What Are Chilling Hours?

November 9th, 2012 at 11:38 am by under Chad's WLFI Weather Blog

With it reaching 70 Saturday & 67 Sunday will Silver Maple buds swell like they do with such days in March?  Well no.  Why?  The trees have not had the required chilling hours.

Chilling hours are very important to trees.  They determine when a tree or shrub has reached a point where it will bud.  Here, we don’t necessarily worry about chilling hours as much, as we rarely see enough warmth in early winter to force trees into bud.  In the southern U.S., it is more of an issue.  If lots of cold air hits the southern U.S. early in the season & there is a warmer period of weather say in January, then species that their chilling requirement is met will bud.  This is a real issue for fruit growers down south.  Many fruit varieties require less chilling hours & thus are more easily forced by southern warmth mid-winter.  This makes the plants susceptible to freezing in February after such a period of warmth.

A chilling hour is an hour where the temperature of 32-45 degrees occurs, while temperatures above 60 take hours away from the accumulated hours.  This temperature range is ideal for required chilling hours for the buds to form properly next year.  You would not want an apple tree that requires lots of chilling hours in your yard in Orlando, Florida.  Lack of chilling hours there would caused very slow budding & the blossoms & fruit to not form properly.  This is also important with tree genotypes.  For example, Tuliptree (Liriodendion tulipifera) has a distinct genotype in central Florida that has smaller blossoms that appear earlier & leaves that have more rounded, usually two-lobed leaves.  The leaves also have veins with a distinct red/rosy hue, but bark, buds are the same, though.  This tuliptree tolerates wetter soils of the Florida flatwoods much better than Indiana tuliptree.  This tree does not need the chilling requirements of the more northern Tuliptree.  In fact, in warmer winters, it may not even loose all of its leaves, but still flowers & re-foliates properly.  Such trees can be found near West Palm Beach & Orlando with several nature preserves boasting very large specimens with clear boles for 80′ of this type.

Red maple genotypes in Florida require less chilling hours than Indiana genotypes.  I have seen Red maples with red seeds dangling after weeks of 70s & 80s after chilling requirements were met in central Florida, while some leaves of the previous season still clung to the tree!

You can calculate the number of chilling hours we have had by recording how many hours we have had temperature 32-45 degrees.  I feel, though, that these hours make a bigger difference when the plant is either bare or semi-dormant with partial, colorful foliage remaining.  I have seen trees bud in the fall in our area, however.  In November 1993, back home, after cold weather in the fall & snow & 20s in late October to early November & lots of 40s, pussy willows, forsynthias bloomed & daffodils sprouted in mid-late November 1993.  In late November 2007, some Red maples blossomed at Terre Haute, Indiana after lots of 40s & 30s early to mid-fall.  This was the first time I had observed such a thing with unseasonable warmth in November.  Several times in the past, I have seen Silver maple buds break in January with unseasonable warmth.  I also witnessed daffodils sprouting way up in January 2006 in Evansville & Terre Haute.  In fact, on a south-facing wall in Terre Haute in early January 2005, I actually witnessed daffodils blooming after chilling requirements were met, followed by record warmth over a couple of weeks with 60s.

A tree loosing its leaves is a form of protection from the cold, lack of light or drought.  Usually cold is needed to force dormancy, but for some crabapples & bradford pears, very dry weather can force dormancy through a different mechanism.  In this case, the trees may re-bloom once rains return.


Almond 500-600
Apple 400-1000 (Low chill varieties are less)
Apricot 500-600
Japanese Pear 400-500
Blackberry 200-500
Blueberry (Northern) 800
Chestnut 400-500
Cherry 700-800
Citrus 0
Currant 800-1000
European Pear 600-800
European plum 800-900
Fig 100-200
Filbert 800
Gooseberry 800-1000
Grape 100+
Japanese Plum 300-500
Kiwi 600-800
Mulberry 400
Peach 600-800
Persimmon 200-400
Plum Cot 400
Pomegranate 100-200
Quince 300-500
Strawberry 200-300
Raspberries 700-800
Walnut 600-700

One Response to “What Are Chilling Hours?”

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