What Maurice meant (I believe, since it would be my answer too) is leave the upper part of the scape above the prolif attached, as long as it is still green. While it is green it is photosynthesizing - remember that the plant's energy food comes from the green parts of the plant so by cutting it off you are potentially reducing its manufacturing capabilities.
I would lower the water level slightly for the prolif in your picture so that there is less of the prolif base submerged.
