Donald and CJ have great advice on growing sweet slips, so I am posting their comments here so I can find them more easily.

I grew these for a few years at my Community Garden Plot in Hamilton Ont. Its a bit cooler here than Boston. To grow the slips, I put three tooth picks in the sweet potato and suspend them in a canning jar full of water. 1 ft slips are ideal. After the slips are 1 ft, put them in the canning jar to root. Space them 2 ft per slip and space the rows 3 ft and use Garden cover to heat the soil. They grow slow and then they they take off and the green leaves spread out. Near the end of summer leave them in until the night time temps are above 10c. Near the end of Summer the tubers will double in size every two weeks. From Donald January 24, 2010 8:55 PM

When the slips get about a foot long. Put them in water to start the roots. If the bare root slips are hardened off properly they won’t suffer much stress when planted. Row covers to heat up the ground will increase the size of your tubers.
From Donald March 03, 2010 2:29 AM

I started my sprouts the day after you did. I took the sprouts off when they were about 4 inches high. I put them to root in water like Donald suggested. Once the roots were about an inch and half long, I planted them in soil. One plant is now about 8 inches tall and the other 3 are about 5. I put 4 sweet potatoes to sprout and as you can see, one did quite well, it has about 15 leaves on it. Two potatoes had no sprouts and in fact started to rot. The other produced the other 3 smaller plants. Can’t wait to see the outcome this year. Sweet potatoes and artichokes are my experiments this year. From CJ March 03, 2010

It sounds like my little sprouts shouldn’t be in water yet. When they are 4 inches or 1 ft long, then I should put them in water to root. The rooting part seems to go very fast when they’re in water. The sprouting part starts slow.