If you want to reduce the risk of a plastic water butt cracking or splitting during the winter you should ensure it is partially drained or, better still, completely empty.
It you leave a full butt outside during a severe cold spell during the winter there's a real risk the water inside will freeze, swell, and actually fracture the barrel.
It's likely to require a period of several days at sub-zero temperatures for a water butt to completely freeze up and potentially split - but it's better to be safe than sorry when the big winter chill arrives.
To keep your barrel safe during the cold winter months you should leave the tap or spigot open. This will allow the water can drain out so it won’t freeze and potentially damage your butt. If the tap is open and the barrel is empty, you can safely leave it outside all through the winter.
If you have a plastic barrel which doesn't have a proper tap attached - maybe one you've made yourself from a plastic dustbin, wheelie bin or other plastic tank - you should consider emptying it and leaving it on its side for the winter.
Some owners of water butts even recommend removing the downpipe, diverter, and connector but most people believe this is being over-cautious and not worth the effort involved when weighed up against the risks of significant freezing.
Another idea used by some owners is to put an old blanket or even a thick layer of bubble wrap on the top of their water containers during the winter.
Rainwater butts are simple devices which don't really need too much care - but the risk of splitting from expanding ice is real enough to consider taking precautions.
It's possible to buy a type of water butt that is capable of expanding when water inside freezes, without and damage being cause, and then return to its original shape when the temperature rises and thawing occurs. It's worth noting, however, that the vast majority of plastic water butts do not have this expansion capability.