A guide to life cycle terminology and some nursery industry quirks

A basic glossary, to begin with---

Annual: a plant that lives for only one season, usually deteriorating rapidly after flowering and going to seed.
Biennial: a plant that lives for two seasons, usually producing only a circle (“rosette”) of basal leaves the first year, followed the second year by a flowering stem.  After blooming and setting seed, the plant dies.
Perennial: a plant that lives for more than two seasons.  This technically includes all trees, shrubs and ferns as well as many grasses and all bulbs.  However, in garden centers, the "perennials" section usually contains only the smaller, non-woody plants.

Herbaceous: generally applied to perennials; an herbaceous perennial is one that does not have woody stems.

Evergreen: often a source of confusion to neophyte gardeners, this does not refer to conifers particularly, but to any and all plants that retain their leaves through the winter, including some short perennials grown for their flowers (Thrift, Hellebore) and many broadleaf shrubs (Rhododendron) and trees (Madrone). 
Deciduous: losing leaves in winter.  Refers not only to trees like maples, but to many shrubs and most herbaceous perennials. 

And now for the quirks:

Many plants sold in nurseries are routinely labeled inaccurately. 

Common nursery plants that are biennial, but nearly always labeled and sold as perennials:  most of the mulleins (Verbascum), all cultivars of Digitalis purpurea (by far the most common species of foxglove found in nurseries).  As these plants most often set seed extravagantly, one can usually expect to have more plants, which will bloom two summers after the parent plant does- but most biennials are sold when ready to bloom, which means that they should if anything be marketed as annuals.  Furthermore, some popular, showy cultivars of Verbascum are sterile hybrids that will not have offspring.  The only way to perpetuate them may be to take cuttings. 

Very tender perennials—these are seldom accurately labeled in nurseries I have checked, but the errors go both ways.  That is, they may be sold without comment in the perennials section, and at perennial prices—or they may be sold in the annuals section without clarification.  These include Cordyline, Purple Fountain Grass, Angelonia, and many of the Salvias.

Indeed, one of the quirkier aspects of any garden center, increasingly, is the relative dearth of true annuals in any annual department.  I suspect that one reason may be the short shelf life of the true annuals.  As a rule they must be sold as very small starts, and by the time they are mature enough that the uninitiated plant buyer can see what they're becoming-- they are ready to quickly bloom, set seed, and die.

By far the majority of plants to be found in any large store are tender perennials, but there are also a significant number of  hardy perennials sold as annuals—one of the most perplexing categories of quirks:

Snapdragons- these are universally sold as annuals; there is not a single truly annual snapdragon on the planet, and most sold in nurseries are completely hardy.  It is true that they appear to be short-lived, though 3 years is not unusual, and generous self-seeding is typical.

Asters- nearly all asters available horticulturally are perennials.  Occasionally, one may find asters in sixpacks or seed packets that seem to be convincingly annual, but all others are uber-perennials, living potentially for many years, and the vast majority of nursery asters come from two extremely cold-hardy species.  Yet, they are often marketed as annuals, and wholesalers’ tags may perpetuate this notion by failing to list hardiness zones.

Diascias-  most sold in nurseries are hardy perennials labeled for zone 7 (occasionally 8), but they are often found in annuals sections.  Like their close relatives the snapdragons, life expectation is short and variable, but plants may survive at least 3 years.

Petunias- this is a complicated group and those in the nursery trade are mostly hybrids, but the parent species are mostly perennials and in truth  petunias often turn out to be perennials in warmer climates.

Chrysanthemums- most, including all of those sold in large numbers at grocery stores in late summer—are perennials, though some may be tender-ish or shortlived.

Rudbeckia hirta cultivars- often sold as annuals, never annual, all quite hardy.  Possibly short-lived or temperamental in cold, wet climates.

Morning glories- several of the most popular cultivars come from a very hardy perennial, Ipomoea tricolor.