The chilling requirement of a fruit is the minimum period of cold

weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmo ...

after which a fruit-bearing tree will

blossom In botany, blossoms are the flowers of stone fruit trees (genus ''Prunus'') and of some other plants with a similar appearance that flower profusely for a period of time in spring. Colloquially, flowers of orange are referred to as such as w ...

productively. 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. Some bulbs have chilling requirements to bloom, and some seeds have chilling requirements to sprout. Biologically, the chilling requirement is a way of ensuring that vernalization occurs.

Chilling units or chilling hours

A ''chilling unit'' in

agriculture Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created ...

is a metric of a plant's exposure to chilling temperatures. Chilling temperatures extend from

freezing point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...

to, depending on the model, or even .Byrne, D. H., and T. A. Bacon (1992).
Chilling estimation: its importance and estimation
''. The Texas Horticulturist 18(8):5, 8-9. Retrieved 2010-05-24.

Stone fruit In botany, a drupe (or stone fruit) is a type of fruit in which an outer fleshy part (exocarp, or skin, and mesocarp, or flesh) surrounds a single shell (the ''pip'' (UK), ''pit'' (US), ''stone'', or ''pyrena'') of hardened endocarp with a seed ...

trees and certain other plants of

temperate climate In geography, the temperate climates of Earth occur in the middle latitudes (approximately 23.5° to 66.5° N/S of the Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ra ...

develop next year's

bud In botany, a bud is an undeveloped or Plant embryogenesis, embryonic Shoot (botany), shoot and normally occurs in the axil of a leaf or at the tip of a Plant stem, stem. Once formed, a bud may remain for some time in a dormancy, dormant conditi ...

s in the summer. In the autumn the buds become dormant, and the switch to proper, healthy dormancy is triggered by a certain minimum exposure to chilling temperatures. Lack of such exposure results in delayed and substandard foliation, flowering and fruiting. One chilling unit, in the simplest models, is equal to one hour's exposure to the chilling temperature; these units are summed up for a whole season. Advanced models assign different weights to different temperature bands. Contrary to popular belief, lack of chilling hours does not usually kill a plant, nor prevent a dormant plant from eventually leafing out. However, it may add stress that can be one factor in a plant’s demise; as such, a previously injured plant may not survive long after a warm winter. Fruit trees that do not meet chilling requirements in a given season will not leaf or flower at normal times or for normal durations, and may only produce a few or no fruit.

Requirements

According to Fishman, chilling in trees acts in two stages. The first is reversible: chilling helps to build up the

precursor Precursor or Precursors may refer to: *Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), hypothesized genus of fossil birds that was composed of fossilized parts of unre ...

to dormancy, but the process can be easily reversed with a rise in temperature. After the level of precursor reaches a certain threshold, dormancy becomes irreversible and will not be affected by short-term warm temperature peaks.

Apple An apple is a round, edible fruit produced by an apple tree (''Malus'' spp.). Fruit trees of the orchard or domestic apple (''Malus domestica''), the most widely grown in the genus, are agriculture, cultivated worldwide. The tree originated ...

s have the highest chilling requirements of all fruit trees, followed by

apricot An apricot (, ) is a fruit, or the tree that bears the fruit, of several species in the genus ''Prunus''. Usually an apricot is from the species '' P. armeniaca'', but the fruits of the other species in ''Prunus'' sect. ''Armeniaca'' are also ...

s and, lastly,

peach The peach (''Prunus persica'') is a deciduous tree first domesticated and Agriculture, cultivated in China. It bears edible juicy fruits with various characteristics, most called peaches and the glossy-skinned, non-fuzzy varieties called necta ...

es. Apple

cultivar A cultivar is a kind of Horticulture, cultivated plant that people have selected for desired phenotypic trait, traits and which retains those traits when Plant propagation, propagated. Methods used to propagate cultivars include division, root a ...

s have a diverse range of permissible minimum chilling: most have been bred for temperate weather, but ''

Gala Gala may refer to: Music * ''Gala'' (album), a 1990 album by the English alternative rock band Lush * Gala (singer), Italian singer and songwriter *'' Gala – The Collection'', a 2016 album by Sarah Brightman * GALA Choruses, an association of ...

'' and '' Fuji'' can be successfully grown in subtropical

Bakersfield, California Bakersfield is a city in and the county seat of Kern County, California, United States. The city covers about near the southern end of the San Joaquin Valley, which is located in the Central Valley region. Bakersfield's population as of th ...

.Hall, Anthony (2001).
Crop responses to environment
'. CRC Press. . p. 87. Peach cultivars in

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

range in their requirements from 100 chilling units (''Florida Grande'' cultivar, zoned for low chill regions) to 1,000 units (''Surecrop'', zoned for high chill regions).Kamas, J.; McEachern, J. R; Stein, L.; Roe, N. (1998).
Peach Production in Texas
', table 1. Texas A&M University. Retrieved 2010-05-24. Planting a low-chilling cultivar in a high-chill region risks loss of a year's harvest when an early bloom is hit by a spring frost. A high-chilling cultivar planted in a low-chill region will, quite likely, never fruit at all. A four-year study of ''Ruston Red''

Alabama Alabama ( ) is a U.S. state, state in the Southeastern United States, Southeastern and Deep South, Deep Southern regions of the United States. It borders Tennessee to the north, Georgia (U.S. state), Georgia to the east, Florida and the Gu ...

peach, which has a threshold of 850 chilling units, demonstrated that a seasonal chilling deficiency of less than 50 units has no effect on harvest. Deficiency of 50 to 100 units may result in loss of up to 50% of expected harvest. Deficiency of 250 hours and more is a sure loss of practically whole harvest; the few fruit will be of very poor quality and have no market value.Powell, A. (1999).
Action Program for Dormex Application on Peaches
''.

Auburn University Auburn University (AU or Auburn) is a Public university, public Land-grant university, land-grant research university in Auburn, Alabama, United States. With more than 26,800 undergraduate students, over 6,100 post-graduate students, and a tota ...

. Retrieved 2010-05-24. Rest-breaking agents (e.g.

hydrogen cyanamide Cyanamide is an organic compound with the formula C N2 H2. This white solid is widely used in agriculture and the production of pharmaceuticals and other organic compounds. It is also used as an alcohol-deterrent drug. The molecule features a ...

, trade name ''BudPro or Dormex''), applied in spring, can partially mitigate the effects of insufficient chilling. BudPro can substitute for up to 300 hours of chilling, but an excessive spraying and timing error can easily damage the buds. Other products such as Dormex use stabilizing compounds. Chilling of orange trees has two effects. First, it increases production of

carotenoid Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, cana ...

s and decreases

chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...

content of the fruit, improving their appearance and, ultimately, their market value. Second, the "quasi-dormancy" experienced by orange trees triggers concentrated flowering in spring, as opposed to more or less uniform round-the-year flowering and fruiting in warmer climates.

Biennial plant A biennial plant is a flowering plant that, generally in a temperate climate, takes two years to complete its biological life cycle. Background In its first year, the biennial plant undergoes primary growth, during which its vegetative structur ...

s like

cabbage Cabbage, comprising several cultivars of '' Brassica oleracea'', is a leafy green, red (purple), or white (pale green) biennial plant grown as an annual vegetable crop for its dense-leaved heads. It is descended from the wild cabbage ( ''B.& ...

sugar beet A sugar beet is a plant whose root contains a high concentration of sucrose and that is grown commercially for sugar production. In plant breeding, it is known as the Altissima cultivar group of the common beet (''Beta vulgaris''). Together with ...

celery Celery (''Apium graveolens'' Dulce Group or ''Apium graveolens'' var. ''dulce'') is a cultivated plant belonging to the species ''Apium graveolens'' in the family Apiaceae that has been used as a vegetable since ancient times. The original wild ...

and

carrot The carrot ('' Daucus carota'' subsp. ''sativus'') is a root vegetable, typically orange in colour, though heirloom variants including purple, black, red, white, and yellow cultivars exist, all of which are domesticated forms of the wild ...

s need chilling to develop second-year flowering buds. Excessive chilling in the early stages of a sugar beet seedling, on the contrary, may trigger undesired growth of a flowering stem ( bolting) in its first year. This phenomenon has been offset by breeding sugar beet cultivars with a higher minimum chilling threshold. Such cultivars can be seeded earlier than normal without the risk of bolting.

Models

All models require hourly recording of temperatures. The simplest model assigns one chilling unit for every full hour at temperatures below . A slightly more sophisticated model excludes freezing temperatures, which do not contribute to proper dormancy cycle, and counts only hours with temperatures between and . The Utah model assigns different weight to different temperature bands; a full unit per hour is assigned only to temperatures between and . Maximum effect is achieved at . Temperatures between and (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. Southwick et al. wrote that neither of these models is accurate enough to account for application of rest-breaking agents widely used in modern farming. They advocated the use of a dynamic model tailored to the two-stage explanation of dormancy.Southwick, S.; Khan, Z.; Glozer, K. (2003).
Evaluation of Chill Models from Historical Rest-Breaking Spray Experiments on Bing Sweet Cherry
'.

University of California, Davis The University of California, Davis (UC Davis, UCD, or Davis) is a Public university, public Land-grant university, land-grant research university in Davis, California, United States. It is the northernmost of the ten campuses of the University ...

. Retrieved 2010-05-24.

References

External links

Harvest Prediction Model for the counties and towns of California
.

University of California The University of California (UC) is a public university, public Land-grant university, land-grant research university, research university system in the U.S. state of California. Headquartered in Oakland, California, Oakland, the system is co ...

Agricultural and Natural Resources.
gardenweb discussion of chilling requirementspermaculture discussion of chilling requirements

chill accumulation calculator for wunderground.com weather stations
{{Webarchive, url=https://web.archive.org/web/20160109162854/http://getchill.net/ , date=2016-01-09 Horticulture Meteorological indices Plant physiology