Outline of a selection

[PixLab]

If I may, another way to do it
Drag and drop the red channel on the canvas
filters > Levels to give a lot of contrast

   

Select the black part > Select > remove holes > fill with black, then select invert > Fill with white
Then duplucate 3 or 4 times the flower layer, on each delete the flower, and move them / transform / flip them horizontally/vertically to remove the holes, then merge them all and blur, and on the top flower layer apply the mask you did created