Botanically, a perennial is any plant that lives for more than a couple of growing seasons. These plants can live for four years or 4,000 and include much more variety than what gardeners commonly call perennials.
Hardiness dictates the survival of a perennial in an area. Temperature is one aspect of hardiness. Different plant parts are hardy only to certain temperatures. (The root system on most plants, for example, is less hardy than the vegetative buds.) Other factors are important to the plant's survival. For instance, highly alkaline soil will kill a rhododendron and soil that's too wet will kill a cactus. Mike Heger of Ambergate Gardens in Waconia, Minnesota, says cold-hardiness ratings are often conservative. With enough protection, many plants will survive in zones lower than their rating.
This is a plant that's able to survive freezing temperatures, usually by going dormant. (Dormancy is a sort of plant hibernation.) These include tulips, peonies, lilacs, pines, and many others.
A tender perennial can't withstand cold temperatures--say under 50 degrees F--without being damaged. Non-hardy plants include weeping figs (Ficus benjamina), many palm trees, and most passionflowers (Passiflora spp.). Some plants we grow as annuals, such as fuchsias (Fuchsia spp.), lantana (Lantana camara), and moonflower (Ipomoea alba) are tender perennials in their native homes, as well.
This is a plant that keeps its stems and other aboveground parts though the dormancy period. Trees and shrubs, such as lindens (Tilia spp.), redwoods (Sequoia spp.), and hollies (Ilex spp.) are all woody perennials.
Herbaceous perennials are the plants gardeners most commonly call perennials. What makes them different from the woody types is that herbaceous types die to the ground when dormant. Herbaceous perennials include monkshoods (Aconitum spp.), bleeding hearts (Dicentra spp.), and lungworts (Pulmonaria spp.).
Short - lived
While some perennial plants have the ability to go on for hundreds of years, others live for only a few. Gardeners call these short-lived perennials. Columbines (Aquilegia spp.) and perennial lupines (Lupinus spp.) are two that tend to die out after several seasons. Many of them reseed, however.
Many perennials benefit from division. Some perennials will choke themselves out if they're not divided; others grow happily without a gardener's intervention. Crown division is common for clump-forming perennials such as daylilies (Hemerocallis spp.) and spiderworts (Tradescantia spp.). To divide these plants, dig the clump and pull sections away. Each section needs roots. Cut pieces from tubers or rhizomes, such as dahlias and irises to divide these plants. Simply cut away rooted sections of the rhizome (the fleshy stem that grows along the ground). As long as each section has roots, it should grow.