The first rule of growing sweet potatoes is that they are not potatoes. That's right, Ipomoea batatas (sweet potatoes) is not even in the same Genus as Solanum tuberosum (potatoes). So, put away most of what you know about the nightshade tuber and embrace the morning glory tuber.
To start, they like heat. And despite what you may have heard, at least in our part of the world - Ottawa, Canada, they can withstand and even thrive in drought (I have two significant droughts in 2012 and 2016 with bumper sweet potato harvests and no irrigation in sandy drought prone soil to attest to that contrasted with poorer harvests in wetter, cooler years). So you won't want to plant out until after last frost in warm soil, even better if you pre-warm it with clear or black plastic mulch. You'll be planting directly into that.
You won't be cutting them up. You won't be cellaring them in cold, moist conditions. And you will ideally not wait until vines have been blackened by frost to harvest.
So how do you start them?
Using a process called slipping. Typically sweet potato sprouts will develop from one end more than the other though there are variations. Therefore you won't want to cut them up.* Some people skewer them and suspend them in glass jars full of water which will work but a more efficient technique, for me, has been placing them half way into a tray of moist starter mix. If they have little sprouts developing, I place those ends upright. Place them in a warm, sunny spot or under lights. Growth will start slowly with the leaves unfurling and them inching upward but suddenly they'll explode into vines.
Once they are about 4-8 inches or so, you can break the slips off the sweet potato. They may have roots on them or they may not. Either way, then you can plant these in a new tray of moist starter mix to grow on until planting. I've also seen people break the new unfurled sprouts off and grow these on. I've not tried it but it would probably work well too if you stay on top of the watering. The slips will keep coming for awhile so don't discard the tray of tubers right away.
Often sweet potatoes in storage will develop sprouts which will stay dormant for quite some time but grow as soon as the conditions are right. The longest I've tried starting a pre-sprouted tuber is 18 months. (Incidentally sweet potatoes, properly cured and stored, will store a long time.)
All sweet potato slips will be similar if not identical to their parent. Despite the fact that each should be clones, occasionally there are sports - mutations - that make the child plant slightly different from its parent tuber.
I usually don't plant out before the very end of May or beginning of June meaning that I'm slipping sometime in April normally unless I have some project up my sleeve meaning I need to start earlier. Harvest in Ottawa is sometime near the end of September or beginning of October, before first frost as the soil begins to cool. They will be damaged by temperatures less than 10C.
Make sure you harden off your rooted slips before planting out and enjoy!
2016 Sweet Potato Harvest
* Advanced observation - broken tubers often sprout more vigorously, earlier. I do not know what this means but I stand by the don't cut them up advice for now. This is probably a function of a stressed plant part desperately trying to survive.
All about growing, selecting and using edible plants in the Ottawa valley.