How to beat the chill in the orchards

Shashi Kumar Sharma

Despite the advancements in agriculture, agrarian output is still weather-dependent. In the subtropical parts of north-west India, frost or low-temperature stress is the weather-related phenomenon which causes more losses to fruit plantations than any other environmental hazard. As per a recent estimate, this region loses more than Rs 1,300 crore during a heavy frost year. Frost is generally perceived as the solid deposition of water vapours from saturated air when the solid surface in contact with air gets cooled below the deposition point due to a sharp decline in temperature. In the subtropical zone, the frost can form on the cooling surface even when the reported air temperature is above 0°C. It is called ‘black frost’ when there occurs no apparent white ice crystal formation due to low atmospheric humidity. The low-lying basal plains are highly frost- sensitive, but due to their high productivity, these are extensively cultivated and utilised for fruit-growing.

The avoidance of frost by way of appropriate site or fruit crop selection is the most important preventive measure against huge economic losses. There occurs high variability in frost intensity with respect to the elevation of the site within an agro-ecological situation. For instance, the sites which are up to 100m of the height from the basal point of the agro-ecological situation are more prone to frost damage than the sites at higher elevations. As the elevation increases, the intensity of frost decreases. Also, the extent of susceptibility to frost is also variable among subtropical fruit species. The observed relative order of susceptibility of fruit crops to frost is: Strawberry < pomegranate < loquat < citrus < phalsa < guava = jamun = litchi < aonla = karonda = mango = jackfruit < custard apple < banana < papaya.

Cold acclimation with the progression of winter is very crucial for plants to tolerate the stress. By modifying their cellular fluids like sugars, sugar-alcohols, lipids and hormones, the plants try to acclimate with the progression of winter. But, unpredictable rise in temperature due to global warming leads to de-acclimation of the acclimated plants. Though acclimation is biomolecule-regulated, plant nutrition also play a major role in developing resilience. Optimum potassium level helps to withstand the stress for a longer period, while phosphorus favours faster recovery of the damaged tissues. Higher levels of nitrogen make the plants more prone to damage. Optimum levels of micronutrients help in maintaining good plant health; healthy plants suffer less damage.

Further, frost is an energy-related phenomenon; solar energy imparts the highest energy to an orchard system. On a cool, clear and calm night, there is a heavy loss of radiation from the earth’s surface. For frost to occur, the loss of radiation energy during night must exceed the gain of radiation energy during daytime. Thus, to maintain the positive balance of net radiations, preventive measures for slowing down loss at night and enhancing the interception of solar radiation during the day can be of great help to the orchardists. There are a number of orchard management practices which can contribute to maintaining radiation positivity. The following practices can be followed for energy conservation at the orchard level:

• Keep orchard soil moist as it stores more solar energy than the dry soil.
• Weed-free, non-tiled orchards store energy better than weedy and loose soil.
• Farm yard manure possesses very low specific heat, therefore it should not be left lying on the orchard floor. It should be mixed well with the soil. 
• Canopy skirt should be kept at least one metre above the soil surface so that light can penetrate beneath the tree.
• Tree branches should be kept at 60° to the vertical axis as the branches or leaves facing the sky lose more energy, and that too at a faster rate and thus suffer the most due to frost. 
• Conical thatch cover to the plants are better, these conserve more heat than flat covers. Slight opening on the southern side should be kept for light and air entrance. Over the nurseries, only flat thatch covers are possible; the height of such covers should be not more than 30 cm above the top of the plants. 
• Trunk wraps of non-hygroscopic material are quite effective for protection of stems of sensitive plants.
• Foliage may be protected to a greater extent by covering with fine meshed synthetic non-hygroscopic fabric.
• Genotypes with entire top canopy should be preferred as denser canopies shield the lower portions from frost damage. Pruning disturbs canopy density, therefore it should be done either in October or February, if required.
• One gram of water releases 20 calories of energy while its temperature drops from 20°C to 0°C. Its freezing at 0°C releases 80 calories in addition. Thus, foliar sprays of water are very effective for frost protection measures under situations where wind velocity remains below 5 km/h during the night. Incorporation of thermos-hysteric compounds in the foliar water spray gives better results by way of restricting the frost-induced freeze initiation and spread. Ingression of these compounds into the plant system through hydrogels may give prolonged advantage.
• Early morning sprays of water are also effective in slower thawing of the frozen plant tissues and prevention of frost damage to a great extent.  

For effective orchard management during winters, growers can take help of the S-Frost Protection Guide Chart, which is a reliable decision support tool for prediction of potential frost threat. This guide chart uses only one input — the sunset time temperature. For example, if the sunset time (here fixed at 5:30 pm for NW Indian conditions) temperature is 10°C; the line pertaining to 10°C passes through the blue zone of the S-Chart, which is the zone of frost-sensitive crops. So, it can be inferred that highly sensitive crops like papaya and banana will be under frost stress and this stress will be effective 8½  hours (corresponding duration on x-axis) after sunset and will last up to 5½  hours after that. It means the critical stress will start at 2 am and last till 7:30 am. Further, the method of protection can be chosen on the basis of the length of the period for which the stress is going to stay. Among the protection methods, affordable water spraying is effective, but its use is limited because overnight sprinkling is quite costly and leads to water stagnation. Under such situations, the S-chart helps in the precise application of spray water and the automation of the equipment.

Weather services to which people have access are generally quite broad, as they use synoptic and/or meso-scale models to provide regional forecasts. The predictions as described above can be calibrated for local conditions for coming to a decision on ‘if and when’ frost protection is needed. It is clear that in the years to come, climatic variability will play an even greater role than in the past and therefore precise knowledge of frost and low-temperature stress will be of great significance.

The author is Principal Scientist, Dr YS Parmar University of Horticulture & Forestry, Solan