Frost, freezing damage and mulch

Australian gardening books and magazines often encourage gardeners to insulate their plants with mulch to protect them against frost damage. Would you like to comment on this advice?
While this advice may be appropriate for gardeners in the Northern Hemisphere where the ground freezes in winter, it is not correct advice for most Australian gardeners. In very cold climates, plant desiccation in winter is a major problem. Gardeners are encouraged to water their plants thoroughly just before the ground freezes because moisture is then not available to the roots. Mulching is often recommended to conserve soil moisture. Dormant plants such as rose bushes and grape vines are often completely buried in mulch to protect plant tissues from the extremely cold air and especially cold winds which are also very drying. In these climates the soil temperature may be higher than the air temperature, so the insulating properties of mulch are an advantage. Snow is also an effective insulator. Damage to plant cells is most likely to occur during rapid changes of temperature. Rapid freezing and thawing is much more damaging than a blanket of snow. The plant's natural cold protection mechanism, which involves chemical cellular changes, may not be able to respond quickly enough to cope when weather changes are rapid. (Plants that are frost tender however eg those from tropical climates, will not have these mechanisms and will quickly die from cell damage as a result of cold and frost.) In severe winters, mulch may help to prevent rapid temperature changes, including occasional warm winter sunshine which can cause frozen plant tissues to split.
Conditions are quite different in Australia. Here, the winter sun is consistently quite warm and temperatures rarely drop below -6ºC. The soil absorbs heat during the day and radiates it at night, so reducing the likelihood of frost. Radiant heat is lost most quickly on still, clear nights and frosts are most likely to occur under these conditions. If garden beds are mulched with organic material such as hay, straw, pine bark, compost, etc, the soil is unable to absorb much heat during the day. (Just as mulch keeps soil cooler in summer by blocking solar radiation, in winter the same blocking means there is less stored heat in the soil to prevent frost.) During the night the heat is quickly lost and the surface freezes. This is why you will often notice frost lying on mulch while bare soil is frost-free.
For Australian gardeners, the best way to avoid frost damage is to have soil that is clear of organic mulch and weeds. In frost-prone areas raking mulch away from plant roots in winter will maximise the heat that can be absorbed during the day and radiated at night. Moist soil absorbs more heat than soil that is dry, and soil that is uncultivated absorbs heat more effectively. (If soil is light in texture or if it is 'fluffed-up' by cultivation, the air pockets act as insulation, preventing heat from being absorbed readily.) Bare, moist, compact soil will most efficiently absorb and radiate heat, so protecting plants against frost. Air temperature in such a case can be 1 – 2C higher than mulched or grassed soil. Mulch can then be reapplied once the danger of frost is over. The precautions discussed here are only worthwhile if you are growing plants that are likely to be damaged by the typical frosts your area suffers. If your plants are hardy there is no need to go to this trouble.
Profile: Dr Peter May is Deputy Head of Campus, University of Melbourne - Burnley College. He is a soil scientist with a range of horticultural interests including plant performance in the landscape, turf and viticulture.