Although ornamental and regular sweet potato plants (Ipomoea batatas) are the same species, the ornamental varieties are grown for their colorful purple, chartreuse, red or variegated foliage rather than for their tubers. With vines up to 10 feet long and 2- to 6-inch-long, heart-shaped or lobed leaves, ornamental sweet potato is perennial in U.S. Department of Agriculture plant hardiness zones 10 through 11 and sometimes as far north as USDA zone 9 when the ground doesn't freeze. Its tubers can be left in the ground in those USDA zones to resprout in spring. Elsewhere, the tubers or plant must spend winter indoors to survive.
Grow the Vine
Ornamental sweet potato grows best in well-drained and slightly acidic soil, with a pH level of 6.0 to 6.5, in full or partial sunlight. Large varieties such as "Blackie" should be planted 2 feet apart and smaller types such as those in the Sweet Caroline series 1 foot apart. "Blackie" and the Sweet Caroline series varieties are perennial in USDA zones 10 through 11. Fertilize with a low-nitrogen fertilizer, scattering 1 tablespoon of a 5-10-10 chemical fertilizer or 2 tablespoons of a 3-8-8 organic fertilizer on the soil around each plant, but don't let the fertilizer touch plant parts. After scratching the fertilizer into the soil surface, water the soil thoroughly. Give each vine's soil about 1 inch of water per week. Ornamental sweet potato seldom blooms, though it is possible for it to produce trumpet-shaped, lavender flowers from late summer through fall.
Preserve the Tubers or Potted Plant
If an ornamental sweet potato vine grows in the ground, then waiting until frost kills its foliage is necessary before digging up the plant's tubers to store them for winter. After removing their dead foliage, take the undamaged tubers indoors, cover them with damp burlap and allow them to sit in a spot that is 80 to 85 degrees Fahrenheit for 10 days or in a spot that is 65 to 70 F for two to three weeks. Following that curing process, place the tubers about 2 inches deep in barely moist peat moss or sand in a shallow, cardboard box. Then the box should be covered with damp burlap and stored it in a dim, cool garage or basement where the temperature stays 55 to 60 F. Moistening the burlap occasionally keeps the peat moss or sand lightly damp.
If you wish to keep a potted ornamental sweet potato vine growing during winter indoors, then place its pot on a warm windowsill where it will receive full sunlight, and treat the vine as you would a houseplant. Refrain from fertilizing it during winter.
Plant the Slips
In spring, six weeks before your location's average annual last frost date, the box holding the tubers can be uncovered and moved to a warm, sunny location in your home. At that point, the tubers should begin to send up sprouts. After the average last frost date passes and the sprouts are at least 6 inches tall, cut the sprouts from each tuber. Plant those cut sprouts, which are called slips, in your outdoor garden, with only the top one-half of each slip protruding above the soil surface. Keep that soil damp until the slips form roots.
Replant the Tubers or Acclimate the Potted Plant
Instead of forcing sprouts to grow on the tubers, you can keep the tubers dormant until planting time. Then cut them up -- leaving one "eye" on each piece of tuber; an eye is a small, dark circle from which a sprout will grow. Plant the tuber pieces 3 inches deep in your outdoor garden's soil, spacing them as you would if they were ornamental sweet potato vines.
An ornamental sweet potato vine that grew in a pot during winter requires a gradual adjustment to outdoor conditions. Take the potted plant outdoors after your area's average annual last frost date in spring, and set it in shade. Each day, move it a little closer to sunlight to accustom the plant to brighter light levels.