Our pots are best described as frost resistant. Generally, they will tolerate being outdoors throughout Winter, however, as they are hand made, it is possible that there are micro imperfections in the clay which, during the most extreme colds of Winter, could potentially allow tiny droplets of water in. When these droplets freeze and expand it is possible the pots could crack. Although we've never actually experience this happen thus far, it is possible and for that reason we cannot say the pots are "Frost Proof" but, for the vast majority, they will survive a normal UK Winter fine. Actually, a common cause of pots cracking arises from allowing the roots of the tree to grow too much, before trimming back and repotting. It is worth checking each of your trees before Winter arrives to ensure they have enough room to grow as Winter subsides and Spring starts to move in. Additionally, if a particularly cold patch of weather is expected, then you can always be extra safe and temporarily move your trees indoors, or into a cold-frame, for that added layer of protection.