Etymology -From the Greekechinos- spiny and opsis - appearance - hence spiny apperance.
The only cactus genus that is more confusing than Echinopsis is that of Opuntia. In both cases, there is a great number of species (over 100) and a tremendous amount of variation. In Echinopsis plants range from very small, flattened-globose plants to quite large, treelike giants. As a result, there is a long list of synonymous names for many of the species. Some synonyms referring to other synonyms that refer to a subspecies of some seemingly distinct species. Sorting through these names often feels like a wild goose chase and is quite frustrating.
In more recent thinking, the previous two genera of Trichocereus and Lobivia are included with Echinopsis. However, it is not at all uncommon for enthusiasts to use all three names in discussion even if their labels read Echinopsis. This usage reflects the general (inexact) situation that the larger, columnar members are distinguished as Trichocereus while Lobivia includes a select group of smaller, not-as-spiny plants which typically flower from low on the plant similar to most Rebutia species. This leaves the bulk of plants referred to as Echinopsis to be mostly spiny, ribbed, globose plants.
The main factor that ties these plants together are their very large, showy flowers. These flowers are all very similar in structure – funnel shaped, with hairy/wooly scaled floral tubes which give rise to hairy, globular fruit filled with a soft, mushy pulp. The flowers seldom last more than a single day and may be diurnal or nocturnal depending on the species. These species hybridize easily and have resulted in a tremendous number of hybrids that some cactus growers specialize in or grow exclusively. There are certainly enough hybrids to keep even ardent hobbyists busy.
Because of their exceptional flowers, many Echinopsis species are found in garden centers and collections world-wide. The larger species (aka Trichocereus) are also popular landscape plants in warmer parts of the world. While there are a number of species common in cultivation, there are as many or more unknown hybrids in the trade. These hybrids are easy-to-grow and produce nice flowers, but buyer beware if exact names are desired. Plants of this genus are widespread throughout South America and inhabit a wide range of habitats and climates.