The term "evergreen" means having foliage that persists (as opposed to dropping) and retains its color throughout the year, rather than changing color according to the seasons. The term is something of a misnomer, as the color in question need not be green. For instance, Colorado blue spruce trees (Picea pungens) are evergreens, but the color they retain throughout the year is a silvery blue. There are also plants with golden foliage that keep that color year round, such as Chamaecyparis pisifera 'King's Gold'. These plants qualify under the definition, even though their leaves (needles) are not green.
What's in a Name?
Trees and shrubs can be classified as either evergreen or deciduous. Another term that comes into play here is "conifer." Many conifer trees are evergreen, but not all, so the two words are not synonymous, even though some people mistakenly use them that way. The Tamarack, or "larch" tree (Larix), for example, is a conifer, but it is not an evergreen.
Yet another distinction is between the needled evergreens and broadleaf evergreens. The latter are represented by such plants as 'Sky Pencil' holly and most other holly shrubs (but not winterberry holly, which loses its leaves in fall and is therefore classified as deciduous). Non-botanists are far more likely to think of the needle-bearers when they hear mention of "evergreens." Examples include:
Is a Pine an Evergreen?
Those who have never paid much attention to plant identification sometimes erroneously treat "pine" and "evergreen" as if they were synonyms. Pines are, indeed, evergreens, but they comprise only one group under the "evergreen" umbrella. If you take a close look at the needles of Canadian hemlock trees or yew shrubs, for example, you will see that they are short and flat -- very much unlike the needles of eastern white pine trees or mugo pine trees. Still, other needled evergreens have needles that are awl-like, scale-like, or blade-like.
Even perennials can be evergreen. Those that are not are called "herbaceous," rather than "deciduous." The warmer the winters in a region, the more evergreen perennials there will be, generally speaking. But even in a chilly region such as the Berkshires of Massachusetts (the northeastern U.S., planting zone 5) there are some evergreen perennials, including:
Uses for Evergreen Trees and Shrubs
But most people, when casually referring to "evergreens," really mean shrubs and trees that bear persistent leaves, which retain their color throughout the year. These are, indeed, some of the most valued plants in the landscape. Why? Because the fact that they hold onto their leaves year-round allows you to do things with them in your landscaping that you cannot do with deciduous shrubs and trees. For example:
Some plants give you a clue as to their evergreen status right in their botanical names. Look for the term, sempervirens, which is composed of two Latin words: semper, meaning "always," plus "virens," which means "green." Examples are: