I haven't seen specific info yet. Your best bet is to download a starter package and run the game locally. I've only played with it a little and my impression

I haven't seen specific info yet. Your best bet is to download a starter package and run the game locally. I've only played with it a little and my impression is that growth rate is an integer that's added to the planet's population every turn, but only occupied planets not neutral ones. (So a planet of yours with a population of 138 and a growth rate of 5 will have a population of 143 next turn (assuming no increase or decrease from friendly or enemy ships).)