Dear Hetty, as you know it can get very cold and wet in the UK. Our rules for all terracotta pots are; never put them on grass (lawns), bark (mulch), or soil. Always keep them on a flat surface (concrete), and if possible supported by fire bricks,
Fire bricks are what you make a barbecue from or something to melt iron, they are readily available as any bricks that overheat when in the kiln become non porous, so they are cheap. This is ideal to stand large pots on as no water can get into the bottom of the porous terracotta pot. No problems in the winter at all, and they of course let the water out of the pots in the summer.
Regards.
Neil.
