In botany, the term perennial (per- + -ennial, "through the year") is used to differentiate a plant from shorter-lived annuals and biennials. It has thus been defined as a plant that lives more than 2 years. The term is also loosely used to distinguish plants with little or no woody growth (secondary growth in girth) from trees and shrubs, which are also technically perennials. An estimated 94% of plant species are perennials. Perennials (especially small flowering plants) that grow and bloom over the spring and summer, die back every autumn and winter, and then return in the spring from their rootstock or other overwintering structure, are known as herbaceous perennials. However, depending on the rigours of the local climate (temperature, moisture, organic content in the soil, microorganisms), a plant that is a perennial in its native habitat, may be treated by a gardener as an annual and planted out every year, from seed, from cuttings, or from divisions. Tomato vines, for example, live several years in their natural tropical/ subtropical habitat but are grown as annuals in temperate regions because their above-ground biomass does not survive the winter. There is also a class of evergreen perennials which lack woody stems, such as Bergenia which retain a mantle of leaves throughout the year. An intermediate class of plants is known as subshrubs, which retain a vestigial woody structure in winter, e.g. Penstemon. The symbol for a perennial plant, based on Species Plantarum by Linnaeus, is .
Life cycle and structure Perennial plants can be short-lived (only a few years) or long-lived. They include a wide assortment of plant groups from non-flowering plants like ferns and liverworts to highly diverse flowering plants like orchids, grasses, and woody plants. Plants that flower and fruit only once and then die are termed monocarpic or semelparous; these species may live for many years before they flower. For example, a century plant can live for 80 years and grow 30 meters tall before flowering and dying. However, most perennials are polycarpic (or iteroparous), flowering over many seasons in their lifetime. Perennials invest more resources than annuals into roots, crowns, and other structures that allow them to live from one year to the next. They often have a competitive advantage because they can commence their growth and leaf out earlier in the growing season, and can grow taller than annuals. In doing so, they can better compete for space and collect more light. Perennials typically grow structures that allow them to adapt to living from one year to the next through a form of vegetative reproduction rather than seeding. These structures include bulbs, tubers, woody crowns, rhizomes, turions, woody stems, or crowns which allows them to survive periods of dormancy over cold or dry seasons; these structures typically store carbohydrates which are used once the dormancy period is over and new growth begins. In climates that are warm all year long, perennials may grow continuously. Annuals which complete their life cycle in one growing season, in contrast with perennials, produce seeds as the next generation and die; the seeds may survive cold or dry periods or germinate soon after dispersal depending on the climate. Some perennials retain their foliage year-round; these are evergreen perennials. Deciduous perennials shed all their leaves part of the year. Deciduous perennials include herbaceous and woody plants; herbaceous plants have stems that lack hard, fibrous growth, while woody plants have stems with buds that survive above ground during dormancy. Some perennials are semi-deciduous, meaning they lose some of their leaves in either winter or summer. Deciduous perennials shed their leaves when growing conditions are no longer suitable for photosynthesis, such as when it is too cold or dry. In many parts of the world, seasonality is expressed as wet and dry periods rather than warm and cold periods, and deciduous perennials lose their leaves in the dry season. Some perennial plants are protected from wildfires because they have underground roots that produce adventitious shoots, bulbs, crowns, or stems; other perennials like trees and shrubs may have thick cork layers that protect the stems. Herbaceous perennials from temperate and alpine regions of the world can tolerate the cold during winter. Perennial plants may remain dormant for long periods and then recommence growth and reproduction when the environment is more suitable, while most annual plants complete their life cycle during one growing period, and biennials have two growing periods. The meristem of perennial plants communicates with the hormones produced due to environmental situations (i.e., seasons), reproduction, and stage of development to begin and halt the ability to grow or flower. There is also a distinction between the ability to grow and the actual task of growth. For example, most trees regain the ability to grow during winter but do not initiate physical growth until the spring and summer months. The start of dormancy can be seen in perennial plants through withering flowers, loss of leaves on trees, and halting of reproduction in both flowering and budding plants. Perennial species may produce relatively large seeds that have the advantage of generating larger seedlings that can better compete with other plants. Perennials also produce seeds over many years. An important aspect of cold acclimation is overexpression of DNA repair genes. In Thinopyrum intermedium a perennial relative of common wheat Triticum aestivum, conditions of freezing stress were shown to be associated with large increases in expression of two DNA repair genes (one gene product a photolyase and the other, a protein involved in nucleotide excision repair).
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