Peach, nectarine, plum and apricot trees all do the same thing every spring: they set far more fruit than they can properly mature. A backyard tree given 4m of space and full sun, the standard planting requirement for all four, will often hold dozens of small fruit clustered along a single branch after flowering. Left alone, that whole cluster ripens into a heavy crop of small, poorly-flavoured fruit, and the sheer weight of it puts real strain on the branch carrying it. Thinning is the job of removing some of that young fruit by hand so the tree can put its energy into fewer, better ones.
Thinning is the job of removing some of that young fruit by hand so the tree can put its energy into fewer, better ones.
The case for thinning is partly about size and partly about the tree's own safety. A branch loaded with fruit it never should have kept can crack or break under the weight, and that risk gets worse as the fruit swells and ripens through summer, right when a snapped branch costs you the whole crop on it rather than just some of it. Crowded fruit also ripens touching each other, which is exactly the kind of damp, bruised contact point that lets brown rot take hold once wet weather arrives, a disease all four of these trees are prone to.
The timing window is short and specific: once the fruit has set and swollen to roughly the size of a marble or a 10-cent coin, about 2-3cm across, it's ready to thin. Depending on the variety and how far south you are, that usually falls somewhere between October and December. Thinning earlier in that window matters more than thinning later, because the tree only redirects the energy it would have spent on a removed fruit toward the ones left behind for however long remains until harvest. Wait until the fruit is nearly full size and thinning still reduces branch strain, but it does almost nothing for the size of what's left.
The actual technique is simple hand work: pinch or snip off fruit until what remains sits spaced roughly 10-15cm apart along the branch, favouring the largest, healthiest, best-positioned fruit and removing the smallest, misshapen or damaged ones first. Peach and nectarine, both usually self-fertile and prone to setting a genuinely heavy crop on a single tree, generally need the more aggressive thin of the two to get worthwhile size gains. Plum sets a naturally lighter crop to begin with, made lighter still by most varieties needing a second, compatible tree nearby to fruit well at all (Santa Rosa is the well-known self-fertile exception), so it usually needs less removed. Apricot crops most reliably in dry, settled-spring regions like Central Otago and Hawke's Bay in the first place, and thinning what does set still produces noticeably larger, better-quality fruit on top of whatever the season already allowed.
None of this is a one-off job. All four trees take 3-4 years from planting to their first proper crop, and once they're there, thinning becomes an annual spring task for the life of the tree, right alongside the winter pruning that shapes it and the copper sprays that keep peach leaf curl off peach and nectarine. If a heavily laden branch still looks at risk of breaking even after a reasonable thin, propping it with a stake or a forked branch underneath is a fair fallback for a grower reluctant to remove any more fruit than they already have. Done properly, thinning is the difference between a mature tree's full 15-40kg seasonal yield arriving as a pile of small, so-so fruit or as genuinely worthwhile eating.