Last fall I planted spinach for a fall crop, but waited too long and cold hit before the leaves had gotten the size of eggs, and it refused to grow any more. I plopped milk cartons over the plants thinking it might grow under the cloche-like thing, but it didn't.
I left them on all winter and they survived thru below zero temps. I didn't know this was possible.
They took off this spring and are now the most lush and thick spinach plants I've ever had, a month ahead of usual.
Charlie