^{(Photograph: Trichocereus pascana, Lleonora Enking)}

Introduction

As a genus, Trichocereus has a long and honorable history. The name was established in 1909. Later discussions (e.g., Britton and Rose, vol 2) refuted the idea of combining Trichocereus with Echinopsis. And the matter was settled for a number of years.

The Details

Unlike Trichocereus, many Echinopsis plants are generally short and globular rather than tall and clearly branched. However, when studied closely there are no clear and delineating differences between the flowers of the two. Similarly microscopic examination the seeds shows no constant way to separate one genus from the other. Thus, after debate and study, the two genera were combined. Because Echinopsis is the older name (established in 1837), it was kept rather than the other way around.

As a result T. pachanoi is now E. pachanoi, at least according to current thinking. Names could change again as new data emerges. In fact, there is no one to “enforce” the change. You are free use call E. pachanoi by the old name, T. pachanoi. No one will misunderstand you.

Additional reading: Molecular Phylogenetics of Echinopsis