If you know it is gonna frost you can put a garbage bag over them over night and this will help it hold its heat. If it goes below 55 degrees it will stress the plant and below 35 degree (fariheight) it possibly could kill the plant. Putting a thick layer of mulch will help keep the soil warm. But if you can (privacy permitting) Find a big bad to put over them. This works well. But be sure to take it off as early as you can the next day after the sun comes out.