Unlike most vegetables, which are started from seeds or tubers, sweet potatoes (Ipomoea batatas) are started by planting slips -- stems that grow out of the eye of sweet potatoes. Sweet potatoes are hardy in U.S. Department of Agriculture hardiness zones 9 through 11, so they will only survive the winter in those climates. This sweet root can be grown as far north as USDA zone 3 with proper planning, however.
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Preparing the Soil
Because they are roots, sweet potatoes need loose, loamy, well-drained soil. Till the space or work it up with a shovel and remove all weeds and grass, but don't add fertilizer yet. Sweet potatoes are tolerant of acidic soils, to 5.0 pH, but prefer a pH between 5.8 and 6.2. To test soil acidity, use an at-home kit or contact a local extension service to see if it offers soil testing. If the existing soil is too alkaline, add 1 1/2 pounds of elemental sulfur per 100 square feet of garden space to lower the pH by 1 point. If soil is too acidic, add 3 pounds of lime per 100 square feet of soil to increase the pH by 1/2 point.
In zones 3 to 5, soil must be warmed up before planting potato slips. In early June, lay clear builder's plastic on the surface of prepared soil. Cut a slit in the plastic 12 to 18 inches apart to insert slips. Whether growing in northern climates under plastic, or uncovered in the south after temperatures have warmed, wait until soil temperatures have reached at least 70 degrees Fahrenheit -- 80 to 100 degrees is better -- before planting slips. In zones 3 to 5, keep the plastic on the soil until sweet potatoes are harvested. Green vines will grow and sprawl on top of the plastic while roots will stay snug at their desired temperatures underneath.
In areas south of USDA zone 5, warm soil by adding clear or black plastic to the bed before planting and remove it when the soil reaches the desired temperature. It is too hot in these areas to grow sweet potatoes under clear plastic.
Purchase sweet potato slips or grow slips by placing a sweet potato in a glass of water, pointy side up. Keep the bottom half submerged in water and the top half out of the water by placing toothpicks in the middle of the potato so that it sits on top of the glass. Only use seed potatoes. When slips are at least 4 inches tall, pinch them off the sweet potato and place them in another glass of water where they will produce roots. When roots are at least 1 inch long, they are ready to plant outside. The process can take up to eight weeks.
While it is possible to sprout store-bought potatoes, they are often sprayed with a chemical to keep them from sprouting.
Dig a hole wide enough to accommodate the roots that are growing on the slip, 2 to 4 inches wide, and deep enough to cover 1/2 inch of the stem above the roots. Water thoroughly immediately after planting and at least once a week or anytime the soil is dry 1 inch below the surface of the soil.
Fertilize sweet potatoes two weeks after planting with a 5-10-10 fertilizer. Use 2 cups of this fertilizer per 30 square feet of space by working it into the top inch of the soil with a rake, but be careful not to go deep enough to disturb the roots.