The term annual refers to those garden flowers that complete their life cycle in one growing season. This means seed is planted in the spring, the plants grow, flower, set seed and then die usually after the frosts in the fall.
Annuals can also be classified based upon their tolerance to and ability to withstand cold. Understanding how annuals are classified based upon their tolerance to cold temperatures can be very useful in extending the gardening season. This allows you to plant well before the last frost in the spring and well past the first frost in the fall.
Hardy annuals are those that can stand the most cold and can tolerate cold soil, cool air and light frosts without being damaged. Ornamental cabbage and kale, pansy, snapdragon, sweet alyssum, calendula and dusty miller fall into this category. These can usually be planted about four weeks before the frost free date in your area.
Half-hardy annuals can tolerate cool soils and cool air but are damaged by frost. Ageratum, geranium, dianthus, lobelia, petunia and gazania are examples. These can be planted about two weeks before the frost free date in your area.
Tender annuals need both warm soil and warm air temperatures to grow well. These do not tolerate any frost and if put out too soon into cold soil, tend not to do much growing until temperatures warm. Begonia, coleus, vinca, salvia, zinnia and impatiens are examples. These are best put out on or a few weeks after the frost free date when both soil and air temperatures warm.
Another class of plants that is being used a lot in today’s gardens are plants that if planted far enough south, in warmer zones, would in fact be perennial and continue to grow and come back each year. When these plants are planted in cooler zones, they will be killed by frost and need replacing. These plants are often referred to as tender perennials used as annuals. These can include geranium, lantana, coleus, various tropical green plants and tropical hibiscus. If these plants are to be saved, they will need to be dug, potted and brought indoors for the winter. For some, taking cuttings in the fall is another way of saving them for next season’s garden.
There are a number of annuals that may act like perennials because of the large amount of seed that is produced and dropped to the ground in the fall. This seed remains viable over the winter and in the spring new plants emerge in and around the area where the annual was planted the previous season. Plants like cleome, snapdragon, cosmos and amaranth are examples. These types of annuals can be useful if one wants to establish a “natural” planting area that continues to provide color year after year with very little to no preparation.