Sweet potatoes aren't started by seed like most other vegetables, they're started from slips. Slips are shoots that are grown from a mature sweet potato. You can order slips from a mail order or Internet catalog or you can start slips from a sweet potato you bought at the store or one from your garden. If you buy a potato from the store, be sure to find out if you're getting a bush type or a vining type.
To start your slips, you need several healthy, clean sweet potatoes. Each sweet potato can produce up to 50 slip sprouts. To create sprouts, carefully wash your potatoes and cut them either in half or in large sections. Place each section in a jar or glass of water with half of the potato below the water and half above. Use toothpicks to hold the potato in place (Image 1).
The slips need warmth, so put them on a window ledge or on top of a radiator. In a few weeks your potatoes will be covered with leafy sprouts on top and roots on the bottom (Image 2).
Once your sweet potatoes have sprouted, you have to separate them into plantable slips. To do this, you take each sprout and carefully twist it off of the sweet potato. Take each sprout and lay it in a shallow bowl with the bottom half of the stem submerged in water and the leaves hanging out over the rim of the bowl. Within a few days roots will emerge from the bottom of each new plant. When the roots are about an inch long the new slips are ready to plant. To keep your slips healthy be sure to keep the water fresh and discard any slip that isn't producing roots or looks like it's wilting.
Before you plant sweet potato slips, you have a little extra work to do. Sweet potatoes need loose, well-drained soil to form large tubers. You don't want the roots to face resistance when they try to expand within the soil. Loose soil is more critical than almost any other factor when it comes to growing sweet potatoes successfully.
Using a small hand trowel, dig a hole about 4" or 5" deep and 3" wide. Place one slip in each hole with the roots pointing down. Position the slip so that the bottom half will be covered with dirt while the top half with all of the new leaves is above ground.
Carefully fill the hole with dirt so that you don't bruise the new plant. Sweet potatoes don't like to be bruised or bumped around too much. When you have completely covered it with soil, gently press the plant and surrounding dirt to set the plant and to remove any remaining air pockets. Continue the same way until all of your slips are planted.
Once all of the slips are in place water them. You'll need to give them a thorough soaking until all of the surrounding dirt is wet. Stop watering before your mound starts to erode. New plants, like slips, need to be watered everyday for the first week and every other day the second week. Each week the watering will get a little farther apart until you're watering once a week. If the ground is very dry or you've had a lot of rain, you may need to adjust this schedule in your own garden. Sweet potatoes can withstand drought but they'll produce less, so make sure you water them during the hottest part of the summer.