Bibaum (divided trunk) pears are grown at the Ferrara fruit research station in Italy
Fruit growers who adopt a spindle style of tree—tall, narrow cylinder shapes—believe the resulting fruiting walls will lend themselves to mechanization and lower their labor costs, with pruning and harvesting being done by workers on mobile platforms and thinning being done by string thinners.
In Italy, growers of apples, pears, and sweet cherries are experimenting with a bi-axis system to develop those fruiting walls. The name being used by one of the largest nurseries producing these trees is Bibaum—literally meaning divided trunk—describing the trees it sells.
Stefano Musacchi, from the University of Bologna, said growers in the Po Valley wanted to solve the vigor problem they have with the slender spindle system, and for about five years have been working with the bi-axis system. It’s greatest application is in pears, where vigor control using rootstocks is less feasible. Apple tree vigor is easier to control with rootstocks, and that is becoming true in sweet cherries, where the Gisela rootstocks are taming tree size.
As Musacchi sees it, the slender spindle system has a number of advantages: It is highly productive, easy to pick, easy to prune, produces high quality fruit because of good light penetration, and has a long life span because of the renewal pruning that continually removes the largest limbs and encourages fruiting on smaller wood.
The disadvantages of slender spindle, he says, are that the fruit distribution could be better—bottom fruit is borne too close to the ground; it can be difficult to get limbs to grow at the wires where you want them; it is costly to prune and bend limbs to wires; sometimes tree vigor is hard to contain on fertile soils; and it’s costly to plant all those trees, sometimes up to 1,600 per acre.
In the bi-axis system Musacchi is working with in Italy, two trunks are grown from each rootstock. The trees are developed in the nursery. The young whip growing from the rootstock is headed high, to get fruit up away from the ground, and two scion buds are grafted on opposite sides, directly across from each other. It is important that the two buds respond equally and that two equally strong leaders grow; one must not take over and act like a dominant central leader, he said.
The trees develop like the perpendicular V sometimes used in peaches, but the dual leaders are oriented parallel along the row, not perpendicular to it. This keeps the fruiting wall narrow. Rather than developing several upright leaders or scaffolds, as in a palmette style, the dual leaders are kept free of large or long side branches, pruned as in the slender spindle system.
Musacchi says that planting on a 3.3- x 1.2-meter spacing (11-foot alleys and trees 4 feet apart) requires about 970 trees per acre, so 485 rootstocks can be used to fill the space with 970 tree trunks. The trees are tied to two trellis wires, one about a foot above the crotch and the other about three feet higher. Tree height is kept to ten feet by heading the trunks at that level.
In the Po Valley, where soil is fertile and the growing season is long, the two-trunk system splits the vigor and makes it easier to contain tree size, he said. In pears, where, unlike apples, good dwarfing rootstocks are not available, the bi-axis system is very useful for restraining tree vigor, Musacchi said.
The concept of diffusing vigor among multiple leaders has long been useful in stone fruits, which also have fewer dwarfing rootstocks available. Training systems like the open vase and Spanish bush diffuse vigor among many branches to keep tree height down.