If you can, add a light on the left at the same level as, and pointed directly at, the arrangement. Just make sure it is not as bright as the main light. This will make the dark green on the left a lighter, more visible green like the right and should eliminate the shadow on the face of the lower left rose.
Also, consider moving the iris from in front of and behind the roses so you can see them. Sometimes you can change the perspective of the image by simply turning the arrangement an inch or or so left or right or raising or lowering the camera.
When you look through the camera at your subject, look at the flowers first to make sure that each flower is as visible as you want it to be. Then check the technical aspects of the photo (lighting, position, etc.).
If you don't have another light, you could reduce the light on the right a little and increase exposure time or open the aperture to achieve a similar effect (if you are using manual controls on the camera)
There is more to this, but it is all about experimentation.
Hope this helps,
Jason
