I'm monitoring my quick hoops carefully this fall to see their limitations.  So far, lettuce with no protection has turned into a slimy mess and tokyo bekana that's not under cover has been moderately nipped.  (Not so much that the chickens don't think it's a treat, though.)

Meanwhile, under the hoops, lettuce and all types of leafy greens are still going strong.  The Persephone Days are here, so my winter crops aren't actively growing, but I planted so many that I'll still have plenty to harvest for weeks yet as long as the quick hoops mitigate the cold.

Broccoli, however, is a goner even under the row cover fabric.  I picked the last head yesterday and had to cut off about half the florets due to freeze damage.  My guess is that broccoli needs more protection than a quick hoop when the outside temperatures get down into the twenties.  Still, it's hard to complain when I can make a whole stir fry (minus the spices and rice) straight out of the garden on the last day of November.