By definition, perennials are non-woody plants that have a life cycle of three years or longer, as opposed to "annual plants" (a portion of one year) and "biennials" (two years). Note that "perennial" is frequently misspelled, with the R doubled and/or one N dropped.
Using life cycle as a criterion dispels three myths about what makes plants perennials:
- Myth #1: Perennials are hardy.
- Myth #2: They die back in winter but return in spring.
- Myth #3: They are the plants you see year after year in your garden.
When you learn the facts about the life cycle of perennials, you will realize that all three of these are merely half-truths. But what exactly is the meaning of "life cycle?" Well, think of it this way. You put a seed in the ground. A plant springs up from that seed. Eventually, that plant flowers and produces seed of its own. It has come "full circle." How long did all that take? How many years thereafter (if any) did the plant continue to bear flowers? If you can answer those questions, then you are well on your way to classifying that particular plant correctly as a perennial, biennial or annual.
Now let's explore those myths in further detail:
Are Perennials Hardy?
It is true that, if you are successful at growing certain perennial flowers in your region as perennials (that is, they live three years or longer), then they must be hardy in your USDA plant hardiness zone. But if you live where it gets cold in winter, many plants from warm climates that are properly classified as perennials will not last more than a year for you. When speaking of such plants, we often say they are "treated as annuals" in regions such as yours.
But this does not change the classification of the plants, botanically speaking, as perennials. They are merely perennials whose life cycle has been cut short. Informally, Northern gardeners sometimes make a distinction between such "tender perennials," on the one hand, and, on the other, "cold-hardy perennials" (see below for examples).
Do Perennials Die in the Winter and Return in the Spring?
Some perennials do die back in winter and return in spring. But that characteristic is not, per se, what makes them perennials. Only one category (albeit a very large one) of perennials follows this pattern: the herbaceous category.
However, there is another category of perennials: the evergreen perennials. Evergreen perennials do not behave in this manner (at least not in climates where the weather cooperates).
Do Perennials Return Year After Year?
Just because you see "the same" plants in your garden year after year, they are not necessarily perennials. They could simply be re-seeding, as some annuals do. The end result is the same (namely, that you get to enjoy the plants year after year), but they do not meet the technical definition of what makes a plant a perennial.
Examples of Cold-Hardy Perennials
The zone to which they're hardy is listed in Parentheses.
Where Do Trees, Shrubs and Bulbs Come In?
If we were to go strictly by lifespan, trees and shrubs could also be considered perennials, so why are they regarded as distinct groups? Trees and shrubs are treated as being different from perennials because they have woody stems. Bulb plants have a greater claim to be called perennials, because they are non-woody and usually last three years or longer. But most garden writers follow a convention whereby bulb plants are treated as a group all unto themselves.
Long-Lived vs. Short-Lived Perennials
When we say that "perennial" means having a non-woody stem and a life cycle of three years or longer, we are really not doing justice to how different one perennial can be from another. Some kinds barely make the three-year requirement, while others are so long-lasting that they may very well outlive you, the grower.
Examples of short-lived perennials (plants that may last only three years) are: