So i cannot personally say how low the temperatures can go, but mine have survived perfectly fine with facing 0C/-1C temperatures, they just really don't do well with really icy winds..
Plus they do go dormant and completely drop all their leaves. The big key i found with them is protecting the roots, you'll find that here in SA they are planted in relatively (by international standards) deep pots, this is done for both high and low temperatures protection, as you'll find, they will drop their leaves if temperatures get high enough as well, learned this the hard way...you would think a african tree would survive in the african sun, but as always a tree in a pot has very different requirements to one in the ground