Leuchtenbergia, also called agave cactus, cob cactus, or prism cactus, is a genus of cactus consisting of a single species: Leuchtenbergia principis. It is endemic to central Mexico. L. principis grows slowly and can reach a height of 2.25 feet (70 cm). The mature roots are 8 to 10 inches (20 to 25 cm) long and 3 inches (8 cm) wide. Yellow flowers and small fruits grow in the middle of the mass of tubercles, which have long spines at the tips.
This solitary plant is called agave cactus because the rosette pattern of its tubercles resemble that of the agave plant. The species was originally found in the desert covering the states of San Luis Potosi and Chihuahua in north-central Mexico, but it has been widely cultivated since the middle of the 19th century. It was named after the duke of Leuchtenberg, Maximilian Eugen Joseph, an amateur botanist.
Leuchtenbergia principis grows slowly from seeds that develop a large tap root. This plant has slender, three-sided, blue-green tubercles that emanate from the tuberous roots. The elongated tubercles can grow to a length of up to 6 inches (15 cm) long and 0.75 inches (2 cm) wide. Tubercles at the sides and base of Leuchtenbergia dry up and fall off as the plant grows in height.
At the tip of each blue-green tubercle are eight to 14 papery spines that can grow 8 inches (20 cm) long. There are one to two central spines, and the rest are peripheral. The long tubercles and gold spines are the distinctive features of this cactus plant.
It takes five years for Leuchtenbergia principis to grow its first flowers. The flowers bloom in late spring to early summer thereafter. They grow from the tips of the youngest tubercles, at the center of the plant. These flowers are fragrant, funnel-shaped, and bright yellow in color. They can grow as wide as 4 inches (10 cm) in diameter.
After blooming season, which lasts approximately two months, the flowers produce a smooth, green fruit. The fruit can grow as large as 1 inch (3 cm) long and 0.8 inch (2 cm) thick. After ripening, this fruit opens up at the bottom and releases hundreds of seeds.
Leuchtenbergia principis grows best in either full sun or partial shade. It needs to be planted in well-drained soil using fertilizers that are low in nitrogen, which inhibits its growth. This plant needs minimal watering, especially during the winter months.