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Let There Be Light

Have you ever thought of your peach orchard as a big solar receptor?  Depending on the time of year, age of the trees, how closely trees are spaced in the row, and how close the rows are to one another, your peach orchard may intercept up to 80% of the sunlight that is incident upon it in a given day.  Given this fact, you may ask why your yields are not higher, why your fruit are not better colored at harvest, or why disease problems are more serious in some orchards than others.  Perhaps you think that growing your trees taller and closer together is the answer.  Maybe they should be shorter and farther apart.  Is there a single formula that will work every time?

Let The Sun Shine In

There are many factors influencing your orchard that you cannot control, but light is something that you can manage.  This begins at planting.  You should orient your tree rows so that they run in a north-south direction to maximize sunlight interception throughout the season.  In the early life of the orchard, irrigate, fertilize, and prune so that trees will fill in the tree row as soon as possible.

Once trees have filled in the tree row, the basic structure of the tree is now set and you are managing the leaf canopy with the goal of producing significant yields of good-size, high-quality peaches.  Since the leaves are the “factory” for manufacturing carbohydrates that are necessary for tree and especially fruit growth, you need a substantial, healthy leaf canopy.  Leaves need to “see” the sunlight to be most effective.  Fruits also need to “see” the sunlight to get adequate coloration prior to harvest.  The bestcolored, earliest-ripening, and largest fruits are typically on the periphery of the canopy near the top of the tree where they receive the most sunlight.  On the other hand, fruits are usually smaller, ripen later, and are poorly colored near the crown of the tree.

Sunlight should be able to penetrate the tree canopy from top to bottom so that the maximum light can get to each part of the tree.  Shading negatively impacts fruit color, size, and return bloom the following year.  Too much foliage in the inside of the tree also creates high humidity conditions that favor disease and limit pesticide penetration.  In addition, too much vegetative growth increases labor costs for pruning during the dormant season.

It is critical to balance canopy development with cropping.  When crop load is high, vegetative growth may be limited due to high competition with fruits for water or nutrients.  However, when crop load is low because of poor pollination or other conditions, vegetative growth may be excessive, causing excess shading.

The combination of dormant and summer pruning can ensure a proper balance between crop load and canopy development to allow maximal sunlight penetration and utilization from the top to bottom of the tree canopy.  Selecting good-quality fruiting wood that will be well-exposed to the sun and removing large diameter non-fruiting wood should be done during the dormant season.  During the growing season, selective removal of five to 10 strongly vigorous, upright, multi-branched, current season shoots per tree can dramatically improve sunlight penetration into the canopy, leading to improved fruit size and color, pesticide penetration, and healthy fruiting wood.

Other Methods

Some additional growing season practices to improve sunlight penetration into the canopy include the use of leaf stripping or “pulling” and reflective mulches.  Although both practices can improve coloration of fruits, there are significant disadvantages and costs. Hand removal of leaves near the fruit is labor intensive, costly, and reduces the leaves most necessary to support fruit growth.  Although fruits may be redder, they will usually be smaller.

Reflective plastic mulches that resemble aluminum foil are rolled out on the ground of the drive row to reflect sunlight back into the tree canopy.  These plastic mulches may result in improved coloration of fruits in the lower part of the tree canopy and may cause earlier fruit ripening.  However, they are costly and difficult to dispose of, and may increase heat and predispose trees to drought stress if rainfall is inadequate.

As you move into the 2005 peach season, are you satisfied with your light management program?  Are you thinking like a peach?  Perhaps some strategic adjustments in your orchard light management program may be worth consideration.

This column by Dr. Desmond R. Layne, “Let There Be Light”, appeared in the May 2005 issue of The American Fruit Grower magazine on page 62.