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 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, follow the steps in the calculator below. The calculator will perform the math and aid in your decision of whether and how much to thin.
Cells highlighted in yellow are editable and can be filled with your orchard specific data.