“All the peaches I bought at the supermarket have split pits. Why does that happen?”
According to gardeningknowhow.com, split pit disorder occurs when the pit of the fruit begins to harden, about forty days after the tree blooms. As the pit hardens, the fruit flesh will cling tightly to it. If the fruit begins to swell at the same time, the pressure on the pit could cause it to split, or even to shatter. Pit split can occur in peaches, nectarines, cherries, and plums.
In her paper “Split Pit,” written for the University of George Department of Agriculture, Kathryn C. Taylor puts forth a few hypotheses as to what causes pit split. When peaches are thinned too much, whether by the farmer or through a frost, the pits are more apt to split or shatter. Too much water (again, whether on purpose of through an act of nature) or too much fertilizing too late in the season can also be a factor.