Ouch. You've uncovered a rather knotty problem. Did you shoot and try to process a raw file or did you shoot a camera JPEG?
Highly saturated colors, especially in the red range, are difficult. On board camera processors have an especially hard time with them but raw conversion software also struggles. Red flowers are the perfect example. Do a Google search on "red knockout rose" and you can look at a few thousand -- many thousands, of digital photos with nuked red channels. Or better yet search for your scarlet lychnis. Here's the histogram of the red channel from your photo:
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You can't get detail in the flower petals until you get that red channel histogram off that right wall.
If you're shooting camera JPEGs you don't have a lot of options.
1. Set a custom white balance -- really helps to have the WB right in camera in this case.
2. You're going to be forced to drop exposure -- minus EC.
3. Consider the picture controls for contrast and saturation and lower them.
4. Problem with steps two and three is that those changes are going to effect the whole photo including all non-red data.
Although this goes against all that I know is holy in photography you do have petal detail in both the green and blue channels and it will help to edit the JPEG and pull back that red channel with a saturation mask which will expose the detail in the other two channels. Like so:
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Best solution for this kind of problem is to start with a raw file. The JPEG options are both half-baked: bad idea to underexpose and bad idea to try and repair a JPEG.
If I may offer an illustration:
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I've yet to see a digital camera that doesn't do this. On left you see the JPEG from my camera (I expose for my sensor not the camera JPEG so my JPEGs tend to look even worse -- red channel blown away). On right the processed raw file. If ever there was an argument for why you need to get a raw file, it's a red rose.
Joe