Freezes aren't that common in the part of the Bay Area where I live, so the garden advice is always repeated: "Do not cover your plants with plastic sheeting because any part of the plant that touches the plastic can freeze." Then they go on to recommend cloth, or plastic sheeting up on sticks away from the plants, etc.
Even in one's own yard, whether a large property, a "standard" suburban quarter-acre lot, the 50'x100' lot DH and I have now, or the 25'x80' (I think
80', might have been 100' from the street) lot we had in Berkeley, there are microclimates (sometimes referred to as micro-zones) where the temps can vary by as much as 5 degrees F (approx. 3 deg C). In December 1990 or 1991, whenever The Big Freeze that destroyed the California citrus crop hit, DH and I kept our cymbidiums and a few other plants alive on the patio in Berkeley in 28 deg F weather for several days, including one night at 25 or 26 deg, by:
1) clustering all the cymbidium pots under the avocado tree,
2) watering them with warm water right before going to bed (the planting medium only
, not the leaves or plants proper),
3) propping old sheets over the orchids and other container plants using sticks so we could get "just a couple more" plants
under shelter, and
4) running a heavy-duty extension cord and attached household heater (meant for use under a desk or table) under the sheet covering the plants while we were asleep. We later added a string of twinkling Xmas lights so that more plants would have something in the way of warmth.
It was quite a bed-time ritual for that week or so: first,
lugging warm water out to the plants in watering cans over multiple trips (that house had only a shower, not a tub, so the kitchen was called into service; the kitchen was farther from the back door than the bathroom was--of course); second,
setting up the sticks-and-sheet "superstructure," once we'd figured out just how many of our plants we could fit under it, and third,
turning the heater on and plugging the lights in.
We saved all but one of the plants, and all of the cymbidiums.
Nothing in the front yard died, though, so I think the front yard must have been 30 or 31 degrees with very little humidity. Certainly not nearly as cold as the back yard, a postage-stamp sized thing surrounded by fence, house, etc. with almost no air circulation.
Maybe some of these tips will help when the temps are threatening again.
And, believe it or not, I had to do Snail Patrol each of those freezing nights! The little beggars were trying to sneak onto the patio for the heat, no doubt, or to eat the other plants. Just made them more visible to me....
Sunset Zone 17, USDA Zone 9