Items that are the most susceptible to being damaged by cold weather are going to be extremely tropical plants, plants that are not used to seeing the temperatures fall below the 40s. You will always run the risk of any tree being damaged however depending on how long it is below freezing and cold it gets, even desert acclimated plants can be damaged if it is cold enough.
You only want to cover your plants on nights that are predicted to be in the freezing zone, you do not want to cover them to early because you want the plant to be able to harden up naturally, and covering it when it’s not necessary can actually make the plant more dependent on the warmth than what may be necessary. If the night is predicted to be cold, wait as long as you can before you cover it, typically later in the night (8-9pm). You want to leave the plant covered until the sun has started to come up; do not leave it covered while the sun is up this can lead to more damage to your plants and trees.
Many people think that covering your plant is the only way to protect it from frost damage; this is not true, there are many other effective means of doing this. One option other than covering is creating a micro-climate in yard. By watering very heavy the night of the free, to the point where you have standing water on the ground, this will create humidity preventing frost from settling on the plants or trees as much.
Homeowners can also use flood lights and clamp lights as an effective way of protecting a tree from frost damage. By putting the lights focused onto the tree it will create heat that will not allow for frost to form. Other effective means of frost protection are using the old style Christmas lights (NOT LEDs) and wrapping your trees and bushes, this can create just enough heat to protect your plants and trees while adding a splash of holiday color to your yard. You can use large osculating fans to help move the air in your yard, simulating wind blowing makes it more difficult for frost to settle. Patio propane heaters are effective if you have a sensitive plant or tree, place the heater near the plant and let it run on low creating just enough warmth to keep the plant safe.
Very rarely do we get temperatures in the valley that will be detrimental to our landscape, but knowing what to do during these few times will keep your landscape looking great throughout the winter and ultimately save you money. Remember if you choose to cover your plants only use burlap or cotton, NEVER USE PLASTIC! Follow these simple tips and keep your landscape green!