It can be grown in an organic program, if planted in healthy soil and in full sun. The soil and air both need to be warm so don't jump the gun in the spring. Obviously we don't have that problem now - it has finally gotten plenty hot. Here's the rest of the information on this beautiful plant.
PERIWINKLE Catharanthus roseus ca-tha-RAN-thus to-SAY-us
Annual – Full Sun Height. 9”-12” Spread 12”-15” Spacing 9”-12”
HABIT: Low, compact annual for dry, extremely well drained areas.
CULTURE: Plant in any well-drained bed in full sun after the weather turns permanently warmer.
USES: Summer color.
PROBLEMS: Too much water or planting too early in the spring is sure death for this plant.
NOTES: Always plant the dwarf varieties so they won’t droop over. Photo shows most popular ‘Bright Eye.’ Native to Madagascar. Many gardeners have stopped planting periwinkles because of the disease Phytophthora, but it can be avoided with the organic program.