Mounding Honeycrisp may overcome weak soils
Mounding might keep Honeycrisp from runting out.
Researchers used a grape hoe to build a berm covering the dwarfing rootstock and protecting it from dogwood borer infestation. They also noticed a boost in tree vigor.
Orchardists growing Honeycrisp apples on weak soils might want to try mounding soil three or more inches above the graft union and leaving it for the first two or three years after planting.
Michigan State University horticulturist Dr. Ron Perry gave that advice while speaking to growers in the Traverse City, Michigan, area, where soils are sandy, even gravelly, and Honeycrisp trees propagated on dwarfing rootstocks often runt out before they fill their space in the orchard. Perry spoke during the Northwest Michigan Orchard and Vineyard Show in January.
“You can grow high-quality Honeycrisp here—probably better than anywhere,” he said. “But it’s a weak-growing variety. You definitely want to keep the precocity of the dwarfing rootstocks, so don’t use MM.106 to get greater vigor.”
Perry noticed that mounding increased the vigor of Honeycrisp trees when he tried mounding of apple trees on dwarfing rootstocks to avoid problems with dogwood borer.
“We are beginning to notice that mounding may also improve canopy vigor on this weak-growing variety,” he said, emphasizing that this is an observation, not the result of a controlled, scientific study.
Growers don’t want to plant trees deeper, because that can cause scion rooting, Perry stressed. He recommends that apple trees be planted with the graft union four to six inches above the soil line. Scion rooting can result in trees that are 20 feet tall after ten years, which makes them problematic in high-density plantings.
Trees settle in the ground following planting. “Overgrowth at the union on dwarfing rootstocks can result in the expansive scion tissue reaching down to the soil and striking roots,” Perry explained. “Scion roots more than one-half inch in diameter will negate the dwarfing rootstock influence, especially after the fifth growing season.”
Taming burr knots
Growers face something of a Catch 22. When the union is set at six inches or higher above the soil, the rootstock shank is exposed, which for most dwarfing rootstocks means the potential development of burr knots, he said. Burr knots are troublesome because they attract damaging insects.
The MSU horticulturists found that covering the graft union will protect newly planted trees from dogwood borers and also from cold weather during the first winter. Borers, and also woolly apple aphid, are attracted to the burr knots, feeding on and laying eggs in these “primordial root” sites, he said. The borer larvae invade and can stunt or even girdle and kill the trees. New York researchers estimate that half of the apple trees on dwarfing rootstocks in that state will be infested by borers, Perry said. He suggested that it is nearly that high in Michigan as well.
Growers now use an annual trunk spray of Lorsban (chlorpyrifos) to control borers, the only chemical treatment available and one that might not survive U.S. Environmental Protection Agency scrutiny in the future. Thorough coverage is needed on the lower trunk in each year of the first five years in late June to mid-July.
MSU researchers reported in 2005 that almost total control could be achieved by covering the rootstock with soil, eliminating the need for the insecticide treatment.
At the same time, covering burr knots will encourage the resting primordial roots to extend into the soil as adventitious roots, and that may add vigor to the growing tree in the early years, Perry said.
In his work with dogwood borer suppression, soil is mounded about three inches above the union within a month after planting. After three years, he noticed, if the mound is still in place, adventitious roots might initiate above the union from scion tissue, and that should be avoided. By the third year, the mounded soil might have eroded and settled to below the union, but if not, it must be removed with high-pressure water or some other method. Adventitious roots that initiate from the scion, once exposed to air, will die or can be clipped off. If woody scion roots have been established, cut them off.
Meanwhile the roots that initiate from the burr knots on the rootstock shank extend into the soil profile and no longer provide a food source for the insect larvae. These roots become woody with bark similar to that seen on branches and trunks. These bark-covered roots do not express phytotoxic symptoms when herbicide treatments are directly applied, Perry said.
Trees in orchards where scion roots have been generated will show excessive vigor after six or seven years, and this problem can’t be rectified, he said.
The higher the bud union is above the ground, the more dwarfing effect there is on the tree. “Europeans have used this knowledge for years in ultra-high density plantings to keep trees weak by planting so that unions are as high as 12 inches above soil,” Perry said.
His “rule of thumb” suggests that, for the M.9 rootstock, every inch the graft union is above the ground translates to 6 to 12 inches reduction in tree height.
In using the practice of mounding to avoid problems with dogwood borer, he has noted that those trees that generated roots on the rootstock shanks have improved vigor.
In the case of weak-growing Honeycrisp on dwarfing rootstocks, this could be an additional benefit beyond avoidance of dogwood borers, he said. “That’s already quite a benefit when considering that forming the mound is only done once at planting time rather than treating the insects each year as they attempt to infest during those first seven years when trees are vulnerable to attack.”