Jaime Ott, UCCE Tehama, Shasta, Glenn, and Butte Counties

The growing degree hours in the 30 days after full bloom (GDH30) is a measurement which predicts reference date, harvest date, and fruit sizing potential. It is counterintuitive, but higher GDH30 (warmer weather after bloom) results in earlier reference date, earlier harvest date, and smaller maximum fruit size.

The Bottom Line

• GDH30 in the Sac Valley is high this year (7,500-8,000 degree hours)
• We expect an earlier reference date and earlier harvest date given a late March bloom timing, and less fruit sizing potential than in a cooler spring.
• You might consider thinning earlier and harder than usual to counteract the effects of our warm spring on fruit size at harvest
• Cropload management, water management, and nutrient management are the most important factors affecting your fruit size at harvest. While GDH30 influences fruit size, it is a comparatively small effect and out of your control. You do control cropload, irrigation, and fertilization.

The Details

The weather in the 30 days after full bloom has a strong effect on the growth and development of prune fruit, and helps us predict reference date, harvest date, and fruit sizing potential. Growing degree hours accumulated in the 30 days after bloom (GDH30) distills temperature data over those 30 days into a single number which is easy to compare from year to year. According to many years of research, lower GDH30 values correspond with later reference date, later harvest date, and larger fruit size at harvest. The opposite is also true: years with higher GDH30 values have earlier reference date, earlier harvest, and smaller fruit size at harvest. This year, GDH30 in the north Sac Valley is high, around 7,700-8,000 degree hours in the orchards I am monitoring. In Sutter County, GDH30 is also high, running about 7500 this year. A rule of thumb: for the same number of fruit per tree, years with GDH30 over 6,000 will have smaller fruit at harvest than a year with lower GDH30. This means that we are likely in for a poor fruit sizing year—in orchards with good fruit set, thinning early and hard will help to counteract the effects of our warm spring on final fruit size. What do I mean by “thinning hard”? Generally aiming for the lower end of a target fruit per tree count.  For example, if you usually target 3500-4000 fruit per tree in a certain orchard, this looks to be a year to try hard to hit 3500 and not settle for 4000 or 4500 fruit per tree.  

It may seem counterintuitive that warmer springs (higher GDH30) result in smaller fruit at harvest, but it can be explained by how prune fruit develop and grow. First, it is important to understand that development and growth are two separate processes, but both contribute to “growth” of the fruit which we might see or measure (Figure 1). Development involves cells in the fruit undergoing cell division (increasing the total number of cells) and undergoing differentiation or maturation (being assigned to a final function and taking on characteristics of that function, like pit cells hardening or the skin cells turning purple during ripening). Growth is simply the expansion of cells which are already present. In the first week or two after bloom, the “growth” we see is dominated by the increasing number of cells. After that, cell division slows rapidly, finishing well before pit hardening. The “growth” we see for the rest of the season is exclusively due to those cells getting bigger and/or heavier, like water balloons being filled. Development does continue throughout the season, but there is no more cell division, and cell differentiation and maturation do not contribute much to visible fruit growth.  Temperature plays a huge role in how long the cell division part of development lasts and how many cells are produced. Warmer temperatures (higher GDH30) result in a shorter, more intense cell division stage. Cooler temperatures (lower GDH30) result in a longer, more moderate cell division stage. Before reference date thinning in a high GDH30 spring, there are not enough resources in the tree to feed all the fruit at the rate they are demanding, so each fruit produces a smaller total number of cells. Over the rest of the season (if cropload, water, and nutrients are not limiting) cells will grow to their maximum size, so fewer total cells results in a smaller maximum fruit size possible at harvest (Figure 2). Thinning earlier than normal can help counteract this by reducing the number of fruit and allowing the tree to deliver more resources to each of the remaining fruit, which should grow faster and be larger at harvest.

Figure 1. A simplified diagram of development (cell division) and growth in prune fruit, both of which contribute to the “growth” we see. Light green circles represent cells, each with a dark green nucleus. During the first week or two after bloom, the fruit “growth” we see is dominated by an increasing number of cells. After that, cell division slows and stops rapidly, and the “growth” we see is due to an increase in cell size.

Figure 2. A representation of how GDH30 affects fruit size. In years with high GDH30, the cell division period is short and intense, leading to a smaller number of cells making up the fruit. In years with low GDH30, the cell division period is longer and more moderate, leading to a larger number of cells making up the fruit. During the season (if cropload, water, and nutrients are not limiting) cells will grow to their maximum size. This results in smaller fruit in years with high GDH30.

With all this talk about GDH30, it is important to remember that cropload management, irrigation management, and nutrient management are the three most important things affecting your final fruit size, and they are all under your control as a grower. GDH30 tells us about the theoretical maximum size your fruit can achieve, but those cells can’t reach their maximum size if they are competing with too many other fruit (heavy cropload), if the tree is water stressed, or if there isn’t enough potassium to go around. GDH30 is a valuable tool because it gives you a sneak peek at what sort of sizing year we are likely to have, helping you make thinning decisions that are tailored to the season. This year, with such a high GDH30, consider thinning as early as possible and erring on the side of taking a little more fruit off the trees. Getting cropload right and following it up with a good irrigation and fertilizer program is the recipe for the highest grower returns at harvest.

More information on GDH30, including links to the research this article is based on.

You can calculate the GDH30 for your orchard using data from your local CIMIS station at the UC Fruit and Nut Research and Information Center. The model is for peaches but also works for prunes.