Mammillaria luethyi is an eye-catching small cactus with solitary or branched stems. It is one of the most spectacular of all Mammillaria s. The stems are very dark-green, almost black, rounded to flattened, and up to 0.8 inches (2 cm) in diameter. It is also commonly cultivated as an ornamental plant.
When you water, the tubercles will expand to allow for increased water storage. The flowers emerge from the axils of these tubercles on the previous year's growth, which accounts for their interesting halo effect. The cactus mustn't be exposed to prolonged dampness and sitting water. Never let your cactus sit in a dish of water.