Tell her to please just wait and see if you end up smoking more and ruining your life. Tell her to give you 3 months after your first harvest... and then talk to you.
You need to make sure that you don't start smoking more though (that is, if you want the relationship to work). If the reason you're growing is so that you can smoke more... then maybe she's just not right for you.
That's my advice. I'm an idiot though, so don't listen to me. My own wife doesn't even know I smoke, much less grow. I started smoking when we went through a tough time and almost got a divorce, and I just never told her. Then I decided to grow for fun.
I'm probably going to tell her in the next few weeks (that I smoke) and see how she takes it. I think she's going to leave my ass.