May is the month to plant warm-season annuals. These annuals need warmth to germinate and long warm days to thrive. Most warm-season flowers prefer days between 70 and 80 degrees F and nights no cooler than 55 degrees F. May fills the bill for temperature requirements and gets the flowers off to a good start.
Sunflowers, cosmos and zinnias are easy to start from seed and grow quickly. Most gardeners prefer the shorter sunflowers that grow to 5 or 6 feet tall rather than the behemoths that reach 12 feet. Sunflowers make good cutting flowers. Cosmos has feathery leaves and flowers in white, pink and purple. There is a variety that blooms in yellows and oranges as well. Cosmos grows to 4 feet high and 3 feet wide. The flowers are on long stems. Zinnias are an old-fashioned favorite of many. Standard zinnias reach 3 feet high with flowers that are 3 to 4 inches across in every color but blue. Rudbeckia, also known as coneflower, includes black-eyed Susan and echinacea, which has purple petals. The flowers bloom with the petals bending back from the centers, resembling cones or badminton birdies.
Short Mounding Flowers
Gazanias have daisy-like flowers with contrasting stripes and centers. The plants have deeply lobed leaves. They don't get much taller than 8 inches. Don't plant gazanias in the shade; they need to have direct sunlight to bloom. The buds won't open on a cloudy day.
Dusty miller has yellow flowers. However, it's used in the garden for its soft, fuzzy gray leaves rather than for its flowers. The plant stays 12 inches high until its ready to bloom. The stems of the flowers are up to 18 inches long. Alyssum is only 6 inches high but is covered in tiny flowers of white, light or dark purple. In mild winter climates, it will reseed itself and bloom year-round.
Sanvitali, also known as creeping zinnia, stays low and spreads. The flowers are small, about the size of a quarter. Galliardia has nondescript gray oval-shaped leaves. The flowers are vibrantly colored with burgundy centers surrounded with pinkish-orange petals edged in bright yellow. The plant reseeds itself and will come back year after year.
Portulaca almost looks like it's made of tissue paper. The petals are thin and vibrantly colored. The leaves are small and look like miniature ice plant leaves. They grow easily from seed, but the seeds are tiny. The plants don't grow more than 4 inches high or so but spread to 12 inches.