This grower is sure of this plant identification.
This is a big agave in a long tom pot.
Century plant, Pulque, American Aloe, Mexican soap plant, Maguey
Agave americana (Century plant). Also known as Agave gracilispina, it will take this species about 10 years to bloom. The flower stems can reach a height of about 10m; they have creamy white to pale yellow flowers. After blooming the mother plant will die and send out new plants around its circumference.
It will prefer an average, well drained soil but will tolerate poor soil and drought.
Greenhouse, indoors, rock garden. The leaves were used medicinally by the native people of Southwestern USA, and this useful plant is also a modern source of steroids.