High union, more dwarfing
Horticulturists recommend planting trees with the bud union well above the soil level to avoid scion rooting, but the higher the bud union, the more dwarfing the tree will be.
Dr. Bruce Barritt, retired Washington State University horticulturist, said it's been well documented in European studies that the higher the trees are planted, the greater the dwarfing effect will be, at least with the Malling 9 rootstock.
Writing in the July 2008, edition of the Australian Fruitgrower magazine, technical editor Dr. Gordon Brown discusses research conducted from the 1970s to the 1990s by Dr. M.S. Parry at East Malling, England, and by Dr. Karoly Hrotko in Hungary. The scientists used M.9, M.26, and Malling-Merton 111 rootstocks in their studies of the effect of bud union height on tree size.
Their findings indicate that the first 10 centimeters (4 inches) of union height has a major impact on the dwarfing ability of the rootstock, and the degree of dwarfing is directly proportional to the height of the bud union from the soil, Brown reports. Hence, the height of the bud union above the ground must be consistent to avoid uneven growth.
With all rootstocks in their trials, the higher the union height above the soil, and the more dwarfed the tree, the higher the yield of fruit per unit of tree size.
So, could you use a more vigorous rootstock and plant the bud unions higher to decrease the vigor?
The European research showed that the growth rate of trees on M.26 rootstocks planted with the bud unions 30 centimeters (12 inches) above the ground was less than of trees on M.9 with the bud unions 15 centimeters (6 inches) above the ground.
But Barritt said several factors need to be taken into account when deciding on planting depth. One is fumigation. Trees in unfumigated replant soil typically don't grow well anyway. "If you don't fumigate and you leave eight to ten inches of rootstock above the soil, you're asking for trouble," he said.
Del Feigal, manager of Auvil Fruit Company's ranch at Vantage, Washington, has trees that were accidentally budded too high—at between 12 and 18 inches above the ground. The trees are so dwarfed that they will never fill their space, and this is the case both with Mark and M.9 rootstocks. "It really shows a big difference in dwarfing," he said