Flowers are very powerful and loaded with meaning. They send the same message as a gift - "I'm interested." You need to be sure they will be favorably accepted before you send them. Roses are the most powerful flowers you can send. A single red rose traditionally means, "I love you." A dozen long-stemmed American Beauties (they're dark red)? Heavy artillery. Start with a small bouquet of fresh flowers. Cost? About fifteen dollars or so. This should be saved until you are convinced that there is something going on. A nice touch is to send them to her workplace. The other girls will make a big fuss over her. Of course, it could backfire if you misread your status. She will have to explain why this guy she just met is coming on so strong, so soon - bad news for you.
Use flowers as an encouraging sign of interest after a few dates. Send them as soon as possible after seeing her, if you're going to do it. Include a simple, light note. Don't use familiar affectionate terms like "Love" just yet. The term is loaded with meaning. Something like "Thanks for a memorable evening" is just fine. Don't overwhelm her with something after every date. Save them for recognition of something special. Flowers need to be unexpected to maximize their effect. Don't mention them after sending them. You are fishing for compliments or reactions - bad form. If she wants to say something, she will. (She probably will.) Don't worry if she doesn't.