With high-density trellised systems for apples, the goal is to maximize yield, and a good fruit-bearing surface is needed to do that, says Andy Arnold, horticulturist with Valley Fruit Orchards in Yakima, Washington. He uses Promalin (6-benzyladenine and gibberellins) either as a spray or as a paint mix with scoring to stimulate branching.
For the spray, he uses 16 ounces of Promalin with 1.2 ounces of surfactant in five gallons of water, aiming for a pH level of 5.5. For the paint mixture, he combines 3.2 ounces of Promalin with 8 ounces of latex paint and 8 ounces of water, keeping the mixture agitated to prevent the paint from settling. The latex paint seals the scoring wounds.
On first-year wood, he uses just the Promalin spray at around bud swell, waiting for about a week of temperatures of at least 80°F, which will help promote the growth of the new branches.
On older wood, he uses the Promalin paint, scoring first with a Sheetrock knife. This is usually done from a platform along with other tasks, such as pruning and tree training. However, results are not always predictable. Sometimes he will paint and score a tree two years in a row, and there is still no branching. Then, the third year, the branches grow. “Sometimes it works, sometimes it doesn’t,” he said. “Every year we’ll go through and hit again, and score and paint again. I find eventually it will put something out. On older than third-year wood, it’s not as effective, but it still works.”
Dr. Don Elfving, Washington State University horticulturist in Wenatchee, said he suspects that higher concentrations of Promalin might be needed on older wood. He stressed the importance of using a cheap interior latex paint. Some paints designed for bathrooms contain a product to prevent mildew, which is toxic to the trees. Exterior latex paints often contain antiweathering agents, which are also toxic.
Jason Matson of Matson Fruit Company, Selah, Washington, said he uses Promalin to help induce branching in young trees. In apples, he sprays Promalin in the spring, using a squirt bottle, about one to two inches below the growth tip, and pinches off the two leaves below the tip, mainly to mark that the tree has been treated. As the tree grows during that season, Promalin is reapplied to stimulate more branching where needed. By mid-July, it is too late in the season to have the desired effect.
The following spring, at bud swell, a slight score is made above the buds where the Promalin failed to stimulate branching.
In cherries, the top three feet of the tree are scored only, as Matson said he thinks that Promalin tends to stunt the growth somewhat. The lower parts of the tree are scored and treated with a Promalin-paint mix.
“The big problem, though, is when all that fails, we end up with blind wood,” he said. He thinks this might be because of weak tree growth or because the Promalin was applied when the weather was too hot or too cold.
He’s noticed that going into the second season of growth, if the terminal in year one has set a fruit bud, the next season’s growth is always weak and difficult to branch.
Elfving said the terminal bud with the flower still exerts a strong influence on vegetative growth, and the little shoot that often emerges coincidentally with the flower bud is thin and weak and doesn’t always grow vertically, so it is disinvigorated. The best branch development occurs on wood that is very vigorous. It’s not possible to get good, vigorous growth on weak trees. The growth regulators can enable the plant to grow, but can’t create growth from nothing, he said, because the growth comes from the plant’s own vigor.
Elfving said the best strategy would be to cut off the terminal flowers, get some vigorous growth and then try to branch that. “You would have a better chance of having an effective branching response,” he said.