Criteria used to identify a species of cactus depend on the approach, with one approach using a process of elimination that focuses on criteria including the habit, flower color, encounterability and habitat, while another approach focuses more on physical characteristics, including spine distribution, fruit and flower color, trunk features and spine characteristics. CactiGuide.com features an online cactus identification tool exemplifying the former method, while DiscoverLife.org features a tool exemplifying the latter method.
CactiGuide.com features a tool for identifying a cactus species by a process of elimination. First, select the overall growth form of an adult specimen, or habit, such as sprawling, shrubby, cylindrical or ovoid, then select the cactus' flower color, geographical distribution and encounterability, such as rare, extremely rare or common worldwide. The search results include pictures of known specimens for comparison with the specimen.
In some cases, this process of elimination may be ineffective, such as when the geographical distribution is unknown or a specimen is not flowering. In these cases, it is more effective to identify a cactus by its physical characteristics. For example, DiscoverLife.org features a tool allowing users to identify a species of Opuntia, a genus of the Cactaceae family, by specifying its physical characteristics and geographic location. This tool places some emphasis on the characteristics of the cactus' spines, such as their number per areole, color and length.