Astrophytum capricorne has grown quite well surviving -2F and even a few survived -17F this year but they were in a verticle crack near our house. This is seed off our original plant, these produce apomixis seed so these should have the same genetics as the rest of our plants. Plants are smaller than I would normally sell so they are priced cheaper than normal.
I do not guarantee these in a zone 5/6/7 because everyones growing conditions are different but this would be a good trial plant to grow in your cold outdoor garden. Try to plant outdoor 90 days before 1st hard freeze.