10 degrees outside and 20 inside my sq foot garden cold frame. I didn't have a blanket big enough to cover it last night. I need to get myself a big painting drop cloth to drape over it. I don't have a thermometer inside the other cold frame, but the ground looks slightly frozen in there as well. The seedlings look unhappy, but not completely dead. 20 is definitely the coldest it has gotten inside one of these frames since I planted in them.
Nice applicable quote from Jeff Ashton's The 12-Month Gardener:
"As you experiment with season extension, you'll inevitably lose plants because of freezing. This isn't failure. If you don't lose plants occasionally, it indicates that you're not taking enough risks. Embrace your failure as a personal learning curve. Seeds are cheap, and success is the promise of great veggies from your garden while the neighbors are eating supermarket produce."