The unconcealed passion the Sergeant Cuff had for the rosery garden initially impressed Betteredge as "trumpery", and evidently inappropriate of a man of his profession. Curiously enough, further reading reveals that whenever the detective was onto some clues, he would stare out the window, and whistle the song "The Last Rose of Summer" to himself. The Sergeant's affection for roses here not only did not impede his "horrid clearness", but symbolized his tenderness for human infirmity, from my perspective. Just like the roses, human nature is lovable and fragile, but with thorns that occasionally stings. As a detective, Cuff didn't have that much opportunity to execute his tenderness, maybe the roses is a vent for this.