Re: 10-12" of branch space per peach.
Scott is correct that most commercial peach experts recommend tighter spacing than that.
However, there are a few things to note. Peach experts are really concerned with yield/acre on peach orchards. This is also in line with most commercial peach orchardists goals. Like almost all agriculture, greater yields = greater revenue, which almost always helps the bottom line which = staying in business.
The other caveat is that those tighter spacings of peach per branch space are generally assumed only the best wood is kept. In other words, all weak short shoots are removed, and trees are aggressively pruned, and trees are irrigated in dry climates (or dry weather) so that size doesn’t suffer from heavy cropping. This is almost never the case with backyard orchards.
What happens in most backyard orchards is that a person reads about a tight branch spacing for fruit, ends up leaving a horrendous amount of peaches on their tree, is rewarded with small, undelicious fruit. Scott has been doing this long enough, even if he starts out at 4" spacing per fruit, he knows how much fruit a certain size peach tree can handle when the fruit is mature. This is hard to communicate in written prose.
Let me just summarize that for good sized tasty fruit, it’s very difficult to overthin fruit. By thinning hard and early, the fewer fruit left on the tree, get that much bigger, and taste much better with higher sweetness. Most commercial growers get paid for poundage, not taste, so lean more towards max number of fruit per branch space, or per tree. I do it different than most commercial peach orchards. Most of my business is repeat. Repeat customers come back for taste, so that plays very heavily in our goals. Plus I like selling fruit which tastes good. So we thin much harder.
We more or less thin to 10-12", but we tend to round up. In other words, if someone new is just starting out thinning peaches on the trees, I cut a 12" stick. I tell them that if the shoot they are thinning is less than 12" (we’ve already pruned off little unproductive shoots of less than 8" at this point) then allow 1 peach. If the shoot is longer than the 12" stick, but shorter than 24" (two sticks worth) then thin to two peaches, etc. So the “rule” is one peach per 12" stick, but in practicality it probably averages to one peach every 10" for globose peaches (flat peaches have tighter spacing). I hope that makes sense.
However, we look at the trees after we are done thinning to see if we’ve left too much on them. A full sized tree (and I’m talking big peach trees) do best with 250 peaches per tree. For sure 300 peaches per tree on a large tree is the very max, before size and quality suffer. With 250 peaches per tree, they will generally produce 1/2 pound peaches which taste good.
Wardog, your peach trees look like about typical for 3rd year trees, except they look more or less unpruned, as there is no height control. I would expect those trees to produce about 100 good tasting peaches/tree max. Probably less than that, given the amount of foliage I see. Note that younger peach trees produce smaller fruit in general, but if thinned appropriately, the smaller fruit will still taste good.
Also all my comments are predicated on a good spray program, so that once the peaches are thinned, you don’t lose a bunch to insect damage or rot. Scott mentioned he leaves more fruit on at first, and thins accordingly as damage occurs. That is a good cultural practice for a light-handed spray program.
You don’t want to thin to 100 peaches per tree on your trees, only to lose 75 more peaches from insect damage. That would be too light a crop for your trees. My comments are based on a spray program which minimizes insect damage.
I hope I have successfully communicated how difficult the thinning guidelines can be, given all the various variables. However, please keep in mind, most new growers under thin, and under thin by a large degree. As mentioned, it is a psychological battle of the mind, in most cases. Pruning peaches is the same way.