The botanical genus Nemesia comprises about 50 species of bushy annuals and perennials native to South Africa. They are closely related to snapdragons and with them are in the plant family Scrophulariaceae. Gardeners use nemesia as cool-season bedding plants in spring or fall for use in containers, hanging baskets or companions to spring-flowering bulbs like tulips and flowering onions.
Three species frequent gardens: Nemesia caerulea, Nemesia strumosa and Nemesia versicolor. Nemesia caerulea grows as a tender perennial that survives only light frost in winter. Nemesia strumosa and Nemesia versicolor are annual plants that wither away after flowering and seeding and are killed by both hot temperatures, over 90F, and a fall frost. According to "Sunset Western Garden Book," most of the plants sold with the name Nemesia strumosa are actually hybrids between it and Nemesia versicolor.
Nemesia caerulea grows with woody lower stems and matures 16 to 24 inches tall and 12 inches wide. The stems grow erect or spreading and bear narrow green leaves. From early summer to autumn, this species bears pink, pale blue, lavender-blue or white flowers with yellow throats that are fragrant. The blossoms are stubby and smaller than those of the annual nemesias. Cultivar Innocence bears white flowers while Joan Wilder blossoms are deep lavender-blue. Perennial nemesia is winter hardy only in U.S. Department of Agriculture hardiness zones 9 and 10.
Flowers of both species of annual nemesias lack fragrance. The flowers are as large as 1 inch wide and appear from late summer to early fall as long as temperatures don't get too warm. Blossom colours are more diverse, ranging from red, yellow, pink, white, blue or purple and the upper and low petal lips may be contrasting colours. Close examination of the flower often reveals tiny purple veins and a yellow throat that looks more like a beard. Numerous cultivars exist with a wide array of colourful flowers.
Regardless of species, plant nemesias in a moist, but well-drained soil that is slightly acidic in pH. The soil should not be heavy or dense in texture and only somewhat fertile. Position them in partial to full sun, six to 12 hours of direct rays daily and water freely in dry and hot weather to promote continual flowering. Gardeners pinch back green stems during spring and early summer growth to create bushier plants. If over watered in too heavy of garden soil, stem base and root rots occur.