Herbaceous perennials are plants that bloom year after year. Their tops die back to the ground each fall, but their crowns and root systems remain alive (though dormant) during winter. They regrow from those roots and crowns when conditions are right the following spring.
Unlike annuals, herbaceous perennials usually don't flower their first season when grown from seed. Instead, they channel their energy into their roots to make sure they make it through the winter. Nurseries and garden centers sell established perennial plants that often flower the first season you transplant them into your garden.
Perennials have many advantages:
They don't have to be planted every year. Once established in a suitable spot, they will come back year after year -- some for decades.
Some herbaceous perennials will only thrive if you dig them up every few years and divide the root systems. Each time you do, you turn what was once a single plant into several (or sometimes many) new plants you can replant or share.
Perennial gardens are dynamic. They change with the season. Each perennial emerges from the ground, grows, and flowers at its own special time -- some early, some late. Unlike annuals, which usually bloom for a long time, perennials bloom for about 1 to 6 weeks, depending on the species. By carefully selecting plants with a variety of bloom times, your garden will change through the growing season and you'll always have something in flower.
In addition to a wide variety of flower colors and forms, many perennials have attractive foliage that adds visual interest even when they are not in flower.
Perennials require an investment in either time or money. They are more expensive than annuals if you buy established plants. If you start them from seed, they are slower to mature and flower. When you consider that they come back every year, however, they are a great value.
Other plants are also perennials. Trees and shrubs can be incorporated into flower gardens, but they are a different category of plants -- woody perennials. They don't die back to the ground each year, but regrow from live buds on their above-ground stems. Spring-flowering bulbs are a type of herbaceous perennial, but they are usually considered a separate category because they have special needs. Perennial ornamental grasses and ferns likewise require unique practices. Some herbs are herbaceous perennials that can be incorporated into flower gardens.
Biennials are a group of herbaceous plants that fall in the gray area between annuals and perennials. When planted from seed, biennials do not flower their first season. The plants are often low-growing rosettes that are easily mistaken for weeds. Their roots overwinter much like perennials. They flower in their second season (often growing quite tall) and frequently reseed themselves (if you allow the seed to mature), then die. Some common biennial flowers include foxglove, hollyhocks, dame's rocket, and lunaria.
The gray area comes because some biennials -- foxgloves, for example -- may flower in their first year grown from seed, just like an annual. Other biennials occasionally overwinter and flower a second year and are often called "weakly perennial." These plants are sometimes lumped in with perennials in catalogs. If you want biennials to reseed and flower in the same spot every year, be sure to plant them there two years in a row. That way, you'll always have some first-year vegetative plants and second-year flowering plants.
It's also possible to mistake some prolifically reseeding annuals for perennials, because every year new plants appear where the old plants were the previous season. But it is the seed -- not the roots -- that are overwintering in the soil. Remove seedheads before annuals mature if you want to prevent them from reseeding.