The trees are developed in the nursery as bi-axis trees, starting from budded bench grafts on Malling 9 rootstocks. Tustin said a bi-axis nursery tree has 30 percent more mass than a single-leader tree, which is a benefit in terms of rapid development of the canopy. After the tree is planted in the orchard, the two cordons are supported until they resume growth and are then laid down in either direction. Upright shoots, or stems, are trained from the cordons every 30 cm (12 inches). The multiple stems per tree help to manage tree vigor by dispersing growth.
Fruit density on the shoots will be managed by spur extinction (bud removal). Early adjustment of the crop load allows the tree to direct more resources into growing the remaining fruit. To achieve a yield of 170 tons per hectare/bins per acre, each stem will need to produce 45 apples, which Tustin said looks like a fairly low crop load. Provided the leaves have enough radiation to drive the growth of that fruit, he is convinced that such yields are possible.
The prototype planting, with Envy as the scion, produced 10 to 15 bins per acre of fruit on the cordons in the second leaf. That was a planned crop because the trees produced plenty of high-quality spurs and were quite capable of cropping, Tustin said. The stems can be grown up to 3.5 meters (11.5 feet) high. “But if we achieve 85 to 90 percent light interception with a canopy height of 2.5 meters (8.5 feet), that’s all we need,” he said.
The trees will be minimally pruned so that they can channel most of their resources into producing fruit. Each year, a tree grows fruit, shoots, and leaves and adds growth to its branches, trunk, and roots. The proportion of the total seasonal growth that ends up in fruit is known as the harvest index.
The harvest index (and thus fruit production) is enhanced by the use of dwarfing rootstocks, reduced pruning, and increased tree manipulation, Tustin said. “The greatest way to get a high harvest index is to reduce cutting of the canopy because the plant will naturally try to regenerate that part of the canopy that’s been removed.
“We will establish the canopy and have very minor intervention with pruning and training, simply because there’s no necessity to do it,” he added. “Then, we’re getting very close to pushing toward the upper limit of what yields might be.”
There will be no rule about renewing the vertical shoots. “If there’s nothing wrong with the fruit, nothing wrong with the buds, nothing wrong with the flowering, why would you replace them?” Tustin asked. “If you’re not replacing them you’re conserving the growth of the tree and the plant won’t have to regrow that and will be more likely to direct more of its seasonal energy into growing the fruit.”
How will the fruit be harvested with the rows so tightly spaced?
“In a whole-system design, anything or everything can or should be changed to optimize the system,” Tustin said.
He sees such challenges as opportunities to engineer new solutions. He envisions that much of the orchard work could be done using small machinery with no more than a 20 horsepower engine. Small, planar canopies are more amenable to mechanization, automation, and ultimately robotics.
“A platform won’t fit down there,” he admitted. “But what’s wrong with a platform with one person on it? It’s a matter of allowing your imagination to range a little bit. Agriculture is the only segment of our society and our economy that hasn’t miniaturized yet. That’s pretty weird when you think about it.” •