Different plants make male/female flowers at different times. This is to prevent the plant from pollinating itself.

I use anime-planet, just because it was the first thing I found back in the day and I'm not switching now. The scoring systems is a bit limited, using the five stars (essentially just 1-10). I do like the simple design, makes it very clear where everything is, and your profile has a nice big overview of how much of your life you have wasted :D