Chilling Units achieved; scanty snow not an issue for apple trees: Expert

Scanty snow though is worrisome for new plantations and Rabi crops

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A view of an apple orchard in Himachal Pradesh during winters; Picture used under Creative Commons License; Image Source: Bharat Justa

Scanty snow and prolonged dry spell has kept the apple growers worried fearing the apple and pear trees may not have received required chilling. However, experts have come forward to quell the fears stating the chilling has been already achieved.

Stone and pome fruit trees rely on enough chilling for flowers and leaf buds to develop normally. If the buds do not receive sufficient chilling temperatures during winter to completely release dormancy, trees may develop physiological symptoms such as delayed and extended bloom, delayed foliation, reduced fruit set and reduced fruit quality. The chilling requirement of a fruit is the minimum period of cold weather after which a fruit-bearing tree will blossom. It is often expressed in chill hours, which can be calculated in different ways, all of which essentially involve adding up the total amount of time in a winter spent at certain temperatures.

Chilling unit in agriculture is a metric of a plant’s exposure to chilling temperatures. Chilling temperatures extend from freezing point to, depending on the model, 7 °C or even 16 °C.

According to Dr SP Bhardwaj, retired professor of Dr YS Parmar Horticulture University Nauni, Himachal follows the ‘Utah Model of Chilling Units’, developed by researchers at Utah State University.

The Utah Model assigns different weight to different temperature bands; a full unit per hour is assigned only to temperatures between 3 °C and 9 °C. Maximum effect is achieved at 7 °C. Temperatures between 13 °C and 16 °C (the threshold between chilling and warm weather) have zero weight, and higher temperature have negative weights: they reduce the beneficial effects of an already accumulated chilling hours.

According to Bhardwaj, fruit plants being grown in temperate or subtropical climatic zone in the state mostly are the high chilling variety, like Apple and Pear, which require fixed annual chilling unit between 600 to 900 and medium range variety up to 600.

Red Delicious and Royal apple needs fixed quota of about 900 chilling unit per annum, which Bhardwaj said has already been achieved with over 1000 chilling units, even without major snowfall in this winter season.

Moreover, farmers opting for new spur and medium chilling strains are further away from its worse impact as varieties like Red Chief, Grainy Smith and Fuzy etc are already in the green zone.

Fruits like peach, kiwi and apricot being grown abundantly in Himachal and bordering states, according to the retired professor require optimum 400 to 600 chilling units.

Farmers, Bhardwaj said, should focus on their routine chores rather be apprehensive about the chilling hours as their orchards have achieved enough number of chilling units despite the erratic weather condition and fluctuating mercury this year.

Low chilling varieties like Gale-Gala too do not require snowfall.

Snow, is considered as ‘white manure’ for apple and other pome fruit, which according to Bhardwaj, helps to keep the temperature low by prolonged evaporation in the season, and regulates the moisture content and helps sustain the water level for longer duration.

Snowfall also helps keep the soil intact as compared to rain, which could lead to soil erosion and flooding. Snow also helps the trees in gradual intake of nutrients through manure and other fertilizers. Rain, in comparison could wash away the fertilizers.

All’s not well though

The state received 62 percent less snow in the months of January and February, this year. Bhardwaj, however, termed it “alarming” for new plantations, which require enough moisture to rise the sap in the new plant. The scanty snow and rain this season could also hamper the growth of Rabi crops, which require more than 14 mm of rainfall as its roots system confines up to nine inches in the land.

Fruit trees in comparison can withstand partly dry weather as their roots system is deep-rooted from nine inches down to 1.50 metre in the soil.

However, with extended dry spell, trees could be infested by root borer, scale, mite, reeds mite, powdery mildew, which could prove drastic for the trees.

Bhardwaj advised the farmers to adopt water conservation techniques to allow sustained irrigation and retain moisture content in the soil.

The dry spell and low rain may affect the Rs 4,000 crore apple economy of the state, which could hamper the livelihood of more than three lakh households in the state. Apple economy contributes about 15 percent to the state GDP, Bhardwaj added.

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