st – an obsolete verb ending that goes with "thou."

(The old informal version of "you.")
Thou grow'st = you grow.

But Shakespeare would change this and use thou grow or growst depending if it fitted the metric for the rhythm of the prose.

Basically if you see an st - stop (st) and cut it off and it will make more sense.

i.e. thou ar'st = thou art - you are.

thou shalt'st = thou shalt - you shall.

ST = STOP CUT IT OFF.