20 degrees? Yowzah!
I lived there (Metro Center area) for 7 years and we never did anything special for our trees...we also never really had weather that was that cold.
I believe the earlier poster is right concerning the wind keeping your trees frost-free. I’d leave stuff be and ride out the cold snap.
We had snow here in Sierra Vista this evening. My fig tree will probably lose its last few leaves tonight.
It is 28 degrees in my backyard this morning.
I was able to cover the ficus but not the jacaranda. The winds died down which I discovered about 1AM.
I guess we will experience freezing temps. again tonight as per the weather forecasters.