Cleistocactus smaragdiflorus is a a much decorative hardy cactus.
Soil: Grow it in a rich and particularly draining substratum, as it is very sensitive to rottenness when in presence of humidity and low temperatures and let the soil dry out between waterings.
Water: In summer, during the vegetative period, it must be regularly watered, but allowing the substratum to completely dry up before irrigating again; in winter, it’s to be kept dry. Preferable not to water on overcast days, humid days or cold winter days.
Hardiness: This is one of hardiest of the woolly columnar cacti from Bolivia that grow at an altitude of up to 1000 meters. It is a frost resistant cactus, hardy to -7°(or even less if very dry). However in cultivation it is better not to expose it to temperatures lower than 0° C, even if in an aerated and protected location, in order to avoid the formation of anti-aesthetic spots on the epidermis. In presence of high atmospheric humidity avoid any frost as it is particularly sensitive to root rot. 
Exposure: It need full sun exposures with ample airflow to produce dense hairs, but, as a former mountain dweller, does not care for extremely high temperatures in summer. 
Maintenance: Repot in the spring, when their roots become cramped. Generally, they should be repotted every other year in order to provide fresh soil. After repotting, do not water for a week or more.