Annuals are plants that naturally complete their entire life cycle--from seed through flowering, seed production, and inevitable death--within one growing season. Most begin developing in spring and die before winter. Besides true annuals, a considerable number of other plants that gardeners grow as annuals for convenience are also called annuals. These include ageratums, petunias, snapdragons, verbenas, wax begonias, and many other favorites.

In regions of severe winters and comparatively short spring-to-fall growing seasons, annuals can be categorized as "hardy" or "half-hardy." Hardy annuals are plants that thrive when sown outdoors in spring as soon as the ground can be brought into condition. They are not harmed by light frosts and make satisfactory growth at fairly low temperatures. Half-hardy annuals, because they are tender to frost or because they grow too slowly in their young stages to come into bloom quickly enough for outside sowing, need fairly warm growing conditions. So they are started early indoors and set in the garden as quite sizable young plants after the weather is suitable for them. In mild climates, the terms hardy and half-hardy are without significance.

Some plants, called winter annuals, germinate in fall in the wild, remain small seedlings through winter, and resume growth in spring. They differ from biennials because they neither grow substantially nor build up considerable food reserves before winter.

Biennials are plants that die at the end of their second growing season.  The first season they grow only foliage, commonly a low-growing rosette of leaves. The second growing season they form flowers and produce seeds; then the plant dies.

Perennials technically include all plants that normally, in their natural habitats and elsewhere, under favorable circumstances, persist for many growing seasons. All trees and shrubs, all ferns, practically all bulb plants, and many vines are perennials.

On the basis of stem tissue characteristics, perennials are further subdivided into woody perennials, which include all trees and shrubs and all vines that have woody (ligneous) stems, and herbaceous perennials, which have stems without woody tissue or are stemless. There is no sharp line of distinction between these groups. Some sorts of plants, often called sub-shrubs, have stems with a certain amount of ligneous tissue but not sufficient to give them the firm, woody character characteristic of more typical shrubs.

Perennials may be deciduous or evergreen, hardy or not. In common garden usage, the term perennial is somewhat loosely employed in a more restricted sense to include only herbaceous plants and sometimes a few sub-shrubby sorts that persist for more than two years. It excludes trees, shrubs, and woody vines.

In temperate regions the term perennial is often still further limited to plants that are sufficiently hardy to live through the winter outdoors. A more precise identification for this category is hardy herbaceous perennials. Familiar hardy herbaceous perennials include anemones, aquilegias, artemisias, asters, astilbes, campanulas, centaureas, chrysanthemums, coreopsises, delphiniums, echinaceas, geraniums, geums, hemerocallises, heucheras, hollyhocks, hostas, iberises, irises, poppies, and many more.

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- Courtesy of NYBG Plant Information Service