This time of year, fruit trees start arriving at the local garden centers because January and February are the months fruit trees need to be planted.
There are many varieties available at the garden center, but having the most success requires planting the right type of fruit and the right variety of that species. It's essential to plant varieties that will fit into each yard or landscape. In a small yard, plant varieties that are dwarves. In large yards, plant standard varieties.
Another factor to look at is disease resistance. This is particularly true of apples and pears. Fire blight is a major disease of these trees, and it's important to plant varieties that are resistant.
The most important factor is to plant a variety that has the proper chill hours or chilling requirement for our area. The amount of cold needed by a plant to resume normal spring growth after winter is commonly referred to as its "chilling requirement."
During the fall and winter, deciduous fruit plants enter a dormant period generally referred to as the plant's "rest period." Plants enter the rest period in the fall as air temperatures start to drop below 50 degrees. The leaves begin to fall and visible growth ceases.
Another less-visible change takes place when different growth regulators in the bud start to increase or decrease. As the temperature cools, the growth-promoting regulators decrease and the growth inhibitors increase. As the chilling requirement of a plant is being satisfied by cold temperatures, the level of promoters increase while the level of inhibitors decrease. The higher levels of promoters in the buds allow normal resumption of growth and flowering in the spring as the chilling requirement is met.
The type of cold temperatures needed to satisfy the rest requirement of fruit plants, especially fruit trees, has been studied carefully. The calculation of chilling hours begins on Oct. 1 and ends Feb. 15. Temperatures of 35 to 55 degrees provide most of the chilling effect needed by fruit plants. The most efficient temperature at which a plant receives chilling is 45 degrees.
Temperatures of 32 degrees and lower contribute little or nothing to the chilling being received by the plant. Therefore, most of the cold that we had last week added very little chill hours to our fruit trees. Also, when we have daily temperatures of 70 degrees and higher for four or more hours it can actually negate chilling that was received by the plant during the previous 24 to 36 hours.
When the plant has received the proper amount of chill hours, it starts collecting heat units. When the heat units have been satisfied, the plant will start to bloom.
Planting a high-chill plant in a low- chill environment can cause spring bud break to be erratic and prolonged. This can lead to poor pollination and fruit set.
Fruit trees all have different chill requirements. The following is a general guide for the different types of fruit that we grow in our area:
- Apples have a range of 800 to 1,100 chill hours. But there are a couple of varieties such as Anna and Dorsett Golden that have fewer chill hours. These varieties are best suited for the lower coastal plain.
- Pears can be divided into three groups, and each group needs different chill requirements. European hybrids need 800 to 1,100 hours while hard pears and Asian pears need 400 to 800 hours.
- Peaches and nectarines need 400 to 1,050 hours.
- Plums need 400 to 700 hours.
- Muscadine grapes need 200 to 600 hours.
- Blackberries need 50 to 800 hours.
- Blueberries need 400 to 700 chill hours.
When planning to buy fruit trees, look at the tag on the plant for chill hours or look in a grower's catalogue for chill hours for that variety.
The total chill hours last year was 1,006 hours. In 2008 it was 889 hours and in 2007 we had 1,008 chill hours. This year through Jan. 4 we have accumulated 572 hours. This compares to 541 hours as of this same period in 2009, 443 hours in 2008 and 467 hours in 2007.
So, when considering different varieties of fruit trees for our area, choose one in the 800 to 900 chill-hour range.
Charles Phillips is a retired Columbia County Extension Service agent. He can be reached at email@example.com.
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