Nice shot
I agree with you on the headroom, but all in all I like this. The complimentary and analogous colors are easy on the eyes. The diagonal line created by the stem pulls the eye right back to the focus point. To often there's an attempt to isolate the flower completely from it's surrounding, but I think the soft focus on the green still gives you the sense of it's belonging, while not overly competitive for attention.
My comment is as much a question as it is a critique; am I seeing some left-right motion blur (looking at full size in Flickr), or is that some kind of lens artifact? It's most noticeable to me at the white edge of the petal where the stem comes in, and the end of the right-most petal. I see a 1/5 shutter speed, so I'm guessing you used a tripod which gives me doubt that it's motion blur.
Smoke offered a pretty good C&C of it. I might see if a tiny,mtiny bit more saturation could be applied. If the headroom is an issue, I suppose the canvas could be enlarged and then cloned it.