[Lesleyk]

A silly question, perhaps, by why shouldn't you pull rhubarb after July?  Is to do with retaining some nutrients for the winter, or does it become horribly toxic or something?  All the advice is not to pull it, but no one says why.....

[Yorkie]

The stems do have an increase in acid so are less pleasant to eat, plus they get more stringy.

But yes, the main reason is that you are allowing the plant to recover itself before the winter break.