TROPO's Organic Info Library

Will my Crop Survive this Winter ?

The first line of defence against frost damage is most obviously not to grow susceptible crops in frost prone areas. Growers wishing to produce tropical crops such as Abiu, Durian, Mangosteen, Rambutan etc. should move to North Queensland.

Site selection is therefore paramount when purchasing a property, as is identifying microclimates within your property for growing frost susceptible crops. On the north coast of NSW, elevated areas are least prone to frost, with north-east slopes the warmest.

The free movement of air to flow down the slope is important, so dense windbreaks such as Barner grass across the bottom of your paddock, should have gaps to let cold air flow out and away.

Growers intending to produce, for example, zucchini in winter, should select these types of low risk, warm locations. Avoid gullies and hollows, even in supposedly frost free areas, as these trap cold air and have proven to be problem areas. Close to forest edges and under large trees tend to create environments where frost doesn't settle. Paddocks protected from the south and west breezes suffer less frost also.

There will always be growers wishing to grow subtropical crops in locations where some frosts occur, especially in the colder winters we can experience during dry years. These may be growers of tree crops only susceptible to damage when young, or the vegie grower hoping to take advantage of a mild winter to make an early planting for their melon or tomato crop.

Protecting Young Trees

There are many ways to aid young trees to survive frost, with the scale of the planting and its associated costs being the only limiting factor of such protection. It may not be good news to organic growers, but mulch will attract ground frost where where bare ground usually will not. This has been demonstrated.

Ground frosts can girdle young trees by burning the bark at ground level. Pulling mulch back away from the trunks of very young trees will help, as will irrigating to keep the soil moist. Wrapping the trunks with materials such as newspaper, cardboard, aluminium foil or sisalation will prevent much of this damage. With more severe frosts canopy death can occur and trunk coverings need to extend up beyond the graft union, so the tree can reshoot from undamaged buds above the graft once the wraps are removed. Tree guards surrounding the entire tree are very effective and may range from the old fertiliser bag to specially made plastic tubes which may become expensive on a large scale.

Although very effective, these tree guards may not prevent tree death with very heavy frosts on sensitive species.

Protective Environments Best for Starting Seedlings

The wisest plan of attack with early vegetable crops is to get a head start by raising your seedlings in hothouses or shadehouses before planting into the field. Since early planting will be the aim, the risk can be greatly reduced by using 4-6 week old seedlings and extendiing the planting date to say early September. The chance of spring frost then becomes the only worry.

Frost Protection Using Overhead Irrigation

Many commercial nurseries, orchards and small croppers utilise overhead irrigation systems to prevent frost damage. The system usually includes temperature sensors within the crop that automatically operate irrigation controllers to begin irrigation at two degrees above zero and continue until four degrees is reached. In multi-area systems the sensor is placed in the coldest area and a brief irrigation is applied to each area, with the monitored area last. This cycle continues to rotate until the sensor reaches the desired temperature. The aim of this system is to prevent frost from forming on the crop. For the early rising grower without automatic systems, a sensor can be used to activate an audible alarm to wake the grower, who starts his irrigation manually.

Nutrition Affects Cold Tolerance

Elevated levels of potassium in the plant has been shown to induce a greater tolerance of frost in many crops. Although this phenomenon is not fully understood or explained, trials have shown the benefits. Excess nitrogen, besides inducing susceptible new growth, can also inhibit the uptake of potassium and so should be avoided on frost susceptilble crops during winter.

Anti-transpirant Works as Frost Protectant

A commercially available product known as Envy has had good results with frost protection of crops. However, it is not a cerified organic product and so is not recommended to certified organic growers. Envy is a non-toxic , water-based, acrylic polymer used for reducing transplanting shock in summer and frost protection in winter. When sprayed onto the leaves it forms a semi-permeable membrane over the leaf surface, which allows the plant to breathe whilst reducing transpiration by up to 60%.

This reduction in moisture loss by the plant raises the level of turgidity in the plant, which has been shown to aid the plant in surviving periods of freezing. Freeze damage to plants occurs when ice crystals form around various bacteria present on all types of foliage. Bacteria such as Pseudomonas syringae cause the ice nucleation that initiates the freeze damage. The polymer coating formed on the leaf by Envy creates a bio-shield against these ice nucleation bacteria. Crops that remain dormant during winter, with no new growth evident, usually only require one application at a lower rate to protect each flush of growth that occurs.

Whilst this product has proven it can work, its success will still depend on the plant species, its level of susceptibility to frost and the degree of frost occurring.

Spring a Danger

After last winter with a moist June, followed by a number of frosts in July, we received heat wave conditions in mid - August, with daytime temperatures in the mid '30's and night temperatures about 15ø. Spring had arrived with a vengeance. Then in early September we received two mornings of late frosts. Until then all my efforts had worked well in frost protection. On those two mornings, I lost a number of young subtropical fruit trees which were surrounded by plastic tree guards. Many other growers lost trees and vegie crops also.

My trees didn't blink during the July frosts, which were greater in number and seemed lower in temperature at the time. The sap had started to flow as growth had sparked by September and these trees had become highly susceptible, even though no new growth was apparent.

So as I sign off late on a June night, I can hear the latest shower outside watering their replacements. They said the drought would end in a flood and it did. Its been raining almost ever since and many are starting to whinge, but not me. Whilst we have moist soil, frosts are unlikely. It's spring I'm worried about.

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