The Pencil Cactus (Euphorbia tirucalli) — also commonly called milk bush due to its thick, white sap — is native to semi-arid tropical regions of Africa and India. In the wild, the Pencil Cactus can grow up to 30 feet tall and well over 6 feet indoors. This stem succulent is not a true cactus and photosynthesizes in its stems, not through the small leaves that appear at the end of its new growth (which are inconsequential to the plant’s health).
As these plants evolved in dry climates with lots of sun, keep your Pencil Cactus in a bright location for maximum growth. This plant will not tolerate low light conditions. Direct sun, or plenty of bright ambient light from a western, eastern, or southern facing window is ideal for this plant to thrive.