As the days become shorter and cooler in fall, deciduous plants stop growing, store energy, lose their leaves and enter a state of dormancy which protects them from the freezing temperatures of winter. Once dormant, a deciduous fruit tree will not resume normal growth, including flowering and fruit set, until it has experienced an amount of cold equal to its minimum “chilling requirement” followed by a certain amount of heat. (Additional factors that affect fruit set include age of tree, nutrition, availability of compatible pollen and weather during bloom.)
Fruit tree chilling requirements can vary widely from one variety to another. In general, excepting the coldest climates (see "Cold Climates" below), for best performance a variety’s chilling requirement should approximately match the amount of chilling normally received where it is planted. Some highly productive varieties, however, will produce well over a wide range of climates and chilling.
If a fruit tree is grown where winter cold is insufficient to satisfy the variety’s chilling requirement, blooming and foliation will be delayed and erratic; fruit set and fruit quality will be poor. Conversely, if a tree is grown where winter cold satisfies its chilling requirement too soon, the end of dormancy and loss of hardiness caused by a warm spell could lead to late-winter freeze damage to the tree and/or a too-early bloom. Subsequent hard frosts could cause crop failure year after year.
Chilling that exceeds a fruit tree’s minimum requirement can lead to a stronger bloom and, all else being equal, a heavier crop. A disadvantage of heavier crops is they require more thinning for best fruit quality and size. Home fruit growers often prefer moderate crops and less thinning work; commercial growers need maximum crops. Heavy crops can also lead to alternate bearing (heavy crops alternating with very light crops).
A fruit variety’s chilling requirement is a key determinant of where it will consistently produce satisfactory crops of fruit. So, how do we measure chilling?
Depending on the method used, fruit tree chilling is expressed either in hours of defined cold temperatures or in other calculated units based on the occurrence of various temperatures. A simple and widely used method is the Hours Below 45°F model which equates chilling to the total number of hours below 45°F during the dormant period, autumn leaf fall to spring bud break. These hours are termed “chill hours”.
Using this model, if a fruit tree were observed to bloom and fruit satisfactorily after winters of 600 or more chill hours, but inconsistently after winters of 500 chill hours and less, the variety would be regarded as having a chilling requirement of 600 hours. In terms of winter cold adaptation, this approach works reasonably well for matching most fruit varieties with suitable climates. It tends to be less reliable, however, for subtropical climates and varieties with very low chilling requirements - and is not especially useful for the coldest fruit tree climates.
How a deciduous fruit tree actually accumulates winter chilling is more complex than represented by the easy-to-use 45°F model. Research indicates fruit tree chilling 1) does not occur below about 30-34°F, 2) occurs also above 45°F to about 55°F, 3) is accumulated most effectively in the 35-50°F range, 4) is accumulated most effectively early in the dormant period, and 5) in early dormancy can be reversed by temperatures above 60°F. Chilling calculation methods such as the 32-45°F model, Utah model, Low Chilling model, Mean Temperature model and Dynamic model incorporate some or all of these findings. To date, all models tend to give significantly different results for different climates.
Note: the perhaps counterintuitive research finding that deciduous fruit trees (and almonds) do not accumulate chilling at temperatures below freezing is nevertheless consistent with the common knowledge that cold winters with plenty of sub-freezing temperatures (lots of “rest” for the trees) lead to heavy crops. Since hours below freezing are typically accompanied by hours above freezing at good chilling temperatures, the correlation is the same: more cold means more chilling, leading to a strong bloom and bumper crops of stone fruits and almonds.
Local fruit tree chilling information and data are available from agricultural universities and extension offices, area retail nurseries and via internet search. For example, the University of California Pomology Weather Services provides chilling data and calculations for 100 California Irrigation Management Information Service (CIMIS) weather station locations. Chill hour data is available yearly for November 1 to February 28/29, the overall fruit tree chilling period in California.
DWN CHILLING REQUIREMENTS
Chilling requirements in the Dave Wilson Nursery fruit and nut variety descriptions are given in chill hours: hours below 45°F during the dormant period.
As noted above, it is very difficult to measure chilling precisely; stated fruit tree chilling requirements are necessarily approximations or estimates. Regardless of the actual chilling algorithm built into a particular variety, the DWN estimated chilling requirement represents the relative amount of winter cold that variety has been observed or is thought to require, expressed in chill hours. The chilling requirement estimate for a new fruit variety is usually based on its behavior in the climate of origin and on parentage, if known. In any case, chilling requirement approximations for a deciduous fruit variety can be improved over a number of years by performance reports which follow dormant periods of varying temperature histories.
FRUIT GROWING IN CALIFORNIA
Much of California is blessed with a wonderful deciduous fruit growing climate: plenty of chilling, no particular spring frost problem and a long, hot, dry growing season. In such areas it is possible to grow almost any kind of deciduous fruit including varieties with chilling requirements anywhere from 100 to 800 or more hours.
“Low-chill” varieties, required in warm-winter climates, are defined by Dave Wilson Nursery as those varieties requiring 500 or fewer chill hours. For coastal southern California, low-chill varieties are considered to be those requiring less than 300 hours. In southern California and the lower deserts of Arizona winters can be short, often lasting less than two months, so it is essential to choose varieties that are low chill.
In warm-winter climates, coastal southern California for example, fruit growers sometimes find that a variety produces well with much less chilling than “advertised”. This could be because their locations receive relatively few hours of chilling below 45°F but plenty in the 45-55°F range (perhaps lots of foggy days), or because the variety’s chilling requirement is actually less than stated. Researchers suggest also that some varieties in the absence of cold are better able than others to "switch" to a heat requirement for triggering bloom and setting a crop. Many "northern" apple varieties can set crops satisfactory for home orchards with far less than the recommended winter chilling (but may not develop best fruit quality and color if summer and fall nights are warm).
Varieties adapted to colder climates usually, but not always, have chilling requirements of 800-1,000 or more hours. In the coldest fruit tree climates, though, growers are little concerned with chilling requirements; almost all varieties receive their chilling requirement early in winter. In these climates the need is for later-blooming, frost hardy varieties. Some of the most cold hardy fruit varieties, in fact, appear to have very low minimum chilling requirements; their cold hardiness derives from a higher heat requirement for ending dormancy – once dormant they are not easily fooled by an early warm spell.
Note: In nursery catalogs, higher stated chilling requirements often mean, essentially, "adapted to colder climates" or "tested in a colder climate". (Note also that bloom timing is not necessarily indicative of a variety's chilling requirement, as the variety's heat requirement for bloom also plays a role.)
THE PROOF IS IN THE PLANTING
A fruit variety is proven for a climate or region by planting and growing it there; promising new varieties are planted by adventurous growers. Note that a deciduous fruit tree's best fruit bud formation and cropping performance can occur only when it is not over-watered, over-fertilized, nor under-pruned.
When choosing fruit varieties to plant, area retail nurseries, agricultural universities, county cooperative extension offices and Master Gardener programs are good resources for local fruit growing information and ideas.
1. In fall, deciduous fruit trees lose their leaves and enter a dormant state in order to survive winter. To end dormancy, bloom and set fruit they require a certain amount of winter cold (their “chilling requirement”) followed by a certain amount of heat. Chilling requirements vary widely among varieties.
2. Fruit varieties with chilling requirements much lower than received at the planting location may end dormancy and bloom too early, subjecting tree, bloom and fruit to freeze damage. Conversely, varieties with chilling requirements much higher than received will suffer delayed, weak leafing and blooming and will not fruit satisfactorily.
3. To quantify fruit tree chilling Dave Wilson Nursery uses the most widely used method, the Hours Below 45°F model. One hour below 45°F during the dormant period (autumn leaf fall to spring bud break) equals one hour of chilling or one “chill hour”.
4. Research indicates fruit tree chilling also occurs above 45°F (to about 55°F), is most effective at about 35-50°F, and does not occur below about 30°F. Chilling temperatures are most effective in early dormancy and during that time accumulated chilling can be negated by temperatures above 60°F. Such findings help explain the response of specific varieties to different climates, i.e. to different patterns of cold and warm temperatures.
5. Generally, fruit growers have most success with varieties that have stated chilling requirements similar to the chilling typically received at the planting location: “high chill” varieties (800-1,000+ hours) for cold winter climates, “low chill” varieties (500 hours and less) for warm winter climates.
6. Published chilling requirements for a fruit variety can be the minimum chilling required to produce a satisfactory home garden crop up to a higher requirement for the consistent, maximum crop sought by commercial growers. Note that many apple varieties can set fruit with far less chilling than recommended, color and quality depending on climate.
7. Fruit varieties can be productive where chill hours are less than recommended if there is sufficient cool weather (45-55°F) during the dormant period.
8. Some cold hardy fruit varieties are widely adapted because they have a low or moderate chilling requirement as well as a cold hardiness deriving from a high heat requirement for ending dormancy.
9. Much of California has an ideal, virtually frost-free fruit growing climate where almost any kind of deciduous fruit can be grown, including varieties with chilling requirements anywhere from 100 to 800 or more hours.
10. A variety is proven for a climate or region only by growing it there. When choosing fruit varieties always refer to area retail nurseries, agricultural universities, county extension offices and master gardeners for local fruit growing information and ideas.