Intellicast - Minneapolis Historic Weather Averages in Minnesota (55401)
Your weather doesn't look terrible. I've seen pex out in the open not freeze when it was 3F.
So I'm sure that in a crawl space with couple layers of insulation will be more than enough.
Also as a warning, if you have a septic tank a small trickle of water overnight, several nights in a row can hydraulically overload you absorption area.