I assume you are using Helicon Focus for the stacking.
The effect around the edge of the petals is due to the settings on Helicon:
Try all three methods A, B and C with the same settings as above. The difference in the output is huge. This flower is a complex shape with lots of edges that would probably benefit most from Method "C" and a bit of tweeking the Radius and Smoothing settings.
If my assumption is wrong and this is what you want then please ignore all of the above.