But why isn't basil as invasive as spearmint? Cuz I'd be okay with that.

Barbara Damrosch lives (I think) in Maine, but submits regular articles to The Washington Post's much-shrunk Home and Garden section.  This week, she has an interesting little column about the mint family.

I did not know that all my favorite herbs are relatives. (Although I can believe it with my unstopable oregano.  Are you trying to grow herbs for the first time?  Try mint or oregano.)  But then why does my basil not come back year after year and spread determinedly around the garden like my spearmint?  C'mon, basil, step up!