building a garden wall in winter

Yes it is possible to build a wall in winter if you follow a few guide lines. Bricks to be laid need to be dry as any wet ones when the temperature drops to freezing or below can blow the bed joint as the water that turns to ice expands. By keeping your bricks covered up when you are not using them will help to prevent this. Always use a frost proofer in the winter months and at the end of the working day cover your wall with a couple of layers of hessian.

Yes, use a frost proofed if it's going to be very cold overnight

as long as its 4 degrees above freezing and rising its not a problem, cover with hessian to keep the frost out overnight