Males were removed after they pollenated the females so there were nurts for the females to develope and finish.
At least thats what I understood from Jeffersons notes:
hemp. plough the ground for it early in the fall & very deep, if possible
plough it again in Feb. before you sow it, which should be in March.
a hand can tend 3. acres of hemp a year.
tolerable ground yields 500. lb to the acre. you may generally count on
100 lb for every foot the hemp is over 4. f. high.
a hand will break 60. or 70. lb a day, and even to 150. lb.
if it is divided with an overseer, divide it as it is prepared.
seed. to make hemp seed, make hills of the form & size of cucumber hills, from 4. to
6. f. apart, in proportion to the strength of the ground. prick about a dozen
seeds into each hill, in different parts of it. when they come up thin them
to two. as soon as the male plants have shed their farina, cut them up that
the whole nourishment may go to the female plants. every plant thus
ended will yield a quart of seed. a bushel of good brown seed is enough for an acre.
You really have to search and read, they don't make it easy:
Thomas Jefferson Papers : An Electronic Archive