Nipped By Frost?

No problem with frost over the last decade for my beautifully-blooming 7-foot wide and tall jade tree/bush!

Australian native plant Eremophila glabra blossoming

Sky-Blue bearded iris blooming

January's nasturtium "ocean" flowing down my hillside garden

Last straggler tomatoes - tonight's salad so I can renovate the bed for more spring veggies.

Loaded Dancy tangerine tree

Cymbidium orchid reblooming for first time

Upright rosemary blooming, although much more brilliantly blue than this shows

Brilliant orange of Leonotis leonurus, variously called Lion's Tail, Lion's Ear, or Lion's Mane.

Broccoli - first main head removed; secondary heads developing at each node.

Harvesting kale, chard, spinach, lettuce - remove all but the top several leaves, which will continue growing for harvests through spring and early summer, when most will bolt and go to seed with the heat.

Lacinata (dinosaur) kale cut into 2" pieces, massaged a bit with oil, lightly salted and dried. Munchies!

Easter Egg Radishes - wonderfully crunchy and mildly biting following all that rain!

Even some early (or late?) mulberries!

     The last several nights have threatened frost, even down here in the mid-lowlands of my garden in the southern section of Pasadena.  While the end of January is the average last frost date for our area, I haven’t worried much about it for quite a few years since we haven’t had even a light frost in more than a decade.  But the possibility is always there at this time of year, especially for more highland gardens than mine.  So, here are some thoughts to help you understand the possibility/probability in your locale.
 
Why Frost Happens
     Frost – or indeed any “bad” thing – actually happening in our gardens is the result of several factors that I’ve adapted from the Plant Pathology “Disease Triangle.”

1. Pathogen – the “disease” – or “pest” or “frost” mechanism. While diseases and pests are pretty much always around in our gardens during their particular life cycles, frost conditions exist only during our winters. But whether or not they result in damage in your garden depends on the other two factors.
2. Environmental Conditions. For pests, this means the temperature, humidity and other elements that they consider close to ideal and so they may thrive. For frost, this means not only being close to 32 degrees Fahrenheit freezing temperatures but also whether the soil is hydrated (our rains confirmed that), the wind is blowing (lowering the freeze factor or keeping the frost from settling), where the plant is located (next to a wall or out in the open), etc.
3. Host Plant Susceptibility. Is the plant more likely to attract the pest or be damaged by frost?If the plant is healthy and of a resistant variety, it’s less susceptible to a particular pest; if it’s worn out from a season’s growth or not resistant to that pest, it’s more susceptible. Regarding frost, if it’s a tender begonia, it is definitely more open to frost damage; but if it’s a deciduous plum tree that’s already dormant, frost is less of a potential problem.

     All three of these conditions must be met for the pest or the frost to affect the plant.  If any of the three isn’t existent, then chances are there’ll be no damage.  Also, time is a limiting factor – long enough for all the conducive conditions to be in effect for the pest or frost to actually inflict damage.
 
How to Protect Against Future Frosts
     You can change a couple of the environmental conditions to preclude frost damage.  First, recognize that frost falls straight down on still nights.

1. Move container plants under eaves so they’re shielded by the roof above.
2. Provide top covering for plants in the open. For example, if you’re still nurturing old tomato plants for those last few slowly-ripening (or at least coloring-up) fruits, provide cover above the plant that extends out just beyond the foliage. Unless you also get freezing winds, no need to wrap the plant entirely; the top protection is most critical.
3. A caveat regarding using plastic, however – don’t let it touch the plant leaves, since this will literally transmit the chill damage directly where it touches.Let there be some open air to act as insulation between the plastic and the plant foliage.
4. Keep plants hydrated. With all of our wonderful rains so far, plants may be fine for at least a couple more weeks before needing irrigation or more rain.

 
If Some of Your Plants Have Already Been Nipped
     You may not know for several days whether a plant has been damaged by frost, since the foliage may not go limp and shrivel for a while. 
     When you do see damage, it’s best to not trim it off immediately, even though this may offend your sense of aesthetics and desire to keep plants neatly trimmed.  The dead foliage will serve as protective interference for the tender interior of the plant from further damage from later frosts.
      Although it may take a month or so for new growth to appear, it will define precisely how far the dieback actually occurred on each branch – sometimes considerably less than you would have guessed when first observing wilted leaves.