Q: I have some plants I want to be sure survive any freezing temperatures. What should I do to protect them?
A: Watering landscape plants 24-48 hours before a freeze can help protect plants. A well watered soil will absorb more solar radiation than dry soil and will reradiate heat during the night. This practice elevates minimum night temperatures in the canopy of citrus trees by as much as 2°F (1°C). However, too much water can contribute to root rots especially if soil material contains heavy organic matter.
Plants in containers can be moved into protective structures where heat can be supplied and/or trapped. Containers that must be left outdoors should be protected by mulches and pushed together before a freeze to reduce heat loss from container sidewalls. Radiant heat loss is reduced by mulches placed around plants to protect the roots but remember to keep the mulch only 2-3 inches deep. I know there is a tendency for us to think a few inches is good therefore 5-6 inches must be better but too much can create an environment for disease.
For perennials, the root system is all that needs to be protected since the plants die back to the ground annually. Be patient, they should return when temperatures become warmer. Coverings protect more from frost than from extreme cold. Covers that extend to the ground and are not in contact with plant foliage can lessen cold injury by reducing radiant heat loss from the plant and the ground. Foliage in contact with the cover is often injured because of heat transfer from the foliage to the colder cover. Some examples of coverings are cloth sheets, quilts, or black plastic. It is necessary to remove plastic covers during a sunny day or provide ventilation of trapped solar radiation. A light bulb under a cover is a simple method of providing heat to ornamental plants in the landscape. http://extension.ifas.ufl.edu/hot_topics/lawn_and_garden/cold_protection_of_ornamental_plants.html