Hi Mat,
Is the original capture RAW or jpg?
It looks to be about a stop and a half under exposed, probably because the metering (and you) did not allow for the subject matter being white and the camera exposed it to be grey
I notice from the EXIF: Canon 5D Mk.II at 28mm, iso 100, 1/25s at f/14
I don't believe the subject needed f/14's worth of DoF, but if there was a breeze, it probably did need a faster shutter speed than 1/25s to stop the leaves from blurring and losing you fine detail - if this was combined with a jpg capture, then you're starting from a less than ideal place to achieve what you wanted.
You could try
Local Contrast Enhancement (LCE) on it, but you really need to start from a properly exposed image.
If you can answer John's and my questions, we'll see what's possible.
Cheers,