
March 9, 2016Thanks, Tom, for your comments.
Kia
August 12, 2017Thanks David for the amazing read…Learned a lot….
David Doll
September 9, 2017You are welcome, I am glad you enjoyed it.
Guy Kemp
October 22, 2017Hi David,
We live in mid-west NSW, Australia in a temperate climate ranging between -5 in winter to 40 degrees in summer. We have two almond trees; one is 7 years old and the other is 4. They are about 5 metres apart on a gentle slope, well-watered but not drowned, and under tall netting from about 75% of petal fall.
They have bountiful blossom early in the spring and in each tree the nut-set that follows is also bountiful but only in their first year. Subsequent years have consistently resulted in almost total fruit drop. Further, the tiny nuts do not appear on the ground under the trees. Could ants be responsible? We would appreciate any advice.
Regards,
an you help?
Regards,
Guy
David Doll
November 5, 2017Dear Guy,
I doubt it is ants. They wont feed on young, immature fruit…at least here in CA. If they are feeding on the nuts, there would be evident damage (i.e. half eaten flowers or fruit). Good chance it could be frost. Any chance of cold temperatures after bloom? It wouldnt take much (-2C).
David
Guy Kemp
November 7, 2017Thanks David,
I guess frost is the most likely cause.
Next year I’ll put clear plastic over the frames as soon as I see blossom. It works on the citrus.
Thanks again.
ernie
September 15, 2019hi guys we have a situation that our almonds are formed to about 5 plus mil in length but are all falling off the tree ,what is the problem the tree blossoms beautifully and are looking very healthy
David Doll
September 29, 2019Ernie,
This would suggest poor fertilization. Was there a frost event? Perhaps a deficiency of some type? Was the crop larger than expected last year and this could be an alternate bearing effect?
David
Carolyn
September 16, 2020Just wondering if we should just leave downed trees, full of almonds (Padre) until its time to knock the other trees. We have so many to clean up during harvest, like 10 trees full of nuts, thinking it will save the almonds. We knocked all the nuts off, then blew them into the rows for pick up. Should we have just knocked the nuts off the downed trees, cut up the trees for wood or was it better to leave the nut on the tree until harvest time…2 months down the road. I’m a grammy, so just curious. Thanks.
David Doll
September 26, 2020Great question! If the nuts have completed hull-split, they can be removed from the tree as they have completed the ripening process. Removing them earlier than will lead to shriveled nuts and adhering hull.
You will need to get the tree out of the way before harvest.
David
Gary
October 27, 2020What are the most Likely deficiencies in terms of nutrients that lead to the “June” drop
David Doll
November 15, 2020Gary,
This is a very complicated question. Any deficiency can lead to an increase in “June Drop.” The most likely deficiency is stored carbohydrates, which could be caused by poor post-harvest practices the year before, over-irrigation in the spring, or various nutrient deficiencies. As far as deficiencies, anything affecting growth would be at the top of my list: nitrogen, zinc, iron would probably influence drop rates. Potassium and boron wouldn’t necessarily effect rates as it would be tied to reduce initial nut set. Phosphorous is questionable as most CA orchards do not express deficiency. Calcium could be an issue depending on soil pH, soil amendment programs. Other micronutrients probably wont strongly influence unless a severe deficiency is observed.
I would want to review your production records and mid-summer leaf samples before jumping to any conclusions that the issue was related to nutrition.
David
Evan
November 24, 2020Where is the source paper / dataset from the M. Jones graph?
David Doll
November 24, 2020Hi, Evan,
The graph by M. Jones was constructed from data we collected from a local rootstock trial near Atwater, CA. The data is not published within any refereed articles, but rather was developed for the blog.
David