Years ago, when I bought my Cast Iron plant, it was considered cold hardy only in subtropical areas (which I am not in). They are even sold as houseplants, everywhere else. PDN sells these plants, and like many things in their inventory, rated as z7b hardy, and up.
For a few years after I got my plant, I grew it as a containerized plant that spent winters, indoors. It was also grown this way, because the start was small, and I wanted to grow it on to get larger. After those initial years, I set it out in my garden, and protected it, as a "z7b and up" plant. It came through those protected years, unscathed.
Cast Iron plants, although relatively slow to do so, produces suckers/pups, often inches away from the main clump. Last year, my plant produced a couple suckers that were outside the range of the two Walls o Water and styrofoam rose cones (I have 2 clumps now, due to pupping).

MY MINIMUM THIS WINTER ROUNDS OFF TO -10F !

However, as what usually occurs, when temps in my area, dip below zero, there is snow cover (often, several inches of snow on the ground). Since Cast Irons are relatively low, or ground-hugging, the exposed leaves were completely covered with snow, when my season lows occurred. Here is what both leaves look like, after the snow melted. You can even see a tip of the rose cone, in one pic. I guess, this plant lives up to its name:


https://photos.app.goo.gl/MQEZt2bX8Ahv267C9


https://photos.app.goo.gl/tXnMB7BSxmSxAdua7


Below are a few other things in bloom. One is a very light pink seed-grown Camellia japonica, and the other shows a pic of a dwarf Loropetalum 'Jazz Hands White', that has decided to bloom, now. I got the plant, back in fall, in a 4 inch pot. So I decided to keep it potted, until I decide on a place for it in the garden:


https://photos.app.goo.gl/JqqwTJairG7KPtE77


https://photos.app.goo.gl/St44f4AGnkcG8g6U8