Garden stalwarts, zinnias have been a favorite for generations. They're a snap to start from seed and their bright colors are wonderful for late-summer bouquets. Dwarf types grow just 6 inches but others can grow up to 3 feet. Zinnias are ideal for attracting butterflies, who feast on their nectar. Common zinnia (Zinnia elegans) is the best known but narrow-leaf zinnia (Zinnia angustifolia) is quickly gaining in popularity because of its low, sprawling habit.