I've had lots of lawyers. Ideally it's best to have one on retainer at all times regardless of you're growing/breaking the law or not.
You don't need to tell them you are growing, I'd advise against it. If they ask why you want to retain a lawyer tell them that you want to be protected in the event something were to happen. (you can cite being threatened with arrest when you recently encountered a cop during a BS traffic stop and you questioned it -- make something up that's plausible) Wealthy people always have lawyers on call, why shouldn't you? Are you not entitled to the same "justice" that people of means enjoy?
The best thing about having a lawyer retained is whenever something happens you don't need to scramble to get one from jail and just having a lawyer in the first place will keep you out of a lot of shit. If someone that you're not comfortable talking with is asking you questions you can just pull out your lawyer's card and say "I don't make any comments about anything without my attorney's guidance". That shuts down a lot of shit on the spot. Sometimes someone will challenge you and ask who your lawyer is and if you don't really have one they will know.
The short answer, yes retain one (put money down this is very important to create a contract or you can be left in the cold if he's busy when you need him), don't need to tell them what you're doing. If you're ever hassled, just say "I have no comment without my attorneys advice".