D@ves Gardens says 20F for Mamillarria spinosissima .Others have it at 25F.I do see where 46F is mentioned as a minimum temperature. That might be a habitat minimum for the natural range that it is in. Mine has seen it below freezing but I take it in when it gets close to 30. All of this is given in a perfect situation, A dry in the ground situation and a brief period of the cold. Deserts plummet at night and warm back up i the day.Was your pot out in the weather and was the ground wet and how long was the freeze. The size of the pot will have the roots much colder than if it was in the ground. Also many cactus will automatically dehydrate themselves as a defense mechanism against the cold if they have been hardened of, and slowly rehydrate as it gets warmer. I do not know if this is what is going on. I would see if your soil its wet. I try to keep my cactus undercover, out of the rain when it gets under 40F. What is the soil mix like. Does it have a lot of drainage? Rot and cold are your enemy. 23 is cold. That is colder than all but the DG lowest rating.

I guess this comes under the heading of learning to question authority. Eh? HINT, Do it politely and quietly after the appropriate research. I guess there is no better time to learn about your parents fallibility. We are all only human. Great lesson for life..

Hidalgo Mexico was mentioned as its natural range. One might see if one could find the plant zone for the area. Remember, many times hardiness can be more than where it lives naturally. But that is a starting place to understand rain patterns and also when to start being cautious. A lot of cactus grow in areas other that the high desert. Hidalgo is not New Mexico or Nevada.

This post was edited by wantonamara on Sun, Nov 23, 14 at 19:12

Mine has frozen many times with no ill effects. However, it was dry and cold for a period of time before that. Going to 23F could be lethal, but you might not know for a few weeks. Unless the plant splits, sometimes the damage is internal and hidden until much later.

Sometimes the growing points are damaged, and the plant becomes monstrose, or at least get several growing points. I have a Mammillaria that is a beautiful crest after a freeze 15 years ago. (Heater malfunctions almost every year.) Some Aloes have several heads, and an Epithelantha micromeris has grown untypical side shoots. Then again, lots of species couldn't take the cold and went to the Big Desert in the Sky.
Maybe I'll start a "Damaged plant returned to vigor" thread. Tuesday.