Where I live winters can freeze the ground solid. No digging possible. However, even in these conditions, in the woods, you can often still dig. The reason I know this is, I'm a metal detectorist, and I like detecting, even if it's cold, but can't dig in the dead of winter, unless I get in a wooded area that has leaf cover.
Snow can insulate the ground too, and create conditions conducive to digging, however, open ground freezes solid.....no metal detecting in the soybean and corn fields.
Tall, heavy grass, when laid over, can also insulate the ground, keeping it warm enough to be able to dig.