In message , Plum
writes
I believe there is a ruling or an old wives' tale that says you must not
pick and eat your rhubarb after a certain date during the summer. Anyone
know if this is the case, and if so why, and what the date is?

Reasons

1) If you keep picking rhubarb all summer you'll weaken the plant.
2) Rhubarb stalks (technically leaf petioles) get rather fibrous later
in the year (tho' I've heard that the 'Timperley Early' variety
doesn't).
3) It's alleged that the oxalic acid content of rhubarb increases later
in the year. (Oxalic acid is why you should eschew rhubarb leaves.)

Any specific date is arbitrary - the line would depend on variety,
locality, and that season's weather.
--
Stewart Robert Hinsley