Well, to begin with, perennials are a type of plants that have a span of life that exceeds two to three years. Once they are planted, they keep coming back year after year until they reach maturity, which varies from 3 to 5 years on average, depending on the plant.
In one season, they grow, bloom and go dormant. They should be trimmed in the fall so that they can start again in spring from the previous root system. Perennials more often than not reseed, and will expand over the years.
They generally bloom for a short time during the season (usually 2 to 6 weeks). If you want to get the best out of your perennials, plant them preferably in spring or fall, 8 to 6 weeks before the winter comes and the ground freezes.
Some the most famous examples of perennials are asters, tulips, echinacea, lilies and lavender.
Now when it comes to hardier perennials, they might last for over 20 years. These perennials have certain structures that help them make it through all these years.