Much of this posting has to do with
1. The light falling on your subject (in this case the flower) and the amount of light falling on the background (which in your case is in shadow)
2. Another facet which impacts your image is that your flower is white and therefore reflects a great amount of light while the background is green and reflects less light.
The way to control this is to "look at and see" the background and how that background will impact your image. IMO, two areas in which accomplished photographers often produce better imagery than less accomplished photographers are:
Selection and control of background AND selection and control of the edges of the image - eliminating portions of people and stray branches, etc., intruding in from the sides of an image...
When your image (in this case this agapantha) is lit by approximately the same type of light as the background and the brightness of the subject is about the same as the background, you do not have the darkened background contrasted with the very bright flower...
However, you can, in PP reduce the brightness of the background by several techniques, including vignetting, which provides a dark BG to isolate the subject...
Here's a totally different question... Why in the name of Ansel Adams
would you select an ISO of 10000 for that shot?
If the EXIF data that you posted is correct, and if the white flower was in sunlight, an exposure of 1/400 @ f/16 is not correct...