Coryphantha nickelsae is a very slow growing cold hardy species found just south of the Rio Grande river. These plants have taken -17F in our garden but have not been trialed further. They were in a very protected spot. So im thinking they are more marginal than that. Plants bloom in mid-summer with large yellow flowers. These are seed grown plants that are 6 years old. They are slow growing and slow to produce seed. Try in your garden at your own risk.