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This week I’m going to talk about chill hours and the affect that cold has on plants. The cold weather can feel very dreary and like there is no life with plants not growing, trees without leaves, and wildlife holing up for the winter. However, winter is a very important and necessary time of the year for plants. Without it there’s a number of important fruits and flowers that would not bloom every year. Let’s talk about the effect that cold weather can have on plants and why it’s so important.

For some plants they are not just responding to the cold, but they are also responding to the change in day length as the days grow shorter in winter and then longer into the spring. These kinds of plants are called photosensitive, because they are responding to the changes in light. Mums and poinsettias are some common examples. A change in the day length can trigger blooming or other responses in these plants.

Many fruit plants require chill hours to be able to put on blooms. Chill hours is a count of the number of hours where the temperature is less than 45 degrees Fahrenheit. After a satisfactory number of chilling hours has been reached, the plant will break into bloom. This chilling process is called vernalization. Without vernalization, we wouldn’t be able to enjoy apples, peaches, tulips, foxgloves, blueberries, and many other plants. Different varieties of plants require different numbers of chilling hours, so it’s always best to pick varieties that are well adapted to our climate so that you have the most success with your fruits and flowers. For example if you select a variety of apples that requires too many chill hours, the apple tree will not bloom. However, if you select a variety of apples that has a chill hour requirement that is too low, it could bloom too early, and the flowers would be killed in a frost.

In these plants, a repressor gene is keeping the plants from blooming. The cold causes that repressor gene to be altered so that it’s no longer controlling the blooming of the flower. That way when warmer temperatures arrive the flowers can bloom out.

The timing of when cold happens is also critical. Oftentimes in the mountains, we’ll have a late freeze after many of the apple trees have already bloomed. Freezing temperatures after bloom can result in flowers dying and falling off. If the blooms fall off a fruit tree, that tree won’t be able to bear any fruit that year. For a freeze to damage blooms the temperature usually need to be 28 or lower. Early freezes can be damaging to plants as well. They are forced into freezing temperatures before being able to harden off properly, and can be severely damaged.

Some plants are able to tolerate cold weather while others can’t. Scientist believe the ability to tolerate cold weather comes down to the makeup of the cell membranes in the plant. Cell membranes are made up of fat molecules. Some of them are saturated and some unsaturated. Similar to what you read on the nutrition facts label for packaged food. Scientists think that plants with a higher ratio of saturated fats can freeze more easily.

Winter can seem like a dead time of the year, really it’s Mother Nature’s way of preparing for spring. If you have questions about the effect of winter on plants contact your County Extension Office or send me an email at Jacob.Williams@uga.edu.

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