Last season the night frost caused a considerable decrease in the European apple harvest. But how can you best protect yourself from this? Jan Peeters (Fruitconsult) and Jef Vercammen (pcfruit) held a presentation on night frost at the Kennisdag Fruitteelt (Fruit Cultivation Knowledge Day).
What is night frost?
There are two kinds of night frost: there is the transport cold and radiation frost (classic night frost). The characteristics of transport cold are: wind that often comes from the east, clouds, often the higher the colder and hard to combat. For radiation frost it is: no wind, clear skies, colder closer to the soil and can be stopped. "Unfortunately night frost is often a mix of both," according to Peeters. The classic night frost is created due to the ground losing a lot of heat on clear and calm nights. There is a negative balance between heat going into and leaving the soil. This causes the air layer just above the ground to cool to below 0 °C. When it is cloudy there will be more heat going towards the surface. This causes a positive balance between the heat exchange which means it doesn't freeze at the ground. In Poland and Czechia tests are currently being conducted with Pulsfog by Brinkman. This is an article cloud (mist) based on water and glycerine. The problem with mist is controlling it. This makes it unsuitable in inhabited areas or near roads.
Blossom damaged by night frost
Factors determining damage
The amount of damage caused by frost is determined by a number of factors. It depends on the length of the frost, the degree of cooling, minimum temperature, the stage of development of the fruit, levels of moisture in the tissue and the previous weather conditions. There are also multiple environmental factors such as the location of the plot, the height, soil type and moisture levels in the soil. Then there are the plant dependant factors such as blooming time, the variety's sensitivity to frost and strength of the buds. There are therefore a large number of factors that play a role in the level of damage to the product.
Influence of soil
Research shows that bare and closed soil releases less heat than vegetated or loose soil. Peeters: "The more vegetation or loose soil, the more cold will build up. Sand soil freezes more quickly than clay. This has to do with sand drying more quickly than clay soil. Moist soil builds up 2 to 3 times more heat and heats the air 5 times more." Jan Peeters' advice: "If you wet the soil 4 to 5 days before the frost, it makes a few degrees difference, as the temperature will decrease less strongly."
There is a big difference between the frost sensitivity of apples and pears and between the different varieties. Gala apples, for instance, can handle more frost than the Elstar and Jonagold. Frost damage can be recognised by a lower production, frost noses, frost rings, ties, roughness and misshapen fruit. After blooming the flowers are the most sensitive and even a short freeze in the morning can cause damage. Due to the late frost in the night of the 19th to 20th of April a large share of the apple harvest failed in the Netherlands and in particular Belgium this year.
Apples with frost damage
Frost protection options
There are various ways to protect the plot against frost. The ways are irrigation, under crown irrigation, windmill (heated or not), paraffin pots, frost guard, frost buster and anti frost agents. Unfortunately irrigation isn't an option everywhere and so other options must be found. Paraffin pots are suitable for small plots. The smoke, however, can cause complaints and it is labour intensive. The Frostbuster is suitable for 8 hectare plots. Theoretically the gain is 0.5 °C and the cost of around 16,000 Euro is very high. If you do this over 8 hectares for 10 years, the cost is 200 Euro per hectare per year. There is also Frostguard, and unlike the Frostbuster this cannot be driven. The Frostguard can protect around 0.7 ha from spring night frost. The cost is around 6,500 Euro.
Jef Vercammen said that Belgica was the only variety in Belgium that had a higher production than in 2016 in 2017. This new variety is less frost sensitive than the other apples varieties as it, like Conference, can produce parthenocarpic fruits (seedless).
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