Will this concrete fence base heave/break during winter cycle?
Best advice is to dig the whole thing down to the level below frost line. It is also the standard practice to put down a bed of gravel at the bottom and lay in a reinforced concrete footing that lies completely below the frost line and is wider that the fence support wall. Say 12 inches wide. This gives the poured wall structure something solid to sit on that does not heave and will also help mitigate problems with ground movement and settling.