Well, there are also flowers that bloom at night. I suspect that they aren't in the Garden of the Sun though. Maybe Yuuka has a Garden of the Moon?
Even so...There are many flowers who will close once night draws near, but this is an ability they have acquired in order to live.Light and darkness are merely two sides of the same coin.The flowers all understand that there's nothing wrong with evening arriving....please don't misunderstand.The flowers don't dislike darkness.