This year, there have been a lot of comments on almond kernel size and the earlier than normal harvest. Since crop load appeared lighter than the past few years, having a smaller nut size may have been a bit of a surprise. This, however, was expected and research within peach, a close cousin to almond, has provided a model for understanding.
Research by Dr. Ted DeJong’s group at UC Davis determined that peach harvest timing and size was influenced by the number of growing degree days experienced within the first 30 days after bloom (GDH30). They were able to determine that for every 1000 GDH30, the number of days from full bloom to harvest decreased by 3.5 days within peaches. DeJong’s group also shown that fruit size was influenced by GDH30. Basically, for every increase of 1,000 GDH, fruit diameter declined by 1.0mm. For peach growers, warmer than normal springs mean that harvest will be earlier and thinning will have to be heavier in order to maintain size…but what does this mean for almonds and why?
As observed this year, almond kernel size is negatively affected by higher than normal heat in the spring (just like peach). This is because photosynthesis is maximized between temperatures of 65° and 90°F while respiration rates generally double in response to every 18°F increase in temperature. In other words, photosynthesis is maximized as long as the temperature is above 65°, but respiration rates continue to increase as temperatures increase. Therefore, when warm (>75°F) days are experienced in March and April, more photosynthate is needed for respiration and less is directed towards crop development, leading to a smaller kernel. This process also compresses stage one of growth, reducing the number of cells that have divided.
Harvest within almond also started earlier than in years past. Again, this is due to the temperatures experienced during March and April and it is thought to be due to the compression of stage one.
In the opposite effect, cooler springs will lengthen the season and increase nut size. A good example of this was in 2010 when ‘Fritz’ were being harvested in late October/early November. In 2010, the spring time degree-days (400 F minimum, no maximum) were the lowest in ten years, in contrast to this year, which was above average.