Zinnias are one of the easiest annual plants to grow from seed (perfect for a child's first garden) and they come in a rainbow of colors, blooming all season long until the frost. Their vivid blooms are the best-kept secret for attracting monarchs and other butterflies to the garden. Zinnia seeds are perfect for starting indoors to have bloom early. But be sure you wait until every frost is gone, and the soil has warmed up a bit before setting them out. They're super-sensitive to frost--after all, these plants are from frost-free regions, and don't like cold soil. If you'd rather, you can just wait until spring arrives, and sow them outside. They're very undemanding.