if they are hardy, strong plants, one frost isn't going to harm your ladies. If frost comes though, unless its extremely early, frost will sure come again soon. If your ladies aren't done flowering, covering them with the burlap will most definitely help keep them warm and frost free. But this effect will only last so long. As the frost continues and temps keep dropping, you may need to pull them early before the frost freezes the plant cells, which will kill them. Good luck! any chance for pics on their flowering?