A growth regulator called Tiberon (cyclanalide) is available to stimulate branching of fruit trees, but it is only registered for use in nurseries and not in an orchard setting.
Dena Ybarra at Columbia Basin Nursery in Quincy, Washington, said growers usually want ¾-inch caliper, highly feathered trees. Good branching occurs naturally with Granny Smith and Braeburn apples, but branching on other varieties is less consistent and varies from year to year.
Promalin (cytokinin and gibberellins) works well in humid areas to stimulate branching but not in Washington’s drier climate, she said. The nursery industry standard treatment is to remove the leaves below the shoot tip when the trees are about 27 inches tall and spray the tip with Promalin.
Tiberon is a new product that is sprayed on trees when they’re between 24 and 36 inches tall. It stimulates growth of small, flat branches at a wide range of heights. However, Ybarra said she has not used Tiberon on pear trees because they are ultra sensitive and it might not produce the desired effects.
Dr. Don Elfving, Washington State University horticulturist, who has studied Tiburon in trials, said that pear trees are five to ten times more susceptible than apples or cherries. So, if the application rate is 100 parts per million on apples, it should only be 10 to 15 ppm on pears.
During a discussion at the Washington State Horticultural Association’s annual meeting, a grower asked if, in theory, it would be possible to plant a high-density block of Bing cherries on Mazzard and do multiple applications of Tiberon over the course of several years and end up with a tree similar in size to one on a Gisela 12 rootstock and with lots of dards. Dards are shoots that are smaller than feathers and are capable of carrying fruit.
“You could do it,” Elfving responded. “But I don’t know if it would be the tree you want. It would be a Bing tree with lots of branches. This product does not suppress the vigor, and if you put the branches on there, with the strength of Mazzard, you would get strong growth with a lack of precocity. We don’t know the effect on flowering.”