Exacum is an annual plant that produces masses of bright blue flowers with yellow in their middle. It does not tolerate cold well, so it is grown as a summer annual in many places and as a houseplant in colder areas. The species commonly grown is Exacum affine, commonly known as the Persian violet or German violet. The plants prefer shade and to be kept moist.
The Persian violet is a rounded plant that commonly grows to be 0.5 to 1 ft (0.15 to 0.3 m) high. The leaves are small, heart shaped, and grow to be 1.5 in (4 cm). The blue flower is 0.5 in (1 cm) and shaped like a star. Different varieties exist that can have white or lavender flowers. A member of the Gentianaceae family, this type of plant can be biennial, but it is usually grown as an annual.
If grown under ideal conditions, the leaves will be tightly packed on the stems. The fragrant flowers will also be produced in great profusion. This combination of attributes, along with its easy growth, has made the Persian violet a popular houseplant and border or ground cover plant for gardens.
Exacum requires partial shade. For growth as a houseplant, bright curtain-filtered sunlight or indirect shade is ideal. The plants should be kept well watered. These types of plants prefer a well-drained soil that has peat moss and perlite in it. Ideal temperatures are 60° to 65° F (15.6° to 18.3° C) at night and 70° F (21° C) or higher during the day.
Cuttings do not grow well, and Exacum is best grown from seed. The fine seeds should be planted in the winter for summer blooms, or in the spring for flowers in the fall and winter. The ideal temperature for growing the seeds is 70° to 75° F (21° to 24° C). They take 15 to 30 days to germinate.
When the seeds have developed their first set of leaves, they can be transplanted into larger pots. Pots that are 4 in (10.5 cm) wide are ideal. The seedlings should be given fertilizer at alternating weeks until flower buds appear. Then, several Exacum plants can be grouped together in larger pots, such as hanging planters. Combining five plants together in an 8 in (21 cm) pot is recommended to make a nice display.
Young plants that are planted in the garden should be planted at 0.5 to 1 ft (15.24 to 30.48 cm) intervals. The plants generally bloom profusely for months. They are often discarded after they are done blooming. Some horticulturalists recommend keeping Exacum plants drier and unfertilized over the winter until growth resumes in the spring.