Does anyone really know what a ‘perennial’ plant is? It is obvious that it is not an ‘annual’ plant that lives only a single year. A ‘biennial’ plant produces vegetative (non-blooming) growth in the first year, and then blooms, develops seed, and dies in the second year. A plant only needs to live more than two years to be a perennial plant, or simply a ‘perennial’. Well, that does not narrow the definition down much. Bristlecone pine can live for thousands of years, but is not often thought of as a perennial.
In simple home gardening terminology, a perennial not only lives for more than two years, but does so without producing significant woody stems. Yes, this also happens to include palms and trunk forming yuccas (which are known as ‘perennial trees’), but that is another topic. Some perennials live only a few years. Some can live indefinitely by replacing their stolons, rhizomes, bulbs, tubers or whatever they regenerate from as their old growth gets left behind.
For example, bearded iris spread by fleshy stems known as ‘rhizomes’. As they grow from the forward tips, the older ends that get left behind will rot away. They are constantly replacing themselves, without leaving evidence of how long they have been doing so. (In other words, it is impossible to cut one down to count the rings.)
Many plants that are known as annuals are actually perennials, but get removed and replaced during their respective dormant season. Busy Lizzy can regenerate each spring if their roots do not succumb to frost in winter. Begonia, chrysanthemum, cyclamen and primrose are just some of the many other annuals that could technically survive as perennials.
Lily-of-the-Nile, African daisy, daylily, canna, penstemon, New Zealand flax and various grasses and ferns are some of the more familiar perennials. They are too diverse to generalize about, but happen to be among the most reliable of plants for bloom and foliage. Because form and mature size is somewhat predictable, properly selected perennials are unlikely to outgrow their particular situations.