That’s a great picture! I think level to the ground works good. The slant of the tulips is not extreme and it looks natural. If you wanted to focus more precisely on zooming in for just a shot of the tulips, then perhaps keeping the stems straight would be more important, but this composition is more of a landscape shot, and the natural slant of the tulip stems looks good. If you tried to straighten them, I think the whole picture would look a little slanted. The aperture value depends on what you are trying to accomplish. If you wanted the background to be blurred to draw more attention to the tulips, then a wider aperture would have been better. Personally, I like it the way you have it because I think the detail in the background adds some interest to the scene. As far as composition is concerned, It would have been nice to either eliminate the closeup flower in the middle or else include it in a more purposeful way.
