(Editor's Note: This article was originally published on December 16, 2007)
In the cold nighttime hours, moisture is squeezed out of the air, decorating your trees and plants in glittering splendor. But once the sun comes up, the frost starts to melt, and evaporate, both of which steal heat from the leaves and bark, lowering their temperature below even that of the chill morning air. Frost may be pretty, but its bite is hard on your bark.
Cover it UpThe simplest answer to frost is to not let it condense on your precious plants in the first place. Row covers on vegetables in the fall or spring provide both insulation and a barrier to frost. Painter's plastic, drop cloths, tarps, or even blankets can be pressed into service to cover your dwarf fruit trees or protect your beloved bougainvilleas when the weather unexpectedly turns freezing. I had a friend who used to buy blankets at the second hand store just for covering his palms on cold nights.
If you're only dealing with frost itself, or perhaps a few degrees in that sudden cold snap, you can't really beat simply covering it up for protection. If you need to protect it from more than that, you can add layers to your protection scheme. The blankets I mentioned above can really add insulation when combined with an outer tarp or plastic to keep the wind out. Remember to never place plastic directly in contact with leaves and plant material for more than a single night because it will encourage fungus and disease, which plants are especially vulnerable to in the cold anyway.
There's nothing like mulch for protecting ground plantings and things which come back from the stalk, like bananas. For a short cold spell you can pile up straw, leaf mould, or other light materials and then remove it when the danger has past to keep from smothering growing plants. Once again, plastic over the top of this, used judiciously, can provide even more protection.
Keep it WarmIn still air, as in a sheltered location, I have successfully protected plants with Christmas lights, using both miniature light strings and the larger socketed lamp variety. Don't use the new LED lights, though. Inefficient lights are what we're looking for here because all of that "lost" electricity is actually turned into heat. Don't forget to wrap the trunk down to the soil line, unless you're only trying to protect foliage.
It is tempting to wrap these up, lights and all, for further protection, but not wise. Covered lights are just as much of a fire hazard in the cold air as they are on your Christmas tree, as they can get very hot in the small location where a lamp touches a surface. I have seen tent-like structures built around plants, but it is difficult to guarantee that it will be sturdy enough not to skew and contact the lights.
Microclimates also provide protection. The area around my front door stays quite a bit warmer than the rest of the yard. Flowers and a potted Valencia orange live there year-round. Siting vulnerable plants and trees close to the house and in protected areas can help ward off future issues with frost and unusual cold spells. Avoid open, exposed areas and low places where cold air can settle.
Areas with morning sun are more vulnerable to frost because the sun helps the frost burn off quickly, imparting greater cold to your plants in the melting and evaporating process. If you've ever gotten out of a swimming pool on a warm day and felt a chill, you know how much evaporation can lower the temperature of something that is wet. The same applies for something with ice crystals on it which suddenly melt.
Move itOf course, if your most vulnerable plants are potted, you can move them into a shed or even the garage to keep them safe. Many fruit trees can be kept in large pots, and casters are available to help roll them around when needed. Beware that pots are more vulnerable to root damage in colder climates, so keep that in mind if you're pushing your zone limits with something more tropical than your winters normally allow.
Think Warm ThoughtsI've had my share of frozen seedlings and dropped buds from a late frost. I've lost that prize tomato that I kept on the vine just a bit too late in the season, hoping for it to ripen before the vines were wiped out. But with a bit of preparation and attention to some of these details, I've been able to extend my growing season and keep tropical trees outside in zones 7 and 8. I've even gotten the neighbors talking about the banana trees in my current zone 7 front yard and my early ripening tomatoes.
Although, it's possible they're just talking about that crazy guy who runs out in the middle of the night to throw bed-sheets on the plants in the fall.