Annuals, biennials and perennials are plants that are classified according to their life expectancy. The life expectancies differ from species to species. An oak or teak tree may live for years together while a sun flower may not.
When many a plant goes through its entire life cycle within a year, such plants are called annuals. Their life cycle starts with germination. Then they flower, produce seeds and finally die within few days, months or a year. Examples of annuals include sunflower, morning glories, zinnia, celosia and many more. While certain annuals like forget-me-not, pansies can tolerate cool temperatures, others like marigold prefer sunny weather. Thus summer annuals germinate during spring and mature by winter and winter annuals germinate during autumn and mature during summer. Annuals like sweat pea can thrive in alkaline soil and poppy can thrive even in infertile soil.
Biennials are plants that live for two years. They germinate, grow and store food in their roots and stems during their first season. The stems are normally very short during the first season. They remain inactive during winter. Then the stem starts to grow at the onset of second season. The plants now flower, produce seeds and die in their second year. There are few biennials compared to annuals or perennials. Though onions, carrots and cabbages are biennials, they are harvested during their first year not allowing them to produce seeds in the second year. Thus biennials that are grown for their edible leaves or roots are grown as annuals while those grown for flowers, fruits, or seeds need to be grown for two years.
The longest-lived are perennials. They can be short lived (more than two years) or long lived like woody trees. They include ferns, orchids, Dahlia grasses as well as trees like maple, apple and pine. Every year, they flower and set seed. Except in the tropical regions, they shed their leaves and stem every autumn. Their roots and other storage structures remain inactive in the soil. They readily send out shoots at the start of next growing season. They reproduce mainly by vegetative reproduction. A plant behaves as an annual or perennial depending on their local climatic and growing conditions.