peachykeen said:god no, not a carnation, she'll think you're cheap! (Unless they are not so common where you are; in the US they are like a cheap filler ingredient.) A nice calla lily might be good though.
I'm with Peachykeen on this one. I've had bad luck with carnations; they never seem to send a good message. A nice alternative might be Lust Engine's idea of a mixed bouquet, though that should probably wait until a later date. (It'd make a great gift if you're invited to her place for dinner though!)
When I'm in doubt about something, I always try to consult an expert, so here's a radical thought: Ask the florist! Explain the situation and seek advice. Then follow it. (That's the point of consulting an EXPERT rather than the average person.) You should definitely set some parameters-- ie cost, no carnations. I wouldn't even mention the rose idea to the florist initially. Get their impression first, THEN perhaps ask about a (colored?) rose.
And definitely keep LE's comment about the cost of roses at Valentine's in mind. I've already told my gf not to expect them; I love her enough to spend my money on her more efficiently!
Good luck!
