How to Germinate Periwinkle

Fill the cells of a plastic bedding flat with a high-quality commercial seed-starting potting mix. Moisten the soil well, and let it drain thoroughly. Do this two to three months before the last frost is expected to occur in your area; this allows the periwinkle seedlings sufficient time to germinate and grow before being transplanted in the spring.

Gently press three or four periwinkle seeds into each cell and cover them with a 1/4-inch layer of soil. Lightly spritz the soil with a mister to avoid disturbing the seeds.

Place the flat into a black plastic trash bag and set it in a warm room. A heating mat, although not mandatory, can help you to keep the soil at an optimal 77 degrees Fahrenheit.

Begin regularly checking the flat for signs of early germination after one week. According to Colorado Sate University Extension, the seeds should not take longer than three weeks to germinate. Remove the trash bag when you see small whitish-green sprouts breaking the soil, and place the flat on a sunny windowsill in a warm room; a consistent temperature of 70 degrees is ideal.

Water the seedlings only when the potting soil becomes dry. Over-watering can leave seedlings susceptible to various fungal diseases, a syndrome known as "damping-off." When it is time to water, irrigate well until water comes out the drainage holes.

Fertilize the periwinkle seedlings 10 days after germination with a water-soluble high-nitrogen fertilizer. Colorado State University Extension advises a 10-4-3 NPK formulation.

Thin the seedlings when they reach 2 inches tall. Select the most robust seedling in each cell, and use a small scissors to cut the others at soil level.

Transplant the seedlings to an undivided plastic bedding flat, spacing them 2 inches apart.

Transplant the periwinkle seedlings to individual 2 1/2-inch pots when they reach 3 inches tall, and return them to a sunny windowsill. Observe them for signs of "legginess," or a stretched-out, vine-like appearance featuring sparse leaves; this is a sign of insufficient light.

Fertilize with the high-nitrogen fertilizer in two weeks.

Select an outdoor planting site in full sun with well-drained soil with a pH of 6.0 to 6.5. Use a small trowel to dig a hole slightly larger around than each seedling's root ball. Upend the pots to gently remove the periwinkles, place each plant in its hole and gently firm the soil around the root ball. For best results, do this on a calm, cloudy day when the soil and air temperatures are consistently above 65 degrees. Use a soil thermometer to check soil.

Water well at soil level and mulch with a 1-inch layer of bark chips to prevent weeds and fungal diseases. Water thereafter when the surrounding soil is dry, and fertilize monthly during the growing season.