Annuals fill the garden with quick, dependable color in every imaginable hue. These are plants that germinate, flower profusely, set seed, and die, all in the course of a single growing season.

In contrast, biennials take two seasons to complete their life cycle, while perennials can live and bloom for many years.

Though the annual-biennial-perennial distinction seems clear on paper, in gardens it's somewhat blurred. For example, some tender perennials — such as geranium (Pelargonium), some kinds of salvia, and verbena — flower year after year in mild-winter climates, but are grown as annuals where winters are cold.

Plants called biennials typically complete their life cycle in 2 years. During the first year, they grow from seed into leafy but nonblooming plants. They live through the winter, then bloom, set seed, and die in the following year. This is the life cycle you'll observe if you start seeds. Biennials you buy from a nursery usually bloom the same year, however; the grower has taken care of the first phase of growth for you.

Plant breeders have worked to push biennials into either the annual or perennial category; there are now annual strains of hollyhock (Alcea), for example, and some foxgloves (Digitalis) are perennials.

To grow biennials, start seeds in pots or directly in the garden at the time indicated on the seed packet — usually mid-spring or late summer.

• WARM-SEASON ANNUALS

Charles Mann
Cosmos bipinnatus

• Warm-season
• Annual
• Full sun or partial shade
• Moderate watering

Showy daisylike flowers nod above lacy foliage from summer through fall. Plants are fast growing, with heights ranging from 2 to 7 feet; tall sorts are good for background planting. C. bipinnatus has flowers in white, bicolors, and shades of pink, lavender, purple, and crimson. C. sulphureus, called yellow cosmos, has bold yellow to deep orange blossoms. Both species grow best in soil that is dry and not very fertile. Space 1 foot apart.

Joanne Pavia
Ageratum houstonianum

FLOSS FLOWER (Ageratum houstonianum)

• Warm-season
• Annual
• Full sun or partial shade
• Regular watering

Fluffy flower tassels come in azure blue, lavender, pink, or white on plants 1 to 2 1/2 feet tall. Blooms from early summer to fall. Space dwarf varieties 6 inches apart, tall ones 1 to 1 1/2 feet apart.

Saxon Holt
Gomphrena globosa

• Warm-season
• Annual
• Full sun
• Moderate watering

Cloverlike blossoms in red, pink, orange, purple, and white are borne on plants 9 inches to 2 feet tall. Easy to dry for winter bouquets. Space plants 8 to 12 inches apart.

IMPATIENS WALLERANA

Susan A. Roth
Impatiens wallerana

Invaluable for providing months of color in sites too shady for most other annuals. Flowers are single or double and come in every color but blue. Dwarf varieties grow 4 to 12 inches high; space these 6 inches apart. Tall kinds reach 2 feet; space 1 foot apart.

MADAGASCAR PERIWINKLE, CATHARANTHUS ROSEUS (Vinca rosea)

Michael S. Thompson
Catharanthus roseus

• Warm-season
• Annual
• Full sun or partial shade
• Moderate watering

Phloxlike flowers bloom in shades of pink, rose, lavender, and white on plants 4 to 24 inches tall, depending on the variety. Thrives in hot conditions, whether dry or humid. Space 8 to 12 inches apart.

MARIGOLD (Tagetes)

Norman A. Plate
Tagetes patula

Robust, fast growing, and virtually trouble free, marigolds come in vibrant shades of yellow, orange, and orange red, as well as white and bicolors. Foliage has a pungent scent. T. erecta, called African marigold (though all garden marigolds are actually descended from species native to Mexico), has large blossoms — fully double in most varieties — on plants that grow 20 to 36 inches tall. T. patula, the French marigold, bears single or double flowers and grows 6 to 18 inches tall. Space dwarf varieties about 6 inches apart, taller kinds 1 to 2 feet apart. Stake tall marigolds early in the season to prevent them from toppling.

Claire Curran
Petunia hybrida

• Warm-season
• Annual
• Full sun
• Regular watering

Richly colored flowers come in red, pink, blue, purple, yellow, cream, white, and bicolors. There are two main flower types, single and double; single blossoms are simple trumpets, while double ones are ruffled blooms resembling carnations. Plants range from 8 to 27 inches high. Space 7 to 10 inches apart.

Tom Woodward
Helianthus annuus

• Warm-season
• Annual
• Full sun
• Regular watering

Huge, radiant blooms in yellow, orange, maroon, creamy white, and bicolors. Depending on variety, plants grow 2 to 12 feet tall, with flower heads from 4 to 12 inches across. The flowers are followed by edible seeds that are much relished by birds. Tall sunflowers should be staked. Plant 1 1/2 feet apart.

Michael S. Thompson
Zinnia elegans

• Warm-season
• Annual
• Full sun
• Regular watering

Borne on plants 1 to 3 feet tall, colorful daisylike flowers bloom in shades of yellow, orange, red, pink, and purple, as well as white and bicolors. Space 6 to 12 inches apart. Zinnias are susceptible to mildew; to help prevent it, water at ground level rather than sprinkling.

• COOL-SEASON ANNUALS

Marion Brenner

• Cool-season
• Annual
• Full sun
• Moderate watering

Bushy, upright plants with pungently scented foliage reach 1 to 2 1/2 feet tall and bear abundant blossoms reminiscent of double daisies. Flower colors include orange and bright yellow as well as white and more subtle shades of cream, apricot, and soft yellow. The petals are edible, with a slightly tangy flavor. Space plants 12 to 14 inches apart.

PANSY, VIOLA (Viola)

• Cool-season
• Annual
• Full sun or partial shade
• Regular watering

Pansy (V. wittrockiana) has 2- to 4-inch flowers in white, blue, mahogany red, rose, yellow, apricot, and purple; the petals are often striped or blotched. Viola (V. cornuta) has blossoms about 1 1/2 inches across, in bicolors as well as many clear solid colors. Both pansy and viola grow 8 to 10 inches high; space both 6 to 8 inches apart.

SNAPDRAGON (Antirrhinum majus)

• Cool-season
• Annual
• Full sun
• Regular watering

Blooming in bright colors, pastel shades, and white, snapdragons range in size from 6- to 8-inch dwarfs to 3-foot-tall giants. There are several flower forms; besides the familiar "snapping" type with upper and lower "jaws," you'll find snapdragons with double, bell-shaped, and azalea-shaped flowers. Space dwarf plants 9 inches apart, taller kinds 15 inches apart. Snapdragons are prone to rust, so choose rust-resistant varieties. Also avoid overhead watering, which can spread rust spores. (Don't let plants dry out, though; the disease spreads faster in hot, dry conditions.)

STOCK (Matthiola incana)

• Cool-season
• Annual
• Full sun or partial shade
• Regular watering

These old-fashioned favorites bear 1- to 3-foot spikes of clustered single or double flowers with a wonderful spicy-sweet scent. Colors include white, cream, pink, lavender, purple, and red. The soft gray-green leaves are long and narrow. Space plants 9 to 12 inches apart.

Charles Mann
Lathyrus odoratus

• Cool-season
• Annual
• Full sun
• Regular watering

Delightfully fragrant sweet peas bloom in shades of pink, purple, blue, salmon, and red, as well as white, cream, and bicolors. Plant size varies: bush types grow 1 to 3 feet high, while vines can reach 5 feet or taller. Space seeds or plants 6 to 12 inches apart.

Provide a trellis or other support for climbing types as soon as you plant them, since seedlings need support as soon as the tendrils form.

Charles Mann
Alcea rosea

CANTERBURY BELLS (Campanula medium)

• Biennial
• Full sun or partial shade
• Regular watering

Another choice for an old-fashioned garden, Canterbury bells sends up leafy, 2 1/2- to 4-foot stems bearing loose clusters of bell-shaped flowers 1 to 2 inches across. Blossoms come in late spring or early summer; besides the traditional blue, colors include purple, violet, lavender, pink, and white. Space 15 to 18 inches apart.

Claire Curran
Campanula medium

• Biennial
• Light shade
• Regular watering

This cottage-garden staple forms clumps of large, furry leaves from which tall flowering spikes (to 4 feet or higher) emerge in spring to early summer. The pendulous, tubular, 2- to 3-inch-long flowers bloom in white, lavender, pink, or purple. Volunteer seedlings often have white or light-colored blossoms. The leaves are a source of digitalis, a valuable medicinal drug (but one that is highly poisonous in the wrong doses). Space 1 1/2 feet apart.

Marion Brenner
Digitalis purpurea

• Biennial
• Full sun
• Regular watering

An old-fashioned favorite with 3- to 6-inch-wide, single to double flowers on stems that range from 2 1/2 feet to a towering 9 feet tall. Blossoms appear in summer; colors include yellow, cream, white, pink, red, and purple. Rust can be a serious problem; choose rust-resistant varieties, remove any rust-infected leaves you see, and avoid overhead watering (it can spread rust spores). Plants self-sow freely. Space 1 1/2 feet apart.