There are a few confusing classifications about perennials. In researching their types, resources use terms like herb, herbaceous, woody, subshrub, and others that blur the lines between annuals, perennials and shrubs. For the perennials listed here, we simplify the classifications as perennials that are either evergreen or herbaceous. Evergreen perennials may or may not have woody older growth but maintain much of their integrity all year long. As examples, Dietes are evergreen and have no woody parts, whereas Romneyas may be woody at their bases but mostly have herbaceous newer growth. These woody plants may or may not benefit from pruning hard, to the ground even, and should be noted to understand how they will perform and be maintained in a design. By contrast, herbaceous perennials will assuredly have a seasonal dormancy, so for California this could mean a summer or winter slumber where they die back to the ground and reemerge the following season.
With the exception of hardy evergreen forms, most perennials can be expected to have a down time, meaning a season or several seasons when they are not looking their best. Designers often forget this, assuming their performance is unending. So, know before you specify, and make sure the landscape or garden (or client) is acceptable with the noted downtime between their showy displays.
Depending upon the perennial, they may be long or short-lived, and this should be identified as well. Agapanthus seems to go forever, whereas some Salvias might only last a few years. How embarrassing would it be to receive a call from a client saying all their plants died after being in the landscape for only five years? When specifying short-lived species, it is wise to provide clients with a succession or rotation plan, so they can be prepared for the decline and refresh the landscape.