The Importance of the Cold

Posted 3/10/19

I remember back many winters ago when I was just a kid, the temperature got so cold even the penguins headed south for the winter (or at least it seemed that cold to me). My father who taught me more …

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The Importance of the Cold

Posted

I remember back many winters ago when I was just a kid, the temperature got so cold even the penguins headed south for the winter (or at least it seemed that cold to me). My father who taught me more about farming and agriculture than any of my formal education ever did, tried to explain to me that the cold weather provided winter chill for our fruit trees and that it was actually good for them – although it didn’t make me feel much better since the house I grew up in had neither air conditioning or central heating. I really thought that all the cold drafts that were blowing through our house had really gotten to him and to be quite honest, I thought it was my dad who was a little bit “fruity”. Many years and two college degrees later, I now know that once again it was my dad who was right about the fruit trees.

The amount of cold needed by a plant to resume normal spring growth following the winter period is commonly called “chilling requirement”. Plant species as well as fruit tree varieties vary greatly in their winter chill hour requirement. Chill hours is one of the most important things to consider when selecting types and varieties of fruit to grow.

During the fall and winter deciduous fruit plants (those that shed their leaves) enter a dormant period which is generally referred to as the plants “rest period”. Plants enter their rest period in the fall as air temperatures begin to drop into the 40’s (give or take), leaf fall occurs, and visible growth stops. 

What happens inside the fruit tree next is nothing short of remarkable. Plants enter their rest period as the level of growth regulating chemicals or hormones changes. As the growth inhibitors increase and the growth promoting hormones decrease, the plant stops growing and goes to rest. As the chilling requirement of the plant is met by cold winter temperatures, the level of growth promoting hormones increase and the growth inhibiting hormones actually decrease in structures we call buds. The higher levels of growth promoting hormones allow the normal resumption of growth, flowering, and fruiting later in the spring.

So what happens when fruit trees do not get enough winter chilling? This is a classic example of what happened during the winter of ’99 ( hard to believe it has been that long ago now) when it stayed warm the entire winter. Fruit trees that do not receive enough chill hours do not go into true dormancy or rest. They are highly stressed and very likely to have disease, insect, growth, and fruiting problems. The stress can be so severe that the tree actually dies as many did in the years following that extremely warm winter. Think how you feel when you go three or even four nights in a row with no sleep and you will know what your fruit trees go through when they do not get their proper number of chill hours.

Temperatures of approximately 35 to 55 degrees provide most of the chilling effect needed by fruit trees; however, the most efficient temperature to provide chilling is around 45 degrees. Temperatures of 32 and under actually do not provide the type of chilling that the plant needs and instead can actually be detrimental if the temperature drops too low. In addition daytime temperatures of 70 or higher for as few as three or four hours can negate several hours of accumulated chill hours

Here is where things get a little complicated. Once the chilling requirement of your fruit tree is satisfied, the buds begin to swell and break dormancy as the temperature climbs above 40 degrees. Each type of fruit and variety has a particular heat unit or “growing degree hour” requirement to reach a given level of bud, flower, and fruit development. The growing degree hours are totaled and can be used to predict the stage of development of your fruit trees. For example, peaches usually require 10,000 to 13,000 GDH’s to reach 50 percent bloom stage AFTER their chill hours are met. The combination of chill hour requirement and growing degree hours will determine whether varieties flower early, mid-season, or late season. For example muscadines, grapes, and figs have a very low chill hour requirement but a very high growing degree hour requirement so they have no trouble getting their chilling and they flower late in the season, this makes them an ideal fruit to grow here in Walker County. On the other hand many plum varieties have not only a low chill hour requirement but also a very low growing degree hour requirement so they are very early bloomers and are frequently bitten back by late frosts.

Apples tend to flower late in the season compared to fruits such as peaches and plums because the standard varieties have high chilling requirements and high growing degree hours. So what can we learn from all this? The idea fruit is one that fits in the average chill hour range for the area that you are going to grow it in combined with a very high growing hour requirement. This will help ensure that you get later flowering and much more consistent fruit crops. Many home orchard owners simply go down to the garden shop or nursery and simply buy the variety of fruit trees that are on hand or else they have their mind set on one particular variety. From now on when you purchase fruit trees, inquire about the variety’s chill hour requirements as well as its growing degree hour requirements. Select only those varieties that fit your chill hour zone and that have the highest growing degree hour requirements. I promise that you will be much happier with your fruit selections.

I once reminded my father about our conversation about fruit tree chilling all those winters ago, and I even told him about chill hour requirements and growing degree hour requirements. I just had to laugh to myself because once again he summed it up in language that transcends education. He simply said “ Bud, I told you those fruit trees needed cold weather”.