I have never grown sweet potatoes, but I will be participating in a community garden project next year that will grow some. The guy with sweet potato experience says they really need to be kept warm. They need a lot of heat. So early on we will cover them with clear plastic (watering will be done via soaker hose) and again near the end of their season if the weather isn't staying warm.
Just thought I would throw that out there for others to consider.
I like to think I know my way around a thriving vegetable garden, but each time I participate in a project with others, I pick up new pointers and try out new veggies.