Periwinkles, also called vinca, produce vibrant blossoms above rich green foliage. Catharanthus roseus is the botanical name for these pert little plants. Periwinkles are perennial plants that grow as annuals in U.S. Department of Agriculture climate zones cooler than zone 11. The best time to plant periwinkle seeds depends on your particular climate and growing conditions.
Since periwinkles perform as annuals in most of the United States, starting them indoors in the early spring provides a longer growing season. Determine the correct indoor sowing time by counting backward about 10 to 12 weeks from your average last freeze of the season. Plant the seeds in biodegradable seed pots filled with seed-starting medium. Sprinkle three to four seeds in each pot and cover them with about 1/4 inch of planting medium. Spray the surface of the soil with a light mist of water and place the seed pots under a black garbage bag to block the light while they germinate. Remove the bag and transfer them to a sunny window after they begin to sprout.
Although they won't experience as long of a growing season and you will have less time to enjoy their bright, colourful blossoms, you can plant periwinkle seeds directly into the flower bed in arid climates or areas that don't experience wet spring rains. Plant the seeds directly into the soil when the outdoor temperatures maintain an average above 18.3 degrees Celsius. Placing a layer of black plastic sheeting over the planted seeds will help hold in heat and provide darkness during germination.
Fall and winter is the best time to plant periwinkle seeds in USDA zone 11, which includes Hawaii and very southern areas in Florida, California and Texas. Planting the seeds directly in the landscape during the dry fall and winter months allows the seeds to germinate and begin growing before the wet weather common in these climates during springtime. Planting during these dry months helps minimise the risk of fungal diseases, such as aerial phytophthora blight, in young seedlings.
While most periwinkle seedlings are prone to damage from fungal conditions in areas with heavy spring moisture, some cultivars are more capable of withstanding the rainy season. The Cora Series periwinkle is a type of periwinkle that is resistant to fungal diseases. Plant these periwinkle seeds any time of the year in USDA zone 11.