Perennial plant

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Red Valerian, a perennial plant.

A perennial plant or perennial (Latin per, "through", annum, "year") is a plant that lives for more than two years. Perennial plants are divided into two large groups, those that are woody and those that are herbaceous. All woody plants are perennials since they form woody tissue that persists from one year to the next. Woody perennials develop a woody base or root system from which the foliage and flower stems grow year after year while the plant is alive. In common usage the term perennial generally describes herbaceous perennials. Since herbaceous plants as a group include all non woody plants, they can be annuals, biannual and/or perennials. Perennial plants are a diverse group, they can be short lived, living for only a few years or they can be long lived persisting for 20 or more years with some woody plants living for over 1,000 years. Perennial plants can vary in size from only a few millimeters in size to over 100 meters tall. They include a wide assortment of plant groups from ferns and liverworts to the highly diverse flowering plants like Orchids and Grasses.

Perennials that flower and fruit only once and then die are termed monocarpic or semelparous. However, most perennials are polycarpic, flowering over many seasons in their lifetime.

Perennials typically grow structures that allow them to adapt to living from one year to the next. These structures include bulbs, tubers, woody crowns, rhizomes plus others. They might have specialized stems or crowns that allow them to survive periods of dormancy over cold or dry seasons during the year. While annuals produce seeds to continue the species as a new generation while the growing season is suitable and the seeds survive over the cold or dry period to begin growth when the conditions are again suitable. Many perennials have specialized to survive under extreme environmental conditions, some have adapted to survive hot dry conditions, or to survive under cold temperatures. Those plants tend to invest a lot of resource into there adaptations and often do not flower and set seed until after a few years of growth. Many perennials produce relatively large seeds, large seeds can have an advantage, with larger seedlings produced after germination that can better compete with other plants or more quickly develop leaves for photosynthesis. Annuals tend to produce many more seeds per plant since they will die at the end of the growing season, while perennials are not under the same pressure to produce large numbers of seeds but instead the same plant can produce seeds over many years.

In warmer and more clement climates, perennials grow continuously. In seasonal climates, their growth is limited to the growing season. For example, in temperate regions a perennial plant may grow and bloom during the warm part of the year, with the foliage dying back in the winter. These plants are deciduous perennials. Regrowth is from existing stem tissue. In many parts of the world, seasonality is expressed as wet and dry periods rather than warm and cold periods. In some species, perennials retain their foliage all year round; these are evergreen perennials.

With their roots protected below ground in the soil layer, perennial plants are notably tolerant of wildfire. They are also less subject to extremes of cold in temperate and arctic winters, with less sensitivity than trees or shrubs.

Perennial plants dominate many natural ecosystems on land and in fresh water, with only a very few (e.g. Zostera) occurring in shallow sea water. They are particularly dominant in conditions too fire-prone for trees and shrubs, e.g., most plants on prairies and steppes are perennials; they are also dominant on tundra too cold for tree growth. In forests, perennial plants are of secondary importance to trees and shrubs, but are often still abundant on the forest floor.

Perennial plants are usually better competitors than annual plants, especially under stable, resource-poor conditions. This is due to the development of larger root systems which can access water and soil nutrients deeper in the soil and to earlier emergence in the spring.

Examples of evergreen perennials include Begonia and banana.

Examples of deciduous perennials include goldenrod and mint.

Examples of monocarpic perennials include Agave and some species of Streptocarpus.