I've had the same thing happen before. About the only thing you can do is to talk to the state and see if they will give you approval to clear enough vegetation to have a clear read of your sign. Some states actually allow this, and have a permit process to get a "vegetation easement". Otherwise, about the only thing you can do is to "rent" that face by putting up a "public service message" and write it off your taxes (seek the counsel of a CPA on this). That's what many of the big companies do in those cases.