This wonderful exemplar of cactus has a cylindrical stem of opaque light green color (almost blue) covered by a whitish plum. P. sp is equipped with numerous well marked ribs and tubercles above which are the white and fluffy areoles. From the latter come out its radial and central spines: the first are white, thin and short while the central ones are longer, more robust and tend to become brown. Its large flowers bloom at the apex of the plant.