Jon this will help explain things about frost.
Im not sure if they have them signs in your area that state this
Caution bridges freeze before roads.
Im sure Cecil when he drove truck experieced this scary situation, i even have in a car right near house where we have a overpass bridge over I-80 that you drive over to get to the ramp, speed limit is 45 and ya better slow down as sometimes that bridge will have slippery surface but the street before and after it is just fine. Here is something interesting.
Think of frost as frozen dew. Dew forms when the temperature at ground level matches the dewpoint and the atmosphere becomes saturated. Frost requires the temperature and the dewpoint to meet and for moisture to condense. Even if the temperatures are low enough, if winds are high enough frost will not form due to evaporative cooling. If the winds are calm or almost calm then dew or frost will have a better chance to form.
Frost forms in several manners but there are two that are more common. Depositional frost is known as hoar frost. This is frost that forms when the dewpoint is already below freezing and the temperature ends up matching the dewpoint. The white crystals that form hoar frost can form and spread out in a crystal-like fashion. During the winter, hoar frost can form frequently on any surface that is exposed and not covered by snow, such as a tree or a car.
The other main type of frost can form often in Fall or Spring. It is simply dew that has formed while the temperature was still above freezing but as the temperature at ground level dropped below freezing the dew at the surface froze and because a frosty (but not a pure white) coating on the ground. This is the frost that either kill your early season plants or put an end to growing season.
It is possible for frost to form while the reported air temperature is above freezing. The air temperature that you hear on TV or radio or see online at various websites is taken at six feet above the ground level. Warm air is typically lighter than cold air, so on cold nights it is possible for the ground to be colder than just a few feet above the ground because of the density of the cold air. Typically as a rule of thumb though, you will not see frost advisories issued unless there's a solid potential for temperatures to bottom out at or below 35 degrees during the growing season.
Also when i watch weather they say something like this
tonight the low will be 40 and in the outlying area's there is a frost warning. Im far enough away from the lake which acts like a heat sink and temps will be colder at night by as much as 10 to 15 degrees.
Sal, im no expert for sure as im in different climate but i would think from what you mentioned about your weather Madeira would do well along with plant called Panache as they reportedly need heat and a longer growing season which you have. As how they would do with rain and humidity that happens in Florida im not sure how they would fare.