Frost Protection

Strangely, in most of the food growing books I’ve read, this issue hasn’t been covered very comprehensively as far as typical Australian conditions go. So here’s a quick note.

The case of frost usually covered is this: When temperatures get low enough, as frost occurs on plants, the cells inside the stem and leaf structures expand and actually shatter. You know this has occurred when you come out in the morning and your plants are totally wilted and keeled over. If this is the case, there is very little you can do except to design so as not to allow the frost to settle on the plants in the first place.

One way is to intercept and divert frost flow as it descends downhill, to stop it settling on plants in the first place. If you are on a slope, frost flows in a similar manner to molasses, and can be lifted or diverted using appropriately placed vegetation, earth banks and water bodies. When designing, consider the mature form of the plants you have placed in the system, and make sure you take into account cold air drainage. Don’t create frost ‘dams’ with dense vegetation at the base of vegie gardens for example.

Another common method is spraying water on the foliage. In the vegie garden, it may be possible to run outside in your dressing gown and spray with the hose as is often suggested. In large orchards, however, this is impossible. A common method used in this situation is to set up irrigation systems with a themostat that turns the sprinklers on when it gets cold enough.

There are many cases in Australia however, where frost settles on plants, but is not actually cold enough to shatter the cell walls. Still, damage occurs, which we refer to as frost burn. The reason this occurs, is that the frost on the leaves acts like a magnifying glass, intensifying the sun’s rays when they hit, hence burning the plant.

Rick Coleman lives in a swamp in Leongatha in Victoria, where every bit of frost from the region runs straight to his place. He can’t drain it away, and has had to find ways to design around this. He has designed and planted Macadamias as a large part of zone 3 in his system, and despite these plants being frost sensitive, he got his first nuts about a year ago. The way he went about this was through design. He first planted out sun traps, and waited until the foliage to the east of each of the Macadamia planting sites was dense enough. By doing so, the sun does not hit the foliage of the plant until about 9.30 or 10am, by which time the frost has already melted, and no frost burn occurs. The sun traps of course are multifunctional which consist mainly of Acacia dealbata or silver wattle: they have light colored foliage which intensifies the sun’s rays through reflection; the inner trees of the sun trap when planted in 2-3 rows can be pruned for timber production; they release nitrogen for the main crop when pruned and thinned; they are improving the soils; they pump down the high swamp water table; and they provide mulch for the Macadamias.

Cam Wilson, April 07