Peach thinning and tree physiology

In the midst of thinning peaches, I once again wish I had taken notes when harvesting last summer, so I would better understand where the biggest peaches grow. Absent that, it would be nice to know the theory behind the 6" rule -- that peaches should end up about that far apart, or more. When thinning I start with the obvious criteria -- peaches too small, or deformed or buggy, or so close they will touch when full size, or in a spot where they will grow into a branch -- those come off first. I am also not a believer in late bloomers when it comes to peaches; if most of the peaches are the size of walnuts, that one the size of a grape is never going to catch up, so it comes off even if it has a whole branch to itself. But I don't know if that is a mistake.

Then I start to apply the spacing rule, with an eye also on the weight the branch will be able to support. And I assume the 6" rule is not meant to be taken rigidly. If you have a 12" branch with two nice large peaches on it 4" apart, leave them alone. I assume the rule is about the supply of resources to the growing fruit, so I look at all the foliage available and use that in part as a guide.

But questions still abound. Is the whole branch supporting all the peaches on the branch? Will the peach at the end of an 8" twig be more likely to get large than one at the base if they are the same size now? When applying the rule, what counts as a branch? Is an 8" twig its own branch, or does the peach on it count against the one on the larger branch 2" along? What about all those 2" twigs? So what I am looking for is what determines peach size and how that should guide thinning.

Turkey Creek Orchard
Solon, Iowa (zone 5A)

I think about this subject too, and was looking at some peaches that I thinned over a month ago now. I discovered small ones that I hadn't really noticed when I first thinned, and they made the branches seem over-crowded now. So I did more thinning today. Also even though I left what seemed like good spacing on some branches, there were so many fruit on a relatively slender branch that the weight was already stressing the branch a lot. I never heard of anyone putting props on peaches, but it seemed like some branches might break so I thinned them even more.

If I tried taking notes I would probably not be able to find them by next season so the next best thing seems to be going back to check the progress of the places on the trees you had questions about to see how they develop over the season. Then if you end up with too many small peaches, the lesson is learned for next year.

Fruitilicious Farm
Zone 9b in California