Sweet potatoes (Ipomoea batatas), sometimes called yams, produce tubers that resemble potatoes in appearance but not in flavor. These summer vegetables grow during warm, frost-free weather from sprouts grown from a stored tuber. Although sweet potatoes are perennial in U.S. Department of Agriculture plant hardiness zones 9 through 11, they are usually grown as an annual vegetable. A raised garden bed in a sunny location provides optimum drainage and soil conditions, which can help improve the production and health of the tubers. Create a temporary raised bed in an existing garden, or plant them in a permanently edged raised vegetable bed.
Test the soil with a self-testing kit or by taking a sample to a testing lab to determine the exact fertilizer needs for your soil. If a soil test isn't done, apply a 6-inch deep layer of compost and 1/2 pound of 16-16-8 fertilizer for every 25 square feet of soil. Mix the compost and fertilizer into the top 6 inches of soil, then pull the soil up with a hoe to form an 8- to 10-inch raised planting bed. Alternatively, add 2 inches of fresh compost and fertilizer to the soil in an existing raised bed to revitalize it for planting.
Plant sweet potato transplants, called slips, in the prepared bed so that the leaves are above the soil surface. Space the slips 12 inches apart in rows set 36 inches apart in a row-style raised bed, or plant one slip per square foot in a block-style bed.
Water the sweet potatoes once or twice weekly for the first month after planting, or when the top 1 inch of soil begins to dry. Supply enough water to moisten the soil to the entire depth of the raised bed. Reduce watering after the plants establish, watering just enough so the soil doesn't dry out completely. Overly moist soil can cause the tubers to crack when they begin to form.
Sprinkle 1/8 pound of 21-0-0 fertilizer beside every 25 foot row, applying it 6 inches from the base of the plants, in early summer about six weeks after planting. Water the bed thoroughly after application so the nutrients soak into the soil.
Pull any weeds that invade the raised bed immediately so they can't form deep roots. Raised beds are usually less prone to weeds, so any weed problems are easily avoided by frequent pulling. Removing weeds and proper watering prevents most pest and disease problems.
Dig up the tubers in late summer or early fall when the foliage begins to yellow. Turn the soil carefully with a spading fork and remove the mature tubers from the loosened soil. Cure the roots for two weeks in an 80 degree Fahrenheit location, then store them for up to four months in a dry 50 to 55 degree location.