Junipers and other evergreens remain metabolically active if ambient temps approach 40 F and need light at this point. Junipers are also generally reliably hardy in pots without any protection in cold that falls into the teens and lower. Back in the day, I always used turkey day weekend in zone 6 a MA to winterize my trees, as the temps were consistently falling into the lower 20's every night and the daytime temps were staying in the lower 40's. I can't speak to your given micro climate but these were the markers I used back in the day.