According to Gary Slusser :

PEX is not harmed by freezing.

Their metal fittings may be.

Plastic plumbing (ie: all plastic irrigation systems) is a lot more
forgiving of freezing than copper or iron because it stretches somewhat.

But, expecting it to stand up to a lot of freeze-thaw cycles
is just asking for trouble. Which is why you drain irrigation systems
for example.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.