It is that time of year again when old man winter blows in and brings a healthy plant’s worst nightmare – frost! Frost forms when the air temperature drops below the dew point and below freezing. Since you have already planted your fall veggies they should not be bothered by the cold, but any plants left over from summer/spring will probably not survive. We had a new crop of corn, tomatoes, and watermelon planted as well as peppers and eggplant leftover from the spring. These are not winter hardy plants, and ended up succumbing to the frost over the last week. You can try to protect your veggies by covering them with sheets, but often this is not sufficient protection and may require a heat source. We have been able to save a couple tomato plants by covering them with several layers of sheets and shining a 60 watt bulb on them to provide warmth.
Last winter we were a bit worried about our fall veggies freezing, especially our lettuce, spinach and peas. We attempted to cover them on several occasions to protect them. However, covered or not, they always ended up freezing but always thawed out later without damage. You can cover your fall veggies if you are concerned, but we found it unnecessary.
If you have young citrus trees it is necessary you protect them during frost. Keep your eyes on the weather report each day during the winter for any frost warnings and be sure you cover your citrus on these days. We cover all our young citrus with sheets and have 60 watt incandescent lamps running to each tree to add warmth. This is how we protected out trees last year, and all of them survived except for a lime tree. Lime and lemon trees are generally not as cold tolerant as orange trees and may require additional measures to protect. For more information about protecting your citrus during the winter see this University of Arizona article.