Green flowers: six images, scroll down to see them all.
All images here J. McKenney
This is Ruscus hypoglossum; the flowers are about a quarter of an inch across:
This is Helleborus multifidus, a species grown as much for its handsomely divided foliage as for its green flowers:
This is Arum hygrophilum. The true flowers are hidden down in the spathe; I don't know what color they are (yet!):
This is Iris tuberosa (aka Hermodactylus tuberosus):
This is Freesia viridis:
This is Hacquetia epipactis .The flowers are yellow, but the bracts are green:
