Understanding and overcoming the tree’s natural tendencies is part of good orchard management.
The optimized orchard system generates high production and high-value target fruit. The trees have little structural wood and are supported by an eight-wire trellis.
“If left alone, an apple tree will not develop a canopy that maximizes both light absorption and light distribution. That is your job and your challenge as a grower.”
Dr. Don Elfving, a Washington State University researcher at the Tree Fruit Research and Extension Center in Wenatchee, makes that assessment.
Light absorption and light distribution are both critical to growers’ ability to produce high yields and a high proportion of “target fruit”—fruit of the size, color, shape, and condition that is in demand by consumers and for which they pay the best price, he says.
In the past, growers had the luxury of being able to sell lesser quality fruit into juice and processed apple product markets. That kind of fruit grew inside the tree canopy and on lower, shaded limbs. Today’s grower can’t justify such a canopy or a tree that shades itself.
In a video presentation at the International Fruit Tree Association annual conference in Grand Rapids, Michigan, last March, Elfving showed a photo of one kind of optimized orchard system that would have high production (80 to 100 bins per acre) and high-value target fruit. The picture was taken by Del Feigal at Auvil Fruit Company in Vantage, Washington. That orchard is, Elfving said, “an example of controlled canopy development that favors optimum light distribution and absorption for maximum production of quality fruit.”
Little structural wood
The orchard was planted at high density (1,800 trees per acre), on a precocious rootstock (Mark), and affixed to an eight-wire, 12-foot-tall trellis that supports the trees. The trees have very little structural wood and cannot support themselves. The limbs are tied to the wires, and there is no interior to the canopy.
“High light absorption is assured by the large number of trees per acre and the canopy each tree has developed,” he said. “The precocious rootstock assures good flowering, both early in the life of the orchard as well as at orchard maturity. Careful regulation of canopy development assures uniform vigor from the top of the canopy to the bottom. The orientation of the canopy, control over vigor distribution, and the careful development of lateral fruiting wood assure near-optimum distribution of light throughout the entire depth of the canopy. Canopy thickness is restricted to limit the shading the tree casts upon itself.”
Over the years, researchers have studied the way apple trees do things and concluded, as Elfving said, that apple trees, left to themselves, won’t deliver target fruit.
“Understanding and overcoming the tree’s natural tendencies is an important component of good orchard management planning,” he said.
Leaves are the factories where the conversion of light energy to chemical energy takes place, he explained, and the typical apple leaf will absorb or reflect 90 to 95 percent of the light that falls on its surface.
“Only a tiny fraction of the light falling on the upper surface of an apple leaf passes through that leaf, and the small amount of light that is transmitted emerges from the bottom side of the leaf depleted in those wavelengths of light that are the most important for powering photosynthesis.
“Leaving openings in the canopy to permit light to penetrate and avoiding too thick a canopy are critical strategies in canopy management that can enhance production of so-called target fruit,” he said.
Using reflective mulch on the orchard floor can also help capture more light and direct it into the lower branches, where shading might occur.
If leaves were merely energy factories and fruit just storages, that would make things easier, too. But that’s not the way it is.
“The formation of flowers and the growth and development of optimum fruit quality also require light,” he said. “It is generally thought that individual spurs need to receive at least 30 percent of full sun equivalent to form flowers that have the potential to set and form fruit.
“Good fruit growth and development of quality fruit color are thought to require as much as 50 percent full sun falling on the spur and the fruit itself.
“Good light distribution in the canopy accomplishes more than simply assuring an adequate supply of fixed carbon.”
Apple trees are quite effective fruit producers, he said. Well-designed orchards can intercept high levels of light energy and the trees can convert much of that into fruit.
“Mature apple trees may routinely allocate half or more of their total seasonal supply of fixed carbohydrates to the growth of their fruit,” he said.
High fruit production comes at the expense of structural wood and roots. “There is an inverse relationship between the earliness and quantity of fruiting and the growth of a strong tree framework and root system,” he said. “Because of this unavoidable fact, growers intent on early production have to install some sort of artificial tree-support system to replace the robust woody structure that such a tree cannot develop on its own.
“In the old days, apple trees became self-supporting because they spent the first several years in the orchard in a vegetative state, growing the roots and wood that eventually would both hold up the tree and carry the fruit,” he said. “Nowadays, we cannot afford to wait the several years it takes for a woody framework to form, and we don’t want to deal with the large trees that approach implies. External tree support is an inevitable consequence of our success in causing apple trees to alter their distribution of fixed carbon from wood to fruit.”