Organic
I've got no trouble sprouting organic sweet potatoes. Just place them in a cup of water and put them in front of a window sill. Nearly every variety of sweet potato is available as organic for a little extra price.
Conventional
However, I have trouble growing conventional sweet potatoes not labeled as organic. In particular, I have never been able to sprout the Okinawan purple sweet potato, because the only place that sells it in California is 99 Ranch and they don't carry an organic version. I have been told that the conventional sweet potatoes are laced with growth inhibitors to increase shelf life. I have washed the outside of this potato before and it still does not sprout. Is there anything else I can do?
I am aware Whole Foods sells an organic Purple Stokes sweet potato. I have indeed sprouted this one easily, but I don't want this variety. I want to the Okinawan variety which is much sweeter and smoother.