I have a couple of exotic trees in pots that I hadn't thought to bring inside, the forecast calling for mid-40s tonight. Now, at 6am, looking at weatherunderground I see that the North Central Phoenix station is reporting 36 degrees, though most other stations are reporting mid-40s into the low-50s. There's such a range! How do you decide when it's time to cover up the roses (guavas, mangos, etc)?
I think you can look back at a history with weather underground and get an idea of the weather patterns.
The way I work is that i'll check basic weather forecast for phoenix which is usually based at the airport. From there you're yard is going to be higher or lower than what is at the airport. And that's pretty consistent. I know my yard is WARMER than the airport so if the weather says 32 then I'm okay. If it's below that I usually cover.
Hope that helps but outside of having an accurate temp in your yard all night it gives you pretty close guesses. You can pretty quickly figure out your yard and then determine cold and hot spots based on microclimate.
I keep an eye out once the weather service hit the low 40's like today I start preparing for it. For me when Sky Harbor says around 37F as a prediction I will have frost on the ground the next morning because it is colder in parts of my backyard than the airport. Just a question of how far one is willing to go and whether there is someone home in the daytime or if protection is left on for winter. I see no point in protecting deciduous trees they are supposed to take care of themselves and our winter freezes are so mild they should die from embarrassment if they cannot survive the winter.
The evergreens are evaluated by known frost limits (for instance I don't bother to protect loquots or mature citrus, except limes). Immature evergreens need protection. Tropicals need protection as a rule past a certain point of frost. Protect the trunk, protect the canopy.
I move evergreen containers into a protected area under eave next to the home south facing and then eventually under a concrete padded covered patio where they still get direct winter sun.
What I do and has always worked for me is not watch the weather reports at all. I wait until someone says, normally at around 9-10pm that night, "wow, it's suppose to get down to 28F tonight". I then try to remember where all my drop cloths are and pretty much think to myself "Screw it. Any plant that dies back gets replaced." Then I see the devastation in the morning and ignore until say Feb. I trim back, see new growth, plants struggle to get back to full glory by say late Oct when I think to myself "no way am I replacing that plant, but this year I'm going to watch the weather and cover up these beauties".
sssshhhhh! Your not supposed to give away our secret. lol
I was surprise to see frost on the roof of my house this morning.