"Simon" wrote in message

This is my first season growing string beans. question: how do I know
when they are ready to be picked? Do they become too hard if I leave
them on the bush for too long?

Do they look like they are big enough and long and plump enough? it won't
hurt if you pick them a bit on the small side but they get coarse if they
get too old on the plant. Do they snap if you bend them because they
should. If you leave them too long you will be able to see fatter bits in
them which is the internal bean inside the pod developing so you should try
to pick them before this happens.