Probably right around 34-35 degrees.
Tedesco covers our greens in the winter, but they put pins in the fairways and large cups so one can play a 2500y 9-hole winter course if there is no snow on the ground.
Many courses on Cape Cod stay open all winter, and there is one 9-holer on the North Shore, Kelley Greens in Nahant, that stays open all winter pending snow. Most of these courses allow play on the greens, but the tee will be in the rough somewhere in front of the real tees. I also noticed in Upstate NY in early January that most courses just heavily sand their greens as opposed to cover for winter.