These Euphorbia leuconeura volunteers survived 3 days of mid-20°f hard freezes overnight.

And they are unharmed. I wasn't aware that this plant had cold hardiness. Each day was about 8-10 hours of hard freeze. These plants are open to the sky directly above the photo without cover. I'll keep observing them, but they seem totally fine. All of the cold damaged plants are looking like boiled spinach already.