Have: columnar cactus
Possibly Myrtillocactus. Individual stems are 2-3" in diameter with six shallow ribs. This cactus forms a clump 10-15' tall when planted in the ground, but are not hardy in Atlanta winters.
Mine are in a 5-gallon pot and grow over 6' tall, limited only by my ability to move their 80-pound weight inside in the winter. Flowers are 6", white, and night-blooming, although I have not been able to get this plant to bloom in Atlanta (five years).
I cut off two 3' long segments in the late fall. They will root readily in dry soil. Requires full sun, will thrive in intense heat and can grow 6-12" per year.
Other cacti, succulents.