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Chilling Requirements: What they mean(700) Three or four digit numbers, such as this example, indicate the approximate chilling hours required for that variety. All fruiting & flowering plants need adequate dormancy before the beginning of their flowering period. For many plants, this dormancy period has been calculated and is known as its chilling requirement. The chilling requirement is the minimum number of hours below 45°F that the plant must be exposed to during its dormancy. Failure to acquire adequate chilling results in buds that do not open or open slowly or unevenly. When the plant has received adequate chilling, it is then ready to bloom, as weather permits. Generally, varieties (within the same species) that have higher chilling requirements bloom later. For example, European plums have much higher chilling needs (700-1000 hrs) than Japanese plums (300-500 hrs), so the European plums will bloom later. This difference between varieties can help the grower select the best peach, apple, etc. to meet the climatic conditions of his/her locale. Frost-prone growers may want plants with longer chilling requirements within a species. Growers in mild winter climates will want to select varieties with low chilling requirements so that the cold needs of the plant will be met in most years. Chilling requirements should not be confused with cold hardiness. A plant with a lower chilling requirement may be grown in cold areas, but it will bloom sooner than high chill varieties and be subject to frosts. A common misunderstanding exists when comparing these numbers between plant species. A species with low chilling requirements, for example figs (100 hrs) is not as early blooming as the higher cold requirements of almonds (400-700 hrs), generally the first fruit or nut bearing tree to bloom. However, within a species, the variety with the lowest numbers blooms first. Information on the average chilling hour accumulation for your area is available from your local farm and garden advisor. Back to Reference Library Index
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