Overwintering Spinach

In these days of climate change and Michigan’s unpredictable springs, I’ve had the most success planting spinach in late fall to over winter. With just a bit of minimal protection the spinach seems to do great. I’ve been saving my seed from the best overwintered spinach for 2 years now. My spinach survived -10 degrees Fahrenheit early this month and is doing great now that it got a few days of warmth and sun.

Basic protection: I put old shower doors over raised cedar bed.Doors don't even cover the entire bed, and snow covered the glass most of the winter. A proper cold frame should yield much better results, but my Spinach is still doing great.

Any body else in cold climates find this to be true? I’m just curious because most information I read and hear does not put Spinach as cold hardy down to 0 degrees Fahrenheit, and definitely not into the negatives.