Press gently with your whole hand: a ripe plum yields all over, not just at the tip. Plums do soften after picking, so a firm one is not a wasted buy, it just needs 2 to 4 days on the counter. What it will not do is get sweeter, so a rock-hard, pale plum picked green stays tart no matter how long you wait.
What to check
Looka dusty white bloom on the skin. That is natural and a sign it has not been over-handled, not dirt to be rubbed off.
Feelan even, slight give across the whole fruit. Soft only at the tip is a false positive, because stone fruit softens at the blossom end first.
Smella sweet, faintly floral scent at the stem end when it is ready.
Buying for whenred and black plums run through early summer. Italian and prune plums arrive late summer into fall, are noticeably sweeter, and are worth waiting for.
Common mistakeputting a hard plum in the fridge to keep it. This is the single most common way good stone fruit gets ruined at home. Held cold while it is still unripe, a plum or a peach suffers internal breakdown: it goes mealy and dry and browns around the pit, and it will never ripen properly afterward. Counter until it gives, fridge only after. The same logic applies in the store, so a dry, unrefrigerated stone fruit table is a good sign and a chilled case full of rock-hard plums is not.
If they are not perfect: a tart plum that has gone soft is never going to sweeten, so stop waiting and cook it. Roasted with a little sugar, or turned into compote, an underwhelming plum is better than a mediocre good one.
Written by the team behind RIPE, a free app that reads a photo of the whole produce pile and rings the ripest ones. Every cue above is one you can check with your own hands, and it all works with no app at all.