Can NiMH batteries charge at cold temperatures?
It's not a hard line. The battery doesn't just quit at -0.00001 degrees. It depends on the battery internal temperature and the status of the liquid electrolyte inside the battery. If it is frozen, no chemical reaction can take place. As Energized states, it will charge at 14 degrees F, 10 C, but it has half the normal capacity it would.

See http://www.robotroom.com/Weather-Station-Data-2.html for a empirical look at temperature vs charging results.