April 2017

The life span of an individual plant depends upon two factors. The first is the innate, genetically determined potential for longevity. The second is the effects of the environment, including soil and weather conditions, competing plants, disease-causing microbes, and herbivores.

Historically, people have classified the life spans of plants into three categories: annuals, biennials, and perennials. Annual plants live for up to one year. Biennials live for approximately two years. Perennials live for more than two years, often for several decades, even centuries.

While this categorization is useful in many ways, botanists have come to recognize that it is inaccurate, especially for plants that grow under natural conditions. Plant life histories are now classified mainly according to the number of times that each individual normally reproduces before it dies. Two main categories are recognized using this system: monocarpic plants and polycarpic plants. Monocarp plants reproduce once before they die. Polycarpic plants reproduce several or many times before they die. Some botanists have defined a third group, the paucicarpic plants that are intermediate between the two. Paucicarpic plants reproduce up to five times.

See also:Monocarpic Plants, Polycarpic and Paucicarpic Plants, Potential and Real Life Spans

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Paucicarpic and polycarpic plants normally reproduce more than once before they die. They are able to survive for at least one year following reproduction and hence are true perennials. Paucicarpic plants are short-lived herbs that may die after reproduction but more commonly live to reproduce two, three, or four times before dying. Paucicarpic plants are therefore intermediate between the true monocarps and the true polycarps. Examples include the common and English plantains, which are weeds found in lawns and fields throughout temperate North America and Europe.

True polycarpic plants survive to reproduce many times during their lifetimes and usually remain alive for at least ten years. Unlike the mono-carps, polycarps do not expend all of their energy in reproduction. They save some of their energy and maintain part of the plant for the post-reproductive period. In seasonal climates, some of that energy must be directed to forming structures that allow the plant to survive the unfavorable season—a cold winter or a rainless period. These structures are called perennating buds, and they differ from plant to plant in their location relative to the ground surface. In some plants, called cryptophytes (the prefix crypto means “hidden”), the perennating buds are buried several centimeters under the ground. Examples of cryptophytes include milkweed, iris, and onion. Conversely, hemicryptophytes (hemi means “partial”) have their perennating buds at the soil surface, a good example is the dandelion. Both cryptophytes and hemicryptophytes are herbaceous plants, never producing an aboveground woody structure.

Phanerophytes are polycarpic plants that do produce an aboveground woody structure—the perennating buds are borne above the ground surface. Some phanerophytes are shrubs that have several shoots. Examples of shrubs include lilac, blueberry, hawthorn, hydrangea, rhododendron, and many dogwoods and willows. A second category of phanerophytes is the trees, which typically have a single woody stem emerging from the rootstock.

In theory, most species of polycarpic plants can live for decades, if not centuries, under ideal conditions. Many do not appear to have a maximum life span because they rejuvenate their tissues with each reproductive period, as in some polycarpic herbs or because the tissues that they accumulate do not put much of an added strain on the plant, as in many phanerophytes.

In nature, such polycarpic plants are not killed by old age. External factors such as herbivory (consumption by animals), fire, severe weather, disease, and competition from other plants contribute heavily to die-off among individuals. Other polycarpic plants form senescent tissue that hastens their death.

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Monocarpic plants have a general life history that involves four separate stages: germination, vegetative growth, reproduction, and death. The period of vegetative growth is very important to the monocarpic plant because during this time the plant manufactures and stores starch, which is rich in energy. When reproduction occurs, all that stored energy is devoted to producing flowers, fruits, and seeds, none is saved for the following year. The plant literally reproduces itself to death. Monocarpic plants vary greatly in their longevity, and it is possible to recognize several subcategories: ephemerals, annuals, obligate biennials, facultative biennials, and long-lived monocarpic perennials.

Ephemerals are plants that germinate, grow, reproduce, and die within a few weeks or months. They are typically found in environments in which conditions favor active plant growth for only a short period of time during the year, such as a desert. Desert ephemerals spend most of the year as seeds. When a heavy rainstorm occurs, the seeds germinate, and the new plants grow quickly and reproduce before the soil dries out. One species in the Sahara Desert can complete its life cycle in as little as ten days.

Annuals are plants that progress from germination to true seed within a six- to twelve-month period. Botanists recognize two major subcategories of annual plants. One is the summer annual, which germinates in the spring. The plant grows vegetatively during the summer and reproduces during the autumn. Examples of summer annuals include touch-me-not, common ragweed, and goosefoot. The second subcategory is the winter annual, in which germination occurs in the fall, vegetative growth occurs in the winter, and reproduction occurs in the spring. Daisy f leabane and winter wheat are examples of winter annuals.

Obligate biennials are monocarpic plants that germinate and grow vegetatively over the course of one year and throughout much of a second year. At the end of the second year, the plant always reproduces, sets seed, and dies (hence the designation “obligate”). During the 1960’s and 1970’s, some botanists doubted that obligate biennials existed in nature. Studies conducted during the 1970’s and 1980’s demonstrated that some plants, such as the white and yellow sweet clovers, are indeed obligate biennials.

Facultative biennials are monocarpic plants that have the ability to germinate, grow, and reproduce within two years. They can behave as biennials only when they grow under favorable conditions, with adequate moisture, light, and soil nutrients. More commonly, these plants grow under stressful conditions either infertile soils or high competition. On such sites, they grow vegetatively for three, four, or even five years before they reproduce. Examples of facultative biennials include wild carrot, foxglove, burdock, teasel, and thistle.

Long-lived monocarpic perennials are able to live for many years or a few decades before they reproduce once and then die. Well-known examples include bamboo and plants from the arid southwestern United States, such as species of Yucca and the century plant Agave. These may reproduce only after they attain an age of sixty, eighty, or even one hundred years.

See also: Types of Life Spans, Polycarpic and Paucicarpic Plants

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