Cosmos (C. bipinnatus) must be one of the easiest annuals ever. Sow its seeds once, and pink or white flowers come back year after year from their own seeds.
Flowers (mostly singles) start blooming in early summer and continue for months until the first hard frost. The wonderful Sensation strain is the best known of the clan, but cosmos come in many other flower forms―some have rolled or frilled petals―and in a range of solid colors and stripes.
OUR FAVORITES: 'Candy Stripe' produces white flowers with crimson borders or stripes and grows to 3 to 31/2 feet tall. Three-foot-tall 'Daydream' has petals of rosy pink that fade to pale pink edges. Psyche mix bears semidouble blooms and grows to 3 feet tall. Seashell mix (to 31/2 ft. tall) has rolled petals in creamy white and shades of red, rose, and pale seashell pink. Sonata mix, a 2-foot dwarf, bears many 3-inch single blooms in white, pink, and mixes. 'Versailles Tetra' (to 3 ft. tall) has 4-inch pink flowers and darker shading around a bright yellow eye.
Yellow cosmos (C. sulfureus) brings yellow and red flowers into the cosmos clan, but at a cost: Its seeds don't germinate as easily as common cosmos, and its flowers tend to be smaller (2 in. in diameter) than other cosmos. Many gardeners find it easiest to grow from nursery seedlings.
OUR FAVORITES: Bright Lights mix has large (21/2-in.) flowers of yellow, gold, orange, or scarlet on 3- to 4-foot plants. 'Lemon Twist' bears clear lemon yellow flowers on stems to 21/2 feet tall. Ladybird mix grows to only 1 foot in height. Sunny Orange-Red and Sunny Gold top out at 15 inches.