Petunias have one of the longest flowering seasons, blooming from mid-spring to late fall. Hybrids such as 'Purple Wave' have a trailing habit and are extremely floriferous. Besides white, they're available in several shades of pink, purple, blue and red. After all danger of frost has past, plant them in a sunny location that drains well or put them in containers or hanging pots. They need regular watering and feeding.
Zinnias are reliable summer bloomers. There are small, single-flowered types, large pom-pom types and everything in between. Grow them in a sunny location. They flourish in hot weather with regular watering and feeding.
Gaillardia is a summer-flowering plant that blooms all through the season and beyond. A North American native, it comes in bright yellows that are often embellished with deep maroon or rust-colored centers. They keep blooming whether you deadhead or not, but deadheading will keep them neat. Short-lived, they will live longer if divided every 2-3 years.
Globe Amaranths are usually covered in blooms all summer and fall. They retain most of their color when dried. Dry bunches in the shade for flower arrangements or potpourri. These deer resistant, drought-tolerant annuals look good in borders. Purple is the most common color, but they're also available in light pink, lilac, white and red.
Hydrangeas have a profusion of long-lasting blooms all spring and summer. Varieties include Bigleaf, Oakleaf, Panicle and Smooth. Hydrangeas prefer morning sun with afternoon shade in places that have very warm summers but take full sun for most of the day in cool regions.
Rose of Sharon is a perennial in USDA zones 5-8. It blooms in various shades of pink, peach, and red. It's a profuse bloomer from late spring until frost. Grow these Hardy Hibiscus in rich, well-draining soil in a sunny location. Give them afternoon shade in areas with hot summers. Mulch and keep the soil moist. Feed occasionally.
Coreopsis is an old-time favorite. It's a perennial in warmer regions but is mainly grown as an annual elsewhere. Once they start appearing in spring, they bloom non-stop until fall. Deadhead for more flowers. Grow these in a sunny area. They are great as ground covers and bedding plants.
French Marigolds are compact in size with a bushy, slightly spreading habit. Their African cousins are taller and upright, producing large pom-poms in yellow, orange and cream. Both these types, as well as the petite ‘Signet’ marigolds, love warm weather and bloom continuously from spring until first frost. Marigolds are easily grown from seeds but may not come true. Regular watering is necessary.
Yarrows can add color and variety to your summer garden with a long blooming season. Grow yarrow in full sun. The flower heads look good in flower arrangements, and cutting them off reduces self-seeding. They retain their leaves all winter in USDA zones 5-9.
Iberis (Candytuft) can grow in full sun as well as partial shade. Keep the soil moist. No garden should be without this native plant. New hybrids offer choices of form and color.
Purple Coneflower is propagated by root or clump divisions. Plant this perennial where you want it to remain. It doesn't like being disturbed. Flowering all through summer and into fall, all parts of the plant have medicinal properties.
Eryngium (Sea holly) has silvery-blue spiky flowers and unusual foliage. Tolerant of neglect, drought, poor soil, and salt, it is are a good choice for xeriscaping. Flower spikes work well in fresh and dried flower arrangements. Grow them in sunny areas.
Asters can be started from seeds, but purchasing young plants is best. Plant in spring for summer blooms that usually extend to fall. Asters do well in both full and part sun but won't tolerate too much heat. Moist, rich soil with good drainage makes them happiest.
Daylilies bloom from spring to fall with flowers borne on long stalks. Different varieties grow to varying heights and have different bloom times. Grow them from divisions. 'Stella de Oro' has a 5-month bloom span.
Rudbeckia is a wildflower that earned a place in the garden because of its large flowers and profuse habit. The contrast between the bright yellow petals and the brownish black center disc makes these large, showy flowers very striking. Rudbeckia is a perennial, but the smaller Rudbeckia hirta can be grown as an annual if started early enough. In most zones they start flowering in early summer and continue until fall. Flowering starts in fall and extends into winter in areas with hot summers.
Snapdragon is another reliable annual with a long flowering season. Snapdragons were an old favorite in summer gardens. Shades of pink, peach, yellow and red blooms open in succession on terminal spikes. Throats usually have a darker or contrasting color. Plant them in spring. Pinch the young plants to induce branching.
Bee Balm/Monarda is a North American native that blooms from early summer to fall with whorls of tubular flowers around the tip of each branch. Flower colors include red and various shades of pink. A perennial in USDA zones 4 to 9, Bee Balm can be planted in fall and early spring. Choose a spot with rich, well-draining soil and full sun to part shade. It attracts butterflies and hummingbirds.
Dahlias come in large dinnerplate size or as smaller plants with pom-pom and ruffled shapes. There’s an endless variety in solids, bi-colors, and variegation too. Although Dahlias are perennials in warmer areas, they need to be started fresh from tubers ever year in most parts of the United States. They can only be planted when the temperature is above 60o F.
Gaura is another plant easily propagated from seeds or by division of rhizomes. You can find colors from white to light pink to shocking pink. It's a North American native that forms large perennial stands. Great for butterflies.
(Credits: Gomphrena photo by Dwight Sipler from Stow, MA, USA [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons; Gaillaria photo byStan Shebs [GFDL (http://www.gnu.org/copyleft/fdl.html), CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) or CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5)], from Wikimedia Commons; Coreopsis photo by I, Hugo.arg [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5)], from Wikimedia Commons)