The survivors of the plant world, perennials often are defined as plants that persist in the garden for at least three years. They can be divided into different categories based on their hardiness, on the makeup of their stems or on how long they retain their foliage.
Hardy and Tender Types
According to Louisiana State University, hardy perennials are those that can survive temperatures of 32 degrees Fahrenheit or less without harm. That category would, therefore, encompass many of the so-called hardy geraniums (Geranium spp.), which are perennial in U.S. Department of Agriculture plant hardiness zones 3 through 10. It wouldn't include the zonal or bedding geraniums (_Pelargonium x hortorum_), hardy only in USDA zones 10 to 11.
The latter, considered tender perennials**, actually may remain undamaged by light frosts -- temperatures as low as 29 degrees Fahrenheit. They seldom survive hard freezes** of temperatures under 25 degrees, however. Usually considered annuals in the north, tender perennials can live on in climates where such freezes rarely occur.
Woody, Semiwoody and Herbaceous Types
The woody category of perennials includes single-stemmed types -- trees -- called "arborescent," and multistemmed types -- shrubs -- described as "fruticose." Oaks (Quercus spp., USDA zones 2 to 11), for example, are arborescent and hydrangeas (Hydrangea spp., USDA zones 5 through 10) fruticose. Although such perennials may drop their leaves in winter, they still retain life in their above-ground woody trunks or stems during that time.
Other perennials, including rosemary (Rosmarinus officinalis, USDA zones 6 through 11) and Russian sage (Perovskia atriplicifolia, USDA zones 6 through 9), show some woodiness at the bases of their stems, but those stems turn tender and green farther up. Such semiwoody plants, described as "suffrutescent," sometimes are called semishrubs or subshrubs and may die back only partway during winter.
The third category, herbaceous perennials, have green stems with no trace of woodiness and usually shrivel down to the ground over winter, retaining life only in their roots and/or bulbs. Perennials of this sort include columbines (Aquilegia spp., USDA zones 3 to 10) and daffodils (Narcissus spp., USDA zones 4 to 10).
Evergreen, Deciduous and Semideciduous Types
Evergreen perennials such as holly (Ilex spp., USDA zones 3 to 11) and hellebores (Helleborus spp., USDA zones 3 to 10) retain at least some of their foliage all year. Deciduous varieties, including maples (Acer spp., USDA zones 3 to 11) and wisteria (Wisteria spp., USDA zones 5 through 10), drop all of their leaves at some point during the year -- usually in autumn. Semideciduous perennials such as rock cotoneaster (Cotoneaster horizontalis) fall somewhere in between and often shed their leaves or keep them based on weather conditions.
These distinctions between perennials aren’t always clear-cut. For example, although many oaks would be classified as hardy, woody, deciduous perennials, some species – such as cork oak (Quercus suber, USDA zones 8 through 10) -- are evergreen and only hardy in the mildest zones.