Carla:

I always thin early and have never overthinned as a result. It is the earliest maturing varieties that need to be thinned first, as early as possible. In fact, research shows that the early maturing cultivars would benefit from bloosom thinning. But even I don't do that. Thinning at penny size is very good. Fruit that large will mostly stay on the tree. Like I say, I never overthin by being early. Usually I have to thin at least twice to get enough fruit off the tree. Sure, if you waited until pit hardening, it would be easier to get the job done in one thinning. But it will cost you fruit size esp on the early maturing fruit if you wait that long.

The Fruitnut

I can imagine blossom thinning for peaches and nectarines. I have a couple of varieties which set ridiculous amounts of fruit. I would have a hard time doing blossom thinning on early plums and Pluots, which don't attract pollinating insects as readily, or on apricots, where blossom disease can take out promising fruitlets.

But I also start to thin early when I am paying attention. It's easier on me psychologically, I think, not to do a drastic thinning all at once. And I can take out all the misshapened or doubled fruit very early. I sometimes use scissors to thin out clusters of tiny plum and Pluot fruitlets, so I don't pull off the whole cluster. I also have a problem with birds chomping the tops off nectarine and apricot fruitlets when they are really tiny, so my eary thinnings leave a few extra to compensate.

I always miss a few branches on the first and second thinnings, and have to go over the tree carefully when the fruitlets get to about penny sized.