All flowering plants belong to a group of plants called angiosperms. The flowers that distinguish angiosperms from other plants are sexual reproductive organs, a survival strategy that has been successful enough to make angiosperms the most abundant type of land plant. The angiosperm life cycle begins and ends with a seed, each seed produced as a result of flowering and pollination.
Angiosperms can be divided into three groups--perennials, biennials and annuals--that refer to how often we gardeners have to replant them, and so, also to the length of their life cycles. Perennials live at least three years--even hundreds of years. Biennials and annuals don't possess the same longevity.
Annuals, as the name implies, only last for one growing season, then need to be replanted; biennials live for two. Because biennials and perennials don't go from seed to seed in one growing season, they rest during a dormant stage, resuming growth in spring.
All angiosperms, including annuals, begin life by sprouting from a seed. One or two cotyledons emerge, looking like small leaves. If two leaves have appeared, the plant is a dicot (short for dicotyledon). Dicots comprise the majority of angiosperms. Angiosperms sprouting only one cotyledon are monocots (monocotyledons). Leaves follow cotyledons, while underground, roots are digging in.
Once the annual is well established, it enters its reproductive phase by flowering. The flowers produce pollen, this pollen containing sperm that needs to make it to female flower parts. These female parts contain eggs awaiting fertilization. Once the pollen reaches its target, fertilization occurs. Fertilization results in a seed encased in a protective covering or fruit. This accomplished, an annual dies.
Biennials, too, sprout, then grow vegetative parts. From here, though, biennials take a detour before arriving at the reproductive stage. Instead of flowering during their first growth season, biennials grow tubers, corms or bulbs--all food storage organs--before going dormant for winter. Examples of such organs are onions and potatoes.
The food storage organs fuel the biennial's new growth come spring, when the plant emerges from dormancy. Once again active, a biennial produces more vegetative growth, then, like the annual, flowers, seeds and dies.
Perennials follow the same sprouting, growing and flowering pattern of annuals, but go dormant instead of dying in the fall and winter. If the perennial is woody, it simply rests. If deciduous, the plant drops its leaves, which grow back when spring arrives to instigate new growth.
If the perennial is herbaceous, its stems, lacking protective bark, die back when faced with winter. Roots survive, though, and from them the plant regrows in spring. Perennials may not flower every year, especially when they are young.
Life Cycle Variations
Sometimes environmental conditions don't favor a plant, and the plant is forced to attempt to live out a life cycle in one season. This phenomenon is called bolting. Gardeners may see it if they plant flowers that aren't naturally suited to the climate.
If a gardener wants to grow a biennial or perennial in a stressful climate, they often treat the plants like annuals, replanting them every year.