Never done it myself, but this is what my book says:
Six weeks before the planting-out date, place tubers in a box of moist sand, sawdust , or chopped leaves. Keep the box at 70-80 degrees F. When sprouts are 6 inches long and have little roots, snap them off the tuber. Cut off the bottum inch of the slip to help reduce tuber-born disease problems.
Sweet Potato, Kumera grows just like a potato doesn't it? The more you earth them up, the more you get. I believe the longer you leave them in the ground, the sweeter they get. I also think they like lighter soil, eg:sandy, than the potato. They also like a warmer climate than the potato, they are a tropical vegetable. Sorry, can't offer you much more help than this, maybe someone else can be more elaborate with details.
Put the sweet potato in water, but leave about half of the potato above the water. We have good luck cutting one side out of a half gallon juice or milk carton and putting the potato in it laying on its side then pouring enough water in to cover the botton half or a little more of the potato. Place it in a well lighted window sill. When the slips get around 5 inches long break them off the potato and stick them into another container that has a couple inches of mud in it. They will not have roots when you break them off the potato, but will grow roots in the mud. Some people put the slips in plain water, but the mud gives them nutrients to grow better. NOW if you are using a sweet potato purchased out of a grocery store there is a chance that it has been treated to prevent it from sprouting. I don't know how to tell other than putting it in water and setting it in light and waiting and waiting and waiting. Then you decide you have waited too long. Safest way is use a potato you know where it came from.
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