In Chapter 32 of Great Expectations, Pip unexpectedly encounters Wemmick in London, and, as he has time to spare, he accompanies Wemmick to Newgate Prison, that notoriously harsh prison of history. As they traverse the grounds and Wemmick walks among the prisoners, Pip notes that Wemmick winds among them much as a gardener among his plants. The subordinate of Mr. Jaggers talks to the plants that he sees,
What Captain Tom? Are you there? Ah, indeed!... Is that Black Bill behind the cistern?
Indeed, the greenhouse is an odd metaphor--almost an oxymoron--for the prison, for life ends at Newgate while life begins in a greenhouse. Nevertheless, when he weaves among them and stops to talk to the prisonesr, Wemmick's mien is much the same, even when he speaks to Colonel who is to executed the next day.
Yet, this manner is in character with Wemmick who is a man of business like his employer. His "post office" mouth, as Pip describes it, merely receives and delivers information without any personal involvement. Keeping his personal life--in which he exhibits great warmth and love--separate from the business world, Wemmick perceives all things equally and dispassionately while he works.
Remember that Wemmick is the clerk for Mr. Jaggers, the lawyer. This can tell you why he would treat the prison as a greenhouse.
So the answer to the first question is that Newgate Prison is Wemmick's greenhouse.
This is an odd metaphor because a greenhouse is a place where things grow. You would not expect to use this metaphor for a prison. Maybe for a school, where kids learn and grow, but not a prison.
But Wemmick works for a lawyer. And he likes to get "portable property" from the criminals. So if you work for a guy who defends criminals, it makes sense to go to the prison and "cultivate" the prisoners so they will hire you again sometime when they need a lawyer.