How to Thin Peaches
When and how to thin peaches depends upon variety and time of maturity. Thin the early-ripening varieties earlier and more heavily than the late-maturing varieties. For early varieties, peaches grow rapidly at first, then seem to cease growing for about two weeks. For late-maturing varieties, they seem to cease growing for about four weeks. This is called "pit hardening" and occurs while the seeds are maturing. Thin early-maturing varieties at any time from full bloom to three weeks later. You want to thin to space fruits six-to-eight inches apart.