Thank you both! Since posting I did find
this from the Unviersity of WIsconsin. But we did have several nights below 0 F, the lowest was probably -15 F. Most of that was with snow insulating the plants but there were one or two nights without it.
Planting a cover crop is a nice idea but I really do want a wheat crop... Is there any reason not to just hoe up half the field (it's about 1500 sq ft) and seed spring wheat?