Question
I am trying to identify my cactus.
It grows rapidly.
I have read that the San Pedro is the fastest growing cactus that will grow a foot a year.
In the past four months mine has grown over 30 inches.  It now stands 54 inches tall.
It is columnar, has six sides on the bottom and about half way up changes to four sides.
Also, my cactus is not as rounded as a San Pedro.
Thanks for your help.
Manny

Answer
Dear Manny,

I got your pictures. What you have is a Cereus Repandus (Peruvianus). It is from South America and is quite common. The reason it went from 6 ribs to 4 and got thinner is from a lack of light. Most cacti need as much light as possible to grow properly. That means about 15 hours per day of ondoor light.

Do not put this plant in direct sunlight however, since it is not used to it, it will sunburn. Otherwise it looks very healthy.

Sincerely,

Greg