While taking down outdoor holiday decorations recently, I noticed daffodils popping up throughout the Jungle. My first thought was "that's not good" before moving on to selfish aggravation that my daffodil display would most likely be less than spectacular this spring due to probable freeze injury to early exposed leaves…what with so much winter still to come. Clearly the daffodils had met their chilling requirement and with continued mild weather were able to start growth. Then I started wondering how many chilling hours the Jungle had actually logged so far and whether other species were waiting with baited breath to break bud at the next warm spell too. Using nearby wunderground weather station data through www.getchill.net, I determined the Jungle had accumulated approximately 759 chilling hours from November 1st - January 1st. What does this all mean?
Normally, with the onset of fall weather, deciduous fruits and nuts have stopped actively growing and have begun to drop their leaves in preparation for a long winter rest. This rest period, termed endodormancy, is necessary to winter survival for not only fruits and nuts, but for many trees, shrubs and bulbs. Endodormancy is a type of dormancy where the buds are dormant because of an internal plant inhibitor system that prevents growth even under ideal external growing conditions. Once a plant has entered endodormancy, it will not grow again until it has received enough cold to overcome the dormancy. However, the best temperature for chilling is not the extreme cold. Effective chilling temperatures to terminate dormancy are generally between 32 to 45OF (it is generally agreed), and approximate hours of chilling required to break dormancy will vary according to species and cultivar.
For the most part, trees adapted to Illinois growing conditions rarely have difficulty attaining the proper chilling hours necessary to break endodormancy. When the chilling requirement has been met, rest (endodormancy) ends, and the plant is ready to grow, regardless of time of year. The continued dormancy of the plant then depends on another form of dormancy called quiescence or ecodormancy. With this type of dormancy, buds are dormant as a result of external conditions unfavorable to growth, i.e., as long as the temperature is cold, the plant will not grow. For this reason, late winter thaws and warm early spring weather in Illinois can be especially disastrous to fruit growers. Early warm weather can overcome ecodormancy, allowing premature growth that can be damaged by subsequent cold and frosts. When purchasing fruit plants, particularly peaches, it is important to avoid low-chill requirement cultivars that have been selected for Mediterranean and sub-tropical climates. Chilling requirement for these cultivars are met very quickly in the Midwest, making them all the more susceptible to early bud break and frost/freeze injury.
In general, the earlier a plant blooms, the more probable it is to suffer some amount of frost damage. A general rating of sensitivity to winter cold and spring frost is as follows: apricots and sweet cherries>peaches, nectarines, and Japanese-type plums>pears and sour cherries>European-type plums and apples. Luckily, fruit trees produce an overabundance of fruiting buds which insures against quite a bit of frost damage and other mishaps. Apples, for example, have a fruit set range of 2-8%. That means 92-98% of the flowers are extra!
Plant Type Approximate Hours of Chilling
Required to Break Dormancy (<45oF)
Apple 800 to 1750
Raspberry 800 to 1700
Pear and sour cherry 600 to 1500
Currant and gooseberry 800 to 1650
Walnut 400 to 1550
Plum (Japanese) 600 to 1600
Pecan 650 to 1550
Cherry (sweet) 500 to 1450
Blueberry (highbush) 800 to 1250
Peach 375 to 1200
Apricot 300 to 1000
Blackberry 350 to 600
Grape 50 to 400 (adequate growth)
400 to 1650 (better and faster growth)
Strawberry 50 to 300