They are not renewable naturally. They spawn with the initial world generation and with bonemeal only.
I have to say in my experience this isn't true, I farmed roses until there weren't anymore, went back a few weeks later in game time and found some more in places that were previously barren. --- A Troll is a Troll, unless it's an ugly Giant. ~Troll Cannibalism 101~
You can make a small structure say 7x7 and 2 high (completely dark) and start through down bonemeal and you'll get loads of flowers and seeds. Just a small little trick to help you get your items faster.