The best time to plant annuals depends on the specific plant and your climate. Annuals are designated as "cool-season" or "warm-season," based on their hardiness and ability to grow in cool soils.
Cool-season annuals, such as pansy (Viola), primrose (Primula), and calendula, grow best in the cool soils and mild temperatures of spring and fall. Most withstand fairly heavy frosts. When the weather turns hot, they set seed and deteriorate. If you live in a cold-winter area (Zones 1-6, 32-45), plant these annuals in very early spring, as soon as the soil can be worked. To bloom vigorously, they must develop roots and foliage during cool weather.
In mild-winter regions (Zones 7-31), many cool-season annuals can be planted in fall for bloom in winter and early spring; or plant them in late winter or very early spring for spring flowers.
Warm-season annuals include marigold (Tagetes), zinnia and impatiens. These plants grow and flower best in the warm months of late spring, summer, and early fall; they're cold tender and may perish in a late frost if planted too early in spring. In cold-winter climates, set out warm-season annuals after the danger of frost has passed. In warm-winter areas, plant them in midspring.
Careful soil preparation will help get your annuals off to a good start and keep them growing well all season. Dig out any weeds on the site and add a 3-inch layer of compost, well-rotted manure, or other organic amendment. It's also a good idea to add a complete fertilizer; follow the package directions for amounts. Dig or till amendments and fertilizer into the soil, then rake the bed smooth.
You can start annuals from seed sown in pots or directly in the garden, or you can buy started plants at a nursery. Nursery plants may be sold in flats, cell-packs, peat pots, or gallon containers; see Planting basics for tips and instructions. For best results, choose relatively small plants with healthy foliage. Plants with yellowing leaves and those that are leggy, rootbound, or too big for their pots will establish only slowly in the garden, and they'll usually bloom poorly.
After planting, water the bed thoroughly. Young seedlings or transplants may need water once a day in warm weather, but as they become established, you can gradually cut back. Apply a 2- to 4-inch layer of mulch (such as compost, ground bark, or pine needles) to conserve moisture and help prevent weeds from becoming established.
Mixing a complete fertilizer into the soil before planting will generally supply your annuals with nutrients sufficient for at least half the growing season. In cold-winter areas, an additional feeding after bloom begins will carry the plants through their season. Where winters are warmer and the growing season correspondingly longer, apply fertilizer again in late summer.