Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties

Summary: Cooler weather in the 30 days following 50% full bloom generally leads to a longer growing season and larger fruit size potential in peaches, plums, and prunes. The heat units used in research comparing early season temperatures, harvest date, and fruit size are called Growing Degree Hours (GDH).  

Generally, GDH30 accumulation looks to be lower this year in the Yuba City area than in the past few years. This means harvest could start in the Yuba City area in the last week of August and sizing potential should be better than in, for example, 2019 when bloom was also late but GDH30 was high. We’ll have to see how the season plays out, but experience favors these predictions (late harvest, better fruit sizing potential). 

Bloom date and cropload per tree still have the largest influence on harvest date and fruit size potential, respectively, but knowing GDH30 accumulation can be used to fine tune harvest and thinning planning. See the thinning article in this newsletter for examples of how GDH30 can be used in thinning planning. 

Details: Almost 20 years ago, UC Davis Professor Ted DeJong, working with fresh market peaches. showed that warmer weather in the first month after full bloom was related to earlier harvest, earlier reference date, and smaller fruit at both reference date and harvest compared to cooler spring weather. Since then, Dr. DeJong and team have shown a similar relationship between GDH30 and harvest date in prunes. 

Why does temperature matter like this? The theory goes like this…Temperature drives development in plants, but growth is influenced most by carbohydrate (sugar) availability. Sugars from winter storage and/or current photosynthesis are both the raw material of plants as well as the energy to fuel growth. When temperatures are warm in the spring, trees develop rapidly but carbohydrate availability lags and growth potential isn’t met even as higher temperatures drive fruit development (pit hardening, reference date, etc.). Conversely, later harvest from cooler springs means the fruit is on the tree longer and so has more time to grow compared to earlier harvests linked to warmer springs. Bottom line? The experience shows later harvests with cooler temps in 30 days after harvest and that can mean slightly bigger fruit at harvest. Check out the GDH30 calculator at the UC Davis website linked above.