Mammillaria Spinosissima cactus is also known as Red-headed Irishman with cylindrical, dark green stem, up to 12 inches (30 cm) tall and up to 4 inches (10 cm) in diameter, almost hidden under a dense covering of spines. Flowers are purplish or deep-pink in spring. The Red-Headed Irishman will do best outdoors, though it may be kept indoors as well. Light/Soil/Water: - Bright light, full sun to partial shade. - Porous and well-drained cactus mix. - Water thoroughly when soil is dry to the touch, then let drain completely. Reduce watering in the winter.