What are chilling hours and what do they have to do with dormancy? Do cultivars differ in their chilling hour requirement?

Answer:  Chilling hours are the number of hours below 45F accumulated by the tree during the winter to overcome dormancy.  Peach cultivars that can be grown in the subtropics (i.e., low-chill types) may require as few as 50-100 chilling hours while those grown in the temperate climates further north or at higher elevations (high-chill types) may require more than 1000 hours.  Knowing the typical chill hour accumulation of your geographic region should be one of the primary criteria you use in choosing cultivars that are suitable to grow there.

Caption: United states map showing approximate minimum number of chilling hours required by state/region.
United states map showing approximate minimum number of chilling hours required by state/region.
Caption: Southeastern U.S. Region map showing approximate minimum number of chilling hours required by state/region.  Note four chilling hour zones for South Carolina with yellow arrows.
Southeastern U.S. Region map showing approximate minimum number of chilling hours required by state/region.  Note four chilling hour zones for South Carolina with yellow arrows.
Carored Peaches chilled for 650 hours Contender peaches chilled for 1050 hours
Carored is a popular SC cultivar that requires 650 chill hours to overcome dormancy.  Because it is somewhat low chilling relative to the majority of cultivars grown in the upstate of SC, it may bloom early and experience cold damage. Contender is a popular cultivar from North Carolina with a 1050 chill hour requirement.  As a result, it would be inappropriate to grow in coastal South Carolina which averages 400-600 chill hours.

 

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