The San Pedro cactus (Trichocereus pachanoi) is native to the Andes, but is now common in the southwestern United States. It is usually grown for ornamental purposes; however, like the peyote catcus, the San Pedro cactus is also grown for the ingredient mescaline, a controlled substance with psychoactive effects if consumed. In many parts of the world, including the United States, it is legal to grow a San Pedro cactus but illegal to extract the mescaline and consume, trade or sell it. The San Pedro cactus is among the fastest growing cacti and can be identified from other cacti by a few distinguishing features.
Monitor its growth. A San Pedro cactus can grow up to a foot a year and can reach up to 20 feet tall and 6 feet wide.
Look at its shape. A San Pedro cactus is a tubular cactus that grows vertically, usually without arms. More than one stem can grow from the same crown.
Count the ribs or columns that run vertically along the cactus. A San Pedro cactus has a total of 6 to 8 rounded ribs or columns with small spikes on top to keep away predators.
Observe its flowers, which bloom during the nighttime hours when it is cooler. San Pedro cacti grows white blooms, usually in July. They can grow to be nine inches in diameter.
Compare a picture of a San Pedro cactus with the cactus that is in question. Alternatively, take a picture of the cactus to your local nursery or county extension office for them to verify if it is a San Pedro cactus or not.