_Vinca minor_, or common periwinkle, is easy to propagate from division, stem cuttings and seed. Dividing established plants is the quickest way to propagate, but if you want lots of new plants taking cuttings or sowing seeds may work better.
Common periwinkle stems are trailing and root easily, which makes them an effective ground cover for partially sun and shaded areas. Because they spread so quickly, periwinkle may become invasive, but this also makes it easy to propagate new plants by division.
Because periwinkle plants bloom in the spring, divide them in fall so the plants can focus their energy on root and leaf growth. Look at the weather forecast to find a cool, overcast day for dividing, and then water the plants well a day or two before you plan on dividing them.
Decide how large a clump you want to separate from the main plant, then use pruning shears to trim back trailing stems that are in the way, and clip any stems that are attaching your division to the rest of the plant.
Using a sharp shovel, dig all the way around the clump and lift it up. The plants are shallow-rooted, so you won't have to dig deep. Plant the division immediately, at the same level it was previously growing. Firm the soil around the plant roots, then water thoroughly.
Stem cuttings are another way to propagate periwinkle plants. Take cuttings from new growth any time during the growing season. Water the periwinkle one hour before taking cuttings, then use sharp scissors or pruning shears to cut the tip of a stem without flowers. Make each cutting 2 to 6 inches long, with at least three sets of leaves. After taking the cuttings, trim the leaves off the bottom half off each cutting.
Fill a pot with a rooting mixture composed of equal parts peat moss and sharp sand or perlite. Use a pot that has drainage holes. Water the mixture and let it drain before planting the cuttings.
Use a pencil to poke a hole in the rooting mixture, then place the cutting in that hole and firm the rooting mixture around the stem so the lower nodes where you trimmed leaves off are covered. You can put several cuttings in the same pot as long as the leaves do not touch.
Cover the pot with a plastic bag to hold in humidity, making sure the plastic doesn't touch the leaves. Leave the bag partially open to allow air circulation.
Place the covered pot in a warm area with indirect sunlight, and water as necessary to keep the soil moist but not soggy. After two to three weeks, the cuttings should have roots and you can remove the bag.
Continue misting the cuttings to keep them moist. After a few months of growth, move each cutting to a larger pot or plant them in the garden if there's no danger of frost and the weather is not excessively hot or dry.
If you want a few more periwinkle plants, it's easier to take cuttings or divide plants than to start seeds. Save seed from periwinkle plants if you have patented hybrids and want to experiment with different flower colors and plant forms, or if you want to start a large number of plants at once. Because only asexual propagation is prohibited under patents, you can propagate hybrid plants from seed.
Hybrid seeds will not produce plants that look exactly like the parent, which means you can get an interesting variety of flower colors and plant shapes from the seedlings. The seeds may be sterile, so consider this an experiment.
Watch the periwinkle plants closely as the flowers are fading and seed pods start to develop. Collect the seed pods as they start to turn brow, sniping them off the plant one seed head at a time.
Place the seed heads in shallow cardboard boxes and let them dry for one to four weeks. Once the pods are completely dry, shake the seeds out into a paper envelope and label them for storage. Store in a cool, dry place until you're ready to plant.
Start seeds 10 to 12 weeks before the last frost date in the spring. Use a damp seed-starting mix to fill a shallow container with drainage holes, then plant the seeds with 1/4 inch deep to block light. Spray the medium with water to settle the seeds.
Keep the tray in a dark or dimly lit room with temperatures close to 77 degrees Fahrenheit for the next one to two weeks until seeds germinate, and continue misting the tray to keep the soil moist.
After the seeds germinate, move the tray to a spot with indirect sunlight and maintain the temperature above 70 F. Water whenever the seed mixture starts to dry out. Transplant seedlings to individual pots when they reach 2 inches tall, then plant them in the garden after all danger of frost has passed.