I grew a huge house plant several feet in many directions from a sweet potato in water in a hyacinth glass (narrowing to hold bulb/plant). Just went on all winter with only water. This was in a west facing bathroom winter in a cold climate but with house heat of course.

As you can notice from my UserName I keep sweetpotatoes over winter by by tissue culture to maintain line with least loss of germplasm. it is easier to just maintain the plant any asexual means by axilary growth. When grown from a root via regeneration (embryogenessis) there is risk of loss of chromosomes etc. Therefore if ou stand on a SP canning line you see shades of yello/orange roots come down the line.
The plant I use for my foundation Beauregard forcertified growers has been growing for 17 years. It is grown in 2 x 2 x 2 foot box under Two compact fluorescent lamps and ithas been down below zero Farenheit in that room several times. I use 2 lamps in case one burns out.

micropropagator ,
I am a total sweet potato fanatic. Could you please tell more about your 17 year growing sweet potato. I have just started a Japanese sweet sprouting.(purple outside/white inside taste like chestnut - dry flesh) it is my plan to use some of the sprouts (slips) in 2008 to plant outside for actual potatoes. I tried to get the Okinawa purple sweet to sprout but after 2 month - no luck. Please tell more about what you are doing. Thanks!!