We planted Queen palms this summer and they have been doing well, however, we've had some cold spells recently and a lot of the fronds are turning yellow. Are they dying? Do we need to fertilize them again? Do we need more mulch? They are 20-30 feet high, with trunks having a 10-12 inch diameter. We're worried about what to do. They were professionally planted with a special mix from the gardeners at Moody Gardens and we don't want to lose them. Can you suggest something?