The closest place, for me, to find a flower was still some distance within the Merrow Ruins. Try your best to clear the path between the Watcher's lair and the warp in Merrow Ruins, then look around on the rocks. All you have to do is locate one and take it with you back to the lair and drop it on top. The flower will pulse (with a signalling sound) and you should unlock the trophy within seconds.