Avoid scion rooting
Scion rooting eliminates the dwarfing effect of the rootstock.
When establishing a high-density apple block, it's best to plant the trees higher in the ground than you actually want them, horticulturists say.
The trees will inevitably sink after planting and irrigating, and if the bud union is below the soil level, adventitious roots will grow from the scion. This will overcome any dwarfing effect of the rootstock so that the trees will in effect become seedlings and grow too big for their allotted space. Once the trees are scion rooted, there is no easy way to address the problem.
"I would say scion rooting is a serious problem, even in good orchards," said retired Washington State University horticulturist Dr. Bruce Barritt. "Even good orchardists have problems."
Dave Allan, a grower in Washington's Yakima Valley, estimates that about five percent of all the apple trees in Washington have a problem with scion rooting, although the effects are less pronounced in replant situations where tree growth is not so vigorous.
Allan said he used to plant trees with the bud union an inch above the ground but has gradually moved up to about eight inches. When trees are planted by machine, they will sink about two inches, leaving the bud union where he wants it—at about six inches above the soil level.
Tom Auvil, research horticulturist with the Washington Tree Fruit Research Commission, said most growers are aware of the danger of scion rooting, but tend not to think about it during the heat of planting. In some cases, their tree planting equipment is not set up properly.
"It's a big issue," he said. "In the last five to ten years, we've been planting increasing volumes of Malling 9 trees at higher densities, so we're just now starting to see that we have trees five to six years old that are becoming out of control with vigor. As the number of trees that have become scion rooted increases and the productivity of the block declines, you start having productivity and fruit quality problems, which impairs the economic vitality of the block."
Dr. Ron Perry, horticulturist at Michigan State University, said when using a mechanical planter, it's advisable to have someone follow-up and check each newly planted tree to ensure that it's at the correct depth. While the soil is still loose, trees that are too deep can be pulled up or trees that are too shallow can be pushed down. He recommends using a two-by-four– or a two-by-six–inch board that can be placed on edge on the ground to help accurately judge the distance between the ground and the bud union.
It's best to err on the side of shallow planting because it's much easier to mound soil up around a tree planted too shallow than to deal with scion rooting, Perry said. "The last thing you need five years down the road is to have trees planted five feet apart in a high-density planting that should have a spacing of 15 to 20 feet."
Barritt said scion rooting can be more of a problem in organic orchards if cultivation for weed control results in soil being mounded up against the tree trunks.
However, trees can scion root even when the bud union is above the ground. Barritt has seen a Gala planting on M.9 where the bud unions were at least three inches above the soil line, but the scion had sent down roots on the outside of the rootstock. The orchard did not have good weed control, which created a humid environment around the base of the trees.
"You need to keep it clean around the trunk, because the scion, given the right conditions, will grow," he said.
Auvil said there should be at least four inches between the bottom of the bud shield and the soil line to avoid scion rooting. That's not four inches from the top of the rootstock to the ground, he stressed. "It's four inches from the bottom of the bud graft to the ground, and for an upright system, that's the minimum."
For angled systems, the bud union should be even higher, he said.
Auvil, who has an orchard at Orondo, Washington, said trees usually need to be planted an inch or two higher than they were in the nursery. "We've actually become a lot more comfortable planting the roots out of the ground and not burying all the roots that came from the nursery," he said.
In some cases, the planting depth is limited by the length of the rootstock shank on the nursery trees, but Roger Adams, vice president of Willow Drive Nursery in Ephrata, Washington, said nurseries have responded to calls from growers for higher-budded trees. Willow Drive aims to have 16 to 17 inches of rootstock below the bud.
Adams said that while it's ideal to have the bud union four to six inches above the ground, he doubts that's the norm. "Where they're using a tree planter and they're putting them in as fast as they can go by the thousands, they're probably all having to pull those trees up a little bit and stomp them down to make sure they don't get planted too deeply."
Plant in place
Del Feigal, manager of Auvil Fruit Company's ranch at Vantage, Washington, said he plants the rootstocks in place, which avoids the problems of sinking. The trees are budded at between six and eight inches above the ground.
In years past, when he used nursery trees, he had problems with scion rooting, he said. For example, in a Gala block on M.26 rootstocks that had scion rooted, he tried to combat it by removing some trees and allowing the others to grow as seedlings. Eventually, the block had to be removed. In Granny Smiths that have scion rooted, he girdles the trees to reduce the vigor and to get them into more of a fruiting mode.
Allan said another solution is to use a hoe to pull the dirt away from the tree and chop off the scion roots. "If you recognize you're scion rooting in the first three years, you might be able to get someplace by moving the dirt away from the tree," he said. "But after that, you're done."
He's also tried scoring, but said there's no good cure for scion rooting. "That's why we're up to six inches. We don't want to go there any more."