Thinning prunes is critical in a year with warm bloom temperatures

Temperatures were warm during prune bloom, which could result in poor sizing. Thinning is necessary many years, but especially critical in years like this.

Below is an excerpt from a prune thinning written by former UC advisor Dani Lightle, you can read the full article here.

Will you need to thin prunes this year? In order to keep fruit from falling through the sizer, you need to do some legwork and estimate your fruit set. If needed, thinning should occur roughly around the same time as ‘reference date’, or the point at which 80-90% of the fruit have a visible endosperm. The endosperm, a clear gel-like glob, will be found in the seed on the blossom end of the prune (Figure 1) and is solid enough to be removed with a knife point. Typically, the reference date occurs in late April or early May, approximately one week after the pit tip begins to harden. The earlier the thinning is done, the greater effect it will have on final fruit size at harvest, though if you thin too early, you may damage the trees without removing the desired number of fruit.

To decide whether to thin, estimate the number of fruit per tree to produce your desired crop, determine the number of fruit on a few (3) representative trees, at or just before reference date, and, using those numbers, decide if you need to thin. Calculate how much fruit needs to come off if thinning is needed. Finally, shake if thinning is needed….