Feeling the frost
Recent frost events have affected some growers with varying degrees of damage to fruit, leaves, branches and flower buds. So, what we can do to minimise frost damage and be better prepared.
The amount of frost damage experienced depends on the cultivar or rootstock’s sensitivity to freezing and the length of time the temperature is below the “critical damage” temperature.
Frost damage results not from the cold temperature but mainly from ice formation inside plant tissue.
Avocado leaves appear wilted during periods of low temperature.This is the tree’s natural protective response to freezing temperatures and does not mean the leaves have been frozen. Leaves will be firm and brittle and often curled when frozen becoming flaccid after thawing. If the frost damage is not too severe, they gradually regain turgor and recover, leaving dark “water stain” marks on the leaves.
A severe frost will affect fruit, flowers, twigs, branches and leaves. Leaves will collapse and dry out and may remain on the tree for several weeks. If, however the leaves are rapidly shed, this is a good sign and means that the twigs and branches have not been severely damaged!
Symptoms of frost damage can take up to a week or longer to show the full extent of the damage. Plant tissue that freezes generally appear dark green and water soaked at first, later becoming brown and necrotic.
FROST PREVENTION AND MINIMISATION
First and foremost is site selection. Cold air is heavier than warm air and will ‘flow’ down slopes and settle in the lowest areas, progressively filtering back up the slopes as more cold air accumulates in these low-lying areas. Avoid planting in these areas if possible.
When planning your orchard, run your rows up and down the slope if possible, to allow for maximum air drainage and don’t plant in the lowest areas. If the gradient is too steep aim for some downward slope to the row. Avoid rows running directly across a slope which trap cold air in pockets. Carefully plan row design in orchards with gently undulating slopes, as they can lead to localised pockets of cold air.
SHELTERBELTS / WINDBREAKS
Anything that restricts the flow of cold air e.g. a dense windbreak, will result in a build-up of cold air within the orchard causing a localised frost event. Ensure shelterbelts are skirted to at least knee height to allow drainage of the cold air out of the orchards.
When long grass or weeds are present in an orchard, sunlight is reflected from the surface. Less heat energy is absorbed by the soil and released at night, making it more prone to frost damage. Mowing or spraying the tree rows with an appropriate herbicide are methods of control.
SOIL MOISTURE LEVELS
Thermal conductivity and heat content of soils are affected greatly by the soil water content. On a daily basis heat is transferred into and out of approximately the top 30cm of the soil. When the soil is wet, heat transfer and storage in the upper soil layer is improved, so more heat is stored during daylight for release during the night.
If your soils are dry or not at field capacity and you are aware of an upcoming frost event, wet the soils one to three days prior to this. Even watering the evening before a frost increases the heat given off by soil during the night. It is unnecessary to wet the soil deeply as most of the daily heat-transfer and storage occurs in the top 30 cm.
PLANT NUTRITION & TREE HEALTH
Trees with poor tree health or nutrient deficiencies are more susceptible to frost damage. The relationship between specific nutrient applications and increased resistance to frost damage however is obscure, and there are many contradictions. In general, though, nitrogen fertilization before a frost encourages growth and increases susceptibility to frost damage.
To enhance hardening of plants, avoid applications of nitrogen fertilizer in early autumn. Potassium has a favourable effect on water regulation and photosynthesis in plants which could be a benefit under frost conditions. However, researchers are divided about the benefits of potassium for frost protection.
Controlling Ice Nucleating Bacteria (INB) with copper-based products is believed to have some affect in protecting plants when temperatures drop and frost occurs. In the case of frost, these bacteria will freeze first on the surface of the plant. The plant tissue itself does not actually freeze until the temperature reaches around -2°C to -4°C. If temperatures drop further, damage will occur irrespective of any application of copper sprays. It is also important to note that adequate time must be given between application and a frost event (about 10 days), so that the ice-nucleating bacteria can be destroyed. Research on this topic however is not conclusive.
APPLICATION OF HORTICULTURAL MINERAL OILS
The application of pesticide oils to citrus is known to increase frost damage and application of oils should rather be avoided shortly before the frost season.
CHEMICAL FROST PROTECTANTS
Chemical sprays such as adjuvants like Vaporgard or frost protectants such as Thermomax claim to improve frost hardiness of plants. Scientific literature reveals few examples of effective spray on frost protection. However, there have been several reports of field applied sprays which have resulted in depressed freezing point for excised leaf, flower and shoot tissue measured under laboratory conditions. Generally, these results have failed to translate into consistent, commercially viable forms of frost protection. However, anecdotal reports of successful field treatments continue to circulate so weigh up the cost of the sprays versus the economic loss due to a frost event and it may be worthwhile trying. Timing though is key.
Apply frost covers to young trees if they are planted in a frost prone area. Side and top cover is required for protection against frost. Stem protectors or trunk guards can also help prevent total tree death by protecting the main stem.
Over-plant or over-head sprinklers provide excellent frost protection. The main disadvantages with using these sprinklers however, are the high installation cost and the large amounts of water needed. This will therefore not be an option where water availability is limited. Excessive use of
“Jack Frost is certainly not a grower’s friend but some positive frosty facts are that it can disrupt pest and disease cycles, and improve soil structure when moisture within the soil freezes, expanding and splitting open soil particles.”
water for frost protection can also lead to soil waterlogging and phytophthora as well as nutrient leaching. The wet canopy can also exacerbate fungal diseases on the fruit. Good management and timing is essential.
For under-plant micro sprinklers, which apply less water than conventional overhead sprinklers, the goal is to keep only the ground under the plants near 0°C in order to concentrate and enhance radiation and heat transfer upwards into the tree.
Heaters provide protection by direct radiation to the plants and surroundings and by causing convective mixing of air within the inversion layer. When heaters are operated, the heated air rises. As the heated air rises, it cools until it reaches the height where the ambient air has the same temperature. Then the air spreads out and, eventually, the air descends again. A circulation pattern is created. However, if the inversion is weak, the heated air cools, but it rises too high and a circulation pattern is not produced. As a result, heaters may not provide adequate protection when there is little or no inversion. Making fires too hot will also make heaters less effective because the heated air rises above the inversion ceiling and the circulation pattern is not created. Heaters can be as rudimentary as metal drums filled with burning wood or as sophisticated as propane heaters.
During cold and still conditions, the down draught from a helicopter flying slowly over the orchards causes the slightly warmer air layered above the inversion layer to circulate downwards amongst the trees, reducing the likelihood of frost damage. A helicopter should pass over the entire crop every 30 minutes during mild freezes and more often during severe freezes. However, if there is little or no inversion, helicopters are not very effective. Remember to notify neighbouring orchards or better yet, combine the flight to share the costs.
For good old fashion kiwi ingenuity refer to http://www. towandblow.co.nz/ for a portable diesel fuelled wind machine for frost protection!
FORECASTING AND MONITORING
Forecasting the minimum ground temperature and how the temperature might change during the night is useful for frost protection because it helps us as growers decide if protection is needed, when to take action and what action to take. Forewarned is forearmed! Go to http://about. metservice.com/our-company/learning-centre/ frost/ for more info on monitoring and forecasting. Jack Frost is certainly not a grower’s friend but some positive frosty facts are that it can disrupt pest and disease cycles, and improve soil structure when moisture within the soil freezes, expanding and splitting open soil particles. So much for the positives...