A true annual is a plant that completes its life cycle in one year. This means it goes from seed to flower and back to seed and then dies off during the course of one growing season. That's what differentiates it from a biennial, which lives for two years, and a perennial plant that is supposed to live for three or more years.
Annuals and Perennials Grown As Annuals: Overview
The entire mission of an annual plant is to produce seed to ensure the propagation of future generations. It sets pretty flowers to attract insects, not humans, so that it can be pollinated. That is why deadheading or removing spent flowers before the seed matures induces the plant to set even more buds and flower, in the hopes of generating more potential seed that will survive.
Some tender perennials, like the popular zonal geraniums (Pelargonium), are grown as annuals in colder climates because they will not be able to survive cold winter. Many perennials only grow foilage their first year and start flowering in their second. For a perennial to be worth growing as an annual, it must flower profusely in its first year of growth. Pansies, lantana, alyssum, and even tomatoes and peppers are all tender perennials grown as annuals.
The distinction between annual plants and perennials can be blurred, but whether your plant is a true annual or a perennial being grown as an annual, you can expect to have to replace it each year.
Hardy Annual Plants
Hardy annuals are annuals that can withstand a little frost without being killed off and will continue to bloom and set seed into the next year, but they do not carry on indefinitely and usually die shortly after their second year begins. Bachelor Buttons and Salvia Victoria are examples.
Cool and Warm Season Annuals
Annuals can be further divided into cool season annuals and warm season annuals. Although they may live for the entire growing season, they may not flower the whole time. For example, pansies will fade as the summer heats up. Zinnias won't even start flowering until the nights stay warm.
Many vegetables and herbs are also annuals, such as beans, basil, cilantro, and cucumbers. Even most perennial vegetables are grown as annuals, partly because they are only hardy in the warmest climates, but also because they are required to continually produce flowers and fruits that are harvested, rather than being allowed to go to seed. All this effort eventually exhausts plants like tomatoes and eggplants.
Why You Should Grow Annual Plants
Annual flowers tend to bloom nonstop, especially if you deadhead the plants. Growing annuals will help keep your garden in bloom all season. They are popular choices for containers and hanging baskets because they remain attractive all season.
Annual flowers also give you the opportunity to have a totally different garden every year. Perennial plants come back each year and remain a constant in your garden. If you want to try a new color scheme or simply experiment with new plants, annuals allow you to do that without making a long-term commitment. They also tend to be much less expensive than perennial plants.