After you tell us zip code and date you saw 20 below 0f in Tx we can get down to details.

I'm not sure if it is necessarily more cogent but this link provides a very thorough explanation of frost damage and recovery. https://www.fao.org/4/y7223e/y7223e0a.htm

And klem1, I don't think dan meant 20 below zero........the symbol he used only infers approximation, so a hard freeze of around 20F.

Yes "~" means "about". No, we don't do below zeroes. Thank you, gardengal48. That's a helpful essay. Yes, I think we're taking about extracellular freezing versus intracellular freezing. In the former, water is actually sucked out of the cells, but without seriously damaging them. The internal pressure in the plant drops, and leaves wilt. In the latter, the cell wall gets fractured, and the plant dies.