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.