If the petals are roughly the same color you can select by color range (Select > Color Range...). Use the add to selection tool (the eyedropper with the plus next to it) and adjust the fuzziness slider to get more or less the area you want.
There have been many times, though, when there's been no way to select what I want to except by using the polygonal lasso and just going around the area, one click at a time. It takes a while, especially when the area is complex, but sometimes there's just no other way.
Oh, one more thing: You do know that when using any of the selection tools (marquee, wand, lasso), you can add to the selection by holding down shift and subtract from it by holding down Option (Alt on a PC).