a plant that completes its life cycle within a single growing season of usually two to five months. The seeds usually sprout in the spring or summer; by autumn (in temperate climates) the plant is bearing fruit and dying. Annuals include flax, millet, buckwheat, spring wheat, corn cockle, and wild oats. Annual plants that develop particularly rapidly are known as ephemerals; they often mature in four to ten weeks. Some annuals, including knapweeds and shepherd’s purse, can survive the winter in the rosette phase if germination occurs late in the season.
Annuals grow only in those regions where they can complete their growth cycle in the period of a single year. Their numbers are greatest in desert areas and fewest in tundras. Annuals are most common where the plant cover is the least dense. In areas where dense plant cover is present, as in meadows, the annuals generally obtain supplementary nourishment through parasitism or symbiosis; these annuals include such hemiparasites as Alectrolophus, eyebright, and cowwheat, as well as such mycotrophic plants as clover, alfalfa, and gentian. As growing conditions worsen (with increasingly higher latitudes in the northern hemisphere or higher elevations in the mountains), the number of annual plants declines. Some species that are not able to complete their life cycle in harsh conditions in one year become perennials; an example is annual blue grass, which becomes a perennial in arctic and alpine settings.