How cold does it need to be before you have to blowout a system? It doesn't get that cold here in Sydney but I did a large project in a cold area this past Winter here (it's late Spring now) and I'm not sure if the system needs to be blown out to prevent damage each Autumn. It gets heavy frost most mornings and a bit of snow that usually melts as soon as it hits the ground maybe 4-5 times per year. It is a poly system running off a pump, and the mains and valves are about a foot and a half deep.
I had never even heard of blowing out systems until I came on Lawnsite.