Clarissa Reyes, Orchards Advisor, Sutter-Yuba, Butte, and Placer Counties

At the end of the day, cling peach growers want high fruit yields with large size. Yield and growth are influenced by two separate, but interdependent processes: assimilation processes that supply carbohydrates and nutrients, and developmental processes that create a demand for these resources. The interaction of these can make it challenging to get the most, largest fruit.

Assimilation processes like carbohydrate movement and photosynthesis do not depend on cultivar genetics, but do depend on leaves and environmental factors, including temperature. The availability of carbohydrates around bloom time affects fruit development – the demand for carbohydrates is high at this time when fruit is growing quickly but leaves are not yet developed to create enough sugar. 

Meanwhile, development processes dictate how large a fruit can grow at a specific time via a relative growth rate function. Relative growth rate is genetically determined per cultivar and can be likened to compound interest rate function. The largest a fruit can become (growth potential) is a function of its size at the beginning of growth phase (akin to a principal deposit) and its developmental pattern (some amount of interest) over time. Ideal conditions for the largest possible fruit size (largest payout) are cool temperatures in the 30 days after bloom with each fruit receiving the maximum amount of carbohydrates and nutrients. However, a high crop load can reduce the amount of nutrients each fruit gets and negatively impact fruit size. 

The timing of thinning is important for sizing because starting with a high principal early in the season allows for a greater amount of interest (growth) to accrue until harvest – think about all the advice about how it’s better to start saving a little for retirement when you’re young vs. trying to save a lot when you’re older! 

When you thin, you reduce the number of fruits on the tree, so that each fruit has less competition for nutrients. In terms of the compound interest rate analogy, think of having $100 to deposit. Split among 10 fruits at bloom, each fruit will start with $10, and then start collecting interest on that. But if you thin later, more fruits will be present, and then maybe you’ll have to split $100 amongst 20 fruits. They will each only start with $5 AND not have as long to collect interest.

Some factors that can limit growth potential: 

• Carbohydrate deficit or water stress during the first stage of fruit growth about 30 days after bloom will reduce your “principal deposit.”
• High temperatures in the 30 days after bloom – fruit will develop faster and harvest will be earlier, which means less time to “collect interest” for growth.
• High crop load can reduce the amount of nutrient delivered to each fruit. 

Data from prior experiments in thinning peaches at different times show that earlier thinning can result in larger fruit size, higher crop load, and higher yields at harvest.

Table 1. Fruit yield data from four clingstone peach cultivars in commercial orchards near Kingsburg, CA that were thinned on two different dates in 1992. Data are means ± standard error for six 4-tree replicates per cultivar and thinning date, adapted from DeJong et al., 1992.

If you wait to thin until reference date comes around, you have already lost out on early season growth potential. Once you lose that potential growth, you can’t regain it because you’re compounding on a principal. Crop load adjustments should be made as early as economically feasible, especially in years of heavy fruit set.