When to harvest leaves off young plants for tea?

I have alot of young perrenials such as persimmon and sea buckthorn and I'd like to take all their leaves to make tea before winter, but I'm afraid if I just go taking their leaves too soon it will be shocked and die.

My hope is that I can collect all the leaves off the plants right before the plant decides to go into dormancy and that won't hurt the plant much. Would that work?

I'm not sure what the result would be of taking all of the leaves right before winter dormancy.  I would suspect that it could hurt the plant, as before they drop the leaves the plants withdraw large quantities of nutrients from those leaves.  The traditional rule of harvesting is not to take more than a third of any plant's leaves.  I would be interested to know if anyone had in fact tried this, and what their experience was.  It may be worth a try as an experiment, if you don't mind risking loss of the plant.

Also, mostly as a point of interest, it is said that the leaves are most potent in the mid- to late-morning just after the dew has evaporated but before the sun has become too strong, and at the point in the season when the plant is just about to produce flowers, but hasn't yet done so.  That is when the energy is in the leaves, when the plant focuses it's primary energy usage on photosynthesis, usually in the spring or early summer.  This time of the year, October, the energy is retreating into the roots, leaving the leaves with less potency.  Just how much of a difference that makes in practice, I have no idea, I harvest at all times of the day and year when I need to and get results, but if I was to make a single large, organized harvest each year, I would certainly make an effort to go by these guidelines.

You pick leaves around midsummer,  first half of June.  Blossoms when they are open, but not quite.  This time of year, the leaves are not potent anymore.  My advice would be to let them be this year.