A covering simply draped over the plant won't protect the plant: the covering will quickly reach the same temperature as the ambient air (i.e. freezing) and in turn freeze whatever it's contacting (i.e. the plant).
Propping the cover up a few inches above the foliage, on bamboo poles or broom handles or steel spears, will, on the other hand, create an insulating cushion of air between frost and plant. A string of holiday lights draped over the plants will further warm the plants.It's more or less a rule of thumb that crops can be damaged when temperatures fall below 28 degrees for about five hours. This particular cold spell may in fact bring those conditions to Palo Alto; but in general, we just don't get that cold. Don't bother blanketing the babies until Wunderground is predicting 30°F or lower.
And finally, leaving those coverings in place all day may be just as bad as not having them on the coldest nights. Despite how cold it's been, we're also having daytime temps in the 50s and 60s. Which means that a plant left wrapped in plastic during these clear sunny days may, in fact, be sweltering! If you just can't bear taking the tarps off every morning and replacing them every evening, at least make sure there's a significant amount of open area near the ground for air to circulate.
There's no shortage of advice on protecting tender plants, so I'll leave that to others. I'll just end with this: much of California is, intrinsically, a desert, with hot arid summers and cold winters.
If you're losing sleep worrying about your Brugmansia and cherimoya specimens, perhaps it's time to ask whether you might be in zone denial? Not that even the best among us aren't… but as the climate continues to remind us who's boss, we might want to start planting a little more appropriately.
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