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Some fruit-growing regions have big advantages over other areas, and one of those advantages is light, according to Dr. Dave Ferree, retired horticulturist at Ohio State University.

For example, orchards in Washington State receive 30 percent more light early in the season than orchards in Ohio, he explained during the International Fruit Tree Association’s 2006 conference.

And while New Zealand orchards have about the same light levels as Ohio orchards early in the season, towards harvest the light levels in New Zealand are much higher, he said. As a result, yields in New Zealand are almost double those in Ohio, and the fruit is 30 percent larger.

Light is an energy source for photosynthesis, during which leaves take carbon dioxide out of the air and combine that with water from the soil to produce sugars. The more light there is, the more photosynthesis takes place—up to a point. At about 30 percent full sunlight, other factors become limiting.

Dr. Don Heinicke, retired U.S. Department of Agriculture researcher in Wenatchee, Washington, has said that the top of a tree intercepts 30 to 60 percent of the light, but in a big tree, a large area of the canopy gets less than 30 percent light. Areas that don’t receive 30 percent of full sunlight don’t produce flowers. Fruit set depends on light interception, and high light interception is needed to produce fruit with high soluble solids and large size.

For good coloring of fruit, 30 to 50 percent of full sun is needed. However, high coloring strains of Red Delicious can color well with as little as 9 percent full sun, though internal quality would be poor, Ferree said.

“Where light is high, we have large size and high quality. Where the light is low, we have small size and poor quality.”

Practices to improve light

An inexpensive way to get the best light interception is to orient the tree rows to run north-south, rather than east-west. This results in more even distribution of light throughout the tree, Ferree said. In trials, north-south rows had 19 percent more flowers, 11 percent more fruit, and 28 percent higher cumulative yields than comparable east-west rows.

Tree shape and height also affect light interception. In an 8-foot-tall tree, only 2 percent of the canopy receives less than 30 percent full sunlight, whereas in a large tree, 30 percent of the canopy can have too little light. However, in a slender-spindle trial where the trees were kept low enough to do all the work from the ground, the trees had to be pruned heavily, which generated a lot of shoot growth and shade in the bottom, Ferree reported.

The ideal tree is 10 to 12 feet tall and 10 feet wide, and is slightly wider at the bottom, he suggested. It should have an open, spreading canopy. The free alley should be half the tree height so that one row of trees doesn’t shade the next. In a tree with good light penetration, spray penetration will also be good, Ferree noted.

Asked about reflective mulches, Ferree said the tree must be well pruned and open for the mulch to be able to reflect the light up through the canopy. “You still need an open canopy to do that,” he said.

Mulches will increase light in the tree, he said, but growers need to look at the cost and whether it can be justified for the cultivar they’re growing.