In nature, things happen when it is their time. Babies come when they're ready, flowers bloom at their leisure, and peaches need thinning when they are marble sized. (Side note: With modern technology, all these can be hurried and disrupted.... usually to our detriment.)
So, it's peach-thinnin' time. We have two peach trees, a J.H. Hale and a Hale Haven, planted about 20 feet apart in our front yard. In the years since they began bearing, they have never failed to deliver a bounteous crop of lusciousness, albeit slightly wormy lusciousness that needs to be cut open instead of eaten whole. This year, as always, the branches are loaded with tiny green fuzzy baby peaches.
60 - 70% of these helpless fuzzies must be eliminated before they grow large enough to crack the branches. Thinning the peaches also lets the others develop to their full size. In the past, I've put off the thinning, or pursued it lightly, or didn't thin the upper branches enough because I didn't want to climb a ladder, and later lived to regret it. Last year, there were several emergency thinning sessions in late May when I noticed the branches literally curved in half. It's soooo much easier to thin baby peaches than adolescent ones. For one thing, they are lighter. For another, they take up less space in the compost pile (which is overflowing, and I haven't started a new one).
The key word is.... ruthless. Follow this rule: thin to one peach every six inches. A slight twist of the peach is better than a yank, which can result in an entire twig ending up in your regretful hand. And remember, peach thinning starts with good pruning.
Pruning should be done several months earlier, in the early spring / late winter, to open up the inside area of the tree to allow easy thinning and harvesting, and allow good air circulation to prevent disease. You DON'T want twigs poking you in the eye while you're harvesting, and you don't want to spend half an hour thinning peaches off branches that shouldn't even be there!
It takes some time to do it right, but on a beautiful spring day out in the fresh air, peach-thinning is actually quite pleasant... if you don't do it so long that your trapezius cramps up and your neck develops a twitch. It's even pleasanter if you remember to put suntan lotion on so you don't get a wicked sunburn, and if you have good company to chat with and some music playing. Add some birds tweeting and it's positively idyllic.
I recommend spreading the thinning out over several days. Most people aren't used to looking up and reaching up for hours at a time. For me, a relatively slow and judicious picker, it probably takes about two hours a tree, considering the time it takes to move the ladder around our fifteen-foot high tree. I spread that out over four or five days, and I'm RUTHLESS so I won't have to do it all again in late May when the peaches start to gain their teen weight.
Now, this only reflects my experience, with these particular peach trees, in this climate. Other people may do it differently, but this system works for me and my trees. How about you?