I’m not a betting man but, given the recent weather patterns, it’s looking like this might be a cold winter coming. After the relatively mild winter last year and the near-tropical summer we just had, this might take a bit of adjustment for the cold-sensitive among us and the more cold-sensitive of our plants.
In our region, radiation frosts are the most common form of frost. This is where the plants lose heat faster than the surrounding environment, resulting in ice crystals forming on the surface of the leaf and into the plant cells. When the ice melts, the punctured cells collapse. A radiation frost can occur on any cloudless evening where the wind drops and the air temperature falls below 4C.
For protection against radiation frosts, the simple answer is cover; whether this be from overhanging foliage, a nearby wall or fence, overhanging eaves, canopy trees (even deciduous trees provide limited protection in a light frost), a greenhouse or conservatory, or any temporary protection the gardener provides.
There are several temporary protection techniques that gardeners can use, with varying degrees (sorry, couldn’t help but use that pun!) of success. Frost cloth hung over the plants is the most common and the most effective. For best effect, use stakes to prop it up or hang it from nearby structures or trees to keep it off the foliage. Any frost cloth touching the foliage will allow damage to occur, although less damage than if no cloth was used.
Other cloths, such as insect-proof mesh, shadecloth, windbreak or even an old sheet or net curtains can be used, though these may be less effective in a hard frost and may need to be removed during the day to let the plants get light and air. Although it seems counter-intuitive, thin plastic film is not as useful as these other covers. In a radiation frost the plastic doesn’t reduce the radiation heat loss to any large degree and any plastic touching the leaves can actually make the damage worse, as water trapped between the plastic and the leaf will freeze.
Don’t also make the common mistake of spraying the plants with water on a frosty morning. This is based on the proven orchardist technique of using water to prevent freezing damage to young growth and flowers. Unless, like these orchardists, you’re prepared to get up before the frost settles (probably about 4am!) you can be doing more harm than good as the rapid temperature rise can rupture even more plant cells than the frost will.
Another technique sometimes employed is spraying the garden with one of several commercially available frost protection products. While these have their avid fans, the claims of effectiveness are anecdotal rather than supported by scientifically based evidence and are unlikely to produce anything more than a feel-good factor for the gardener!