We seem to have a good crop of plums on our little tree this year. But half of them are on the ground already (must remember to pick them up before DD eats them...), and those that are on the tree don't seem ripe yet. From which I conclude that, as with so much in life, there must be an optimal 5 second period between hard and squishy. Can I pick the unripe ones off the tree, and leave them to ripen inside? Would that work?
A very unscientific way of telling is to shake the tree gently and see which ones fall off - if they're ripe they should be easily plucked and fall off easily. But I don't see why you couldn't pick them a bit early - they'll ripen quickly if you put them in a fruit bowl with some bananas.
And don't forget to make jam with all the squishy ones! Yummy