You might not think that a sweet potato plant (Ipomoea batatas) is tropical, but it's native to South America and grows as a long, trailing vine with fleshy, yellow-to-orange swollen roots. It can grow year-round in U.S. Department of Agriculture plant hardiness zones 9 through 11, but makes a good garden plant during the warm summer months in colder areas and can also grow as an attractive vining houseplant. You can start a plant by suspending a sweet potato in water; once you see sprouts, take cuttings called slips from its vines to make new plants.
To start a sweet potato vine, choose a large sweet potato at your local food store, picking one that's firm and without soft spots. Suspend the potato in a jar of water by placing three toothpicks into the potato, spacing them evenly around it, and using these to keep the bottom one-half to two-thirds of the potato in the water. Ensure the potato's pointed end is downwards, into the water, and use a jar large enough to provide free space around the potato.
Place the jar in a warm spot; a temperature of 75 to 95 degrees Fahrenheit is best. Since light isn't needed for initial sprouting, you can keep the jar in an empty cooler or other lidded container made of heat-resistant material, with a light bulb suspended inside for warmth; for safety, prop the lid slightly open to let excess heat escape. Check the potato every few days for new growth; it usually takes four to six weeks for slips, or sprouts, to appear and you can expect between 15 and 50 separate slips per potato; this depends on the number of "eyes" it has.
Producing and Rooting Slips
Once you see leaf clusters develop on the slips growing from the potato and the stems are about 12 inches long, you can remove slips and use these to start plants. Use your fingers to twist each slip off at its point of origin, or use shears to cut them free, but clean the blades by wiping them in rubbing alcohol between each cut to prevent spreading plant diseases.
To root each slip, remove bottom leaves so that none are under soil when planted and set the slip 3 to 6 inches deep in a 4- to 6-inch pot that has a drainage hole and is filled with moist potting soil. Keep soil lightly moist until you see new growth. You can also root several slips by placing these in a glass of water, but remove any bottom leaves that might be submerged and change the water every few days to keep it clean. With either method, roots should develop in a few weeks; for slips in soil, check this by tugging gently on the stem and feeling for resistance.
If you plan to grow a sweet potato vine as a houseplant, plant a slip rooted in water in a pot with at least one drainage hole that's half-filled with potting soil. Fan the roots out on the soil's surface, then fill the rest of the pot with soil, but first remove lower leaves so none are below the soil; new roots form from the upper part of the buried stem. For a slip already rooted in soil, transplant it into a larger pot with a drainage hole. Keep the new plant in a sunny, south- or west-facing window and water whenever the top inch of soil feels dry to your fingertip.
To grow a sweet potato vine outdoors in the garden, wait until weather warms and the soil is at least 65 degrees F. Choose a spot that gets full sun for most of the day where soil is well-drained. Add an inch of coarse sand to the area if your soil is high in clay to improve its drainage, and mix about 2 inches of compost into the soil to boost its fertility.
Set rooted slips about 15 inches apart and allow about 3 feet between rows so plants can spread well. Because sweet potato plants thrive in heat, you can also cover beds with hoops covered in plastic sheeting designed for use in the garden to raise the temperature, but slit the covers to allow excess heat to escape. Keep the soil evenly moist, watering whenever the top inch of soil feels dry to your fingertip.
Fertilize the plants once, about two weeks after planting them, using a 5-10-10 granular formula. Mix 2 cups into the soil beside each 10 feet of planted row, gently scratching it into the soil, then water it in well.
If you grow sweet potatoes in the garden, you can increase the size of your planting by taking cuttings and planting these in new spots, but do by early summer so there's still time for a crop to grow. Cut a piece of a vine that's about 12 inches long, remove all the leaves except for those at the tip and lay the cutting horizontally in a shallow trench in the soil, then cover the stem with soil. When planted properly, only the tip's leaves should show. Keep the soil moist and roots will form all along the buried stem.
You can also take cuttings in late summer or early fall to overwinter indoors as houseplants; these can be starter plants for the next spring. Plant cuttings in pots in moist potting soil, keeping them in a partially shaded spot outdoors until you see new growth. Then move them indoors into a brightly lit spot to over-winter.