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Apple chilling requirement - winter dormancy Photo: Natalia Sevruk · Pexels License · source ↗

Apple chilling requirement and chilling units

Apple is a deciduous fruit tree that requires a defined period of winter cold ("chilling") to break bud dormancy and produce a uniform spring flush and flowering. The chilling requirement is the minimum number of hours of low temperature a cultivar needs after leaf fall and before it can break dormancy. Inadequate chilling produces erratic budbreak, sparse flowering, delayed bloom, weak shoots and ultimately yield loss. The metric is critical for site selection, cultivar choice and predicting climate-change risk in the Indian apple belt.

Principle

There are three commonly used models in India:

  1. Hours-below-7.2 °C model (Weinberger): count hours between 0 °C and 7.2 °C from October to February. Simple but inaccurate when temperatures fluctuate.
  2. Utah model (chilling units): weights temperatures by effectiveness — 1 chilling unit (CU) per hour at 2.5-9.1 °C, 0.5 CU at 1.6-2.4 °C or 9.2-12.4 °C, 0 below 1.5 °C, negative for hours above 16 °C.
  3. Dynamic model (Erez): a temperature-portion model used in modelling literature; more accurate for warmer winters.

ICAR-CITH and HRRS Mashobra report results in both hours-below-7.2 °C and Utah CUs.

Apple cultivar chilling requirements

Cultivar Chilling hours below 7.2 °C Notes
Royal Delicious 1,000-1,200 Standard Kashmir / HP cultivar
Red Delicious 1,000-1,200 Same group
Granny Smith 800-1,000 Slightly lower
Gala (Royal Gala) 600-800 Low-chill, suited to lower elevations
Fuji 800-1,000 Late-storing
Ambri 1,000-1,200 Traditional Kashmir
HRMN-99 (low-chill) 100-400 Bilaspur (HP) low-chill selection; bears in plains
Anna 200-300 Low-chill, suited to Karnataka hills

Procedure: how growers use it

  • Site selection: an automatic weather station or temperature logger records hourly winter temperatures from October to February. Total chilling hours / units are computed at the orchard site for the previous 5 years.
  • Cultivar matching: cultivars are chosen whose chilling requirement is ≤ 90% of the site's mean annual chilling supply. For example, a site averaging 1,300 hours below 7.2 °C can safely grow Royal Delicious (1,000-1,200 h) but not a cultivar needing 1,500 h.
  • Marginal sites: where chilling is inadequate in warm winters, growers apply rest-breaking sprays — hydrogen cyanamide (0.5-1% Dormex) or potassium nitrate (3-5%) — 30-40 days before normal budbreak.

Where it applies

The Indian apple belt is concentrated between 1,500 and 2,500 m elevation in J&K (Shopian, Pulwama, Baramulla, Kupwara), HP (Shimla, Kullu, Kinnaur, Mandi) and Uttarakhand (Chamoli, Uttarkashi). Below 1,500 m, only low-chill cultivars such as HRMN-99 and Anna succeed.

Limitations and climate change

Climate-change models project a decline of 15-30% in chilling hours in the Kashmir Valley by 2050, which threatens Royal Delicious and Red Delicious. ICAR-CITH and HRRS Mashobra are evaluating low- to medium-chill colour-strain Delicious bud sports and Gala/Fuji as climate-resilient alternatives for mid-elevation sites. Low-chill HRMN-99 has enabled apple cultivation in Bilaspur (HP) and parts of Karnataka and Manipur.

See also: Royal Delicious, Red Delicious, Ambri, apple rootstocks, HDP.

Sources

  1. Chilling requirement in temperate fruits. ICAR-CITH Srinagar.
  2. Apple climate requirements. HRRS Mashobra, Dr YS Parmar University.
  3. Apple cultivation in India. National Horticulture Board.