Tree ferns can stay outside all year in the UK but they need protection to survive. It’s best to take small tree ferns to a frost free shelter but once they get too big to store then leave them in the ground but wrap the trunk and growing tip at the top. Fleece and hessian sacking and straw are all good insulators but it’s a good idea to cover the whole set up with a waterproof outer layer as the growing point can rot if it stays wet overwinter