Dwarfing cherry rootstocks promote smaller trees, boosting cultural efficiencies
By Renee Stern
Bigger isn't always better.
With cherries, a dwarfing or semi-dwarfing rootstock offers orchard efficiencies that pay off while still producing desirably large fruit. What's more, some variety selections known for their problems turn into winners when paired with dwarfing rootstocks.
"To me, it's a no-brainer," says Matt Whiting, assistant horticulturist at Washington State University's Irrigated Agriculture Research and Extension Center in Prosser. "The advantages have been demonstrated clearly."
Dwarfing rootstocks are more precocious, reaching production levels in three or four years rather than the five or six years needed with standard rootstocks such as Mazzard and Mahaleb, says Lynn Long, Oregon State University's Wasco County Extension horticulturist.
Workers accomplish orchard chores--thinning, pruning and harvesting--more easily with smaller trees that don't require extensive ladder time, Whiting says. That's a key factor in a crop where labor accounts for 40 percent to 50 percent of production costs.
Long's harvest comparisons show workers picking 100 pounds per hour in a traditional Mazzard orchard and 170 pounds per hour in a "pedestrian" orchard planted on dwarfing Gisela rootstocks.
"Pickers love (smaller trees) because they can make more money," he says. And that pays off for growers not only by reducing labor needs but also making their operation more attractive when competing for workers.
Those productivity gains help recover planting expenses in almost half the time--typically 15 years with a standard rootstock compared to eight years on a precocious dwarfing rootstock, Long says.
Standard rootstocks tend to be "overly vigorous," leading to a greater percentage of shaded leaves that aren't earning back their growth costs to the tree, Whiting says. Dwarfing rootstocks offer better light distribution through the canopy to bring the system into more efficient balance.
A too-dense canopy also creates obstacles for optimal spray penetration, he says. A more balanced canopy, in comparison, eases pest and disease control.
Match rootstock to variety
But market preferences push cherry growers toward the largest fruit possible to maximize profits, holding many back from switching to dwarfing rootstocks. Smaller trees have a smaller leaf area to support the fruit.
The trick is to manage the tree so it produces that desirable nine- or 10-row cherry instead of smaller 11- or 12-row fruit, says Greg Lang, horticulture professor at Michigan State University.
High-density plantings used in combination with dwarfing rootstocks add up to more trees per acre than in a traditional cherry orchard, and thus mean a much higher potential yield per acre, Lang says. That's countered by the need to reduce crop load to match the smaller leaf area and gain fruit size.
"In a well-managed orchard, there should be roughly equal yields," whatever the rootstock, he says.
Matching rootstocks with variety selections plays a big role, Long says. Precocious varieties such as Sweetheart, Lapins or Skeena require more management when matched with a precocious rootstock such as Gisela 6.
Gisela 6 produces trees not much smaller than Mazzard, but the semi-dwarfing rootstock responds better to hard pruning, making it easier to keep under control, he says.
In tests, the Sweetheart-Gisela 6 combination produced packouts with 76 percent of the fruit at 9 1/2-row or larger.
Mazzard may have produced slightly more of the large Sweetheart cherries, but over time the Gisela trees won out in total yields.
"The returns were probably greater over time with Gisela over the Mazzard," Long says. "And that's with the most difficult (rootstock-scion) combination."
While Tieton fared poorly in evaluations on Mazzard rootstock, it shined when combined with a dwarfing rootstock, Whiting says.
'No combination is necessarily better'
But, he cautions, "No combination is necessarily better. Some may require a little more management or a little less."
Other rootstock options include Gisela 3, 5 and 12, Edabriz, Weiroot 158 and 72, Krymsk 5 and 6, and an Oregon rootstock bred from Mazzard and Mahaleb dubbed Maxima 14.
Learn new management approaches
To ensure fruit size, growers must learn new management approaches, Whiting says.
You can't prune a tree on Gisela rootstock as if it were a Mazzard tree, he says. Instead, he recommends plenty of heading cuts and limiting shoot growth to remove future fruit sites.
At the same time, growers should plant trees closer together and prune aggressively. Leaving more space and allowing the trees to grow vigorously to fill in the gaps only produces more fruit sites and leads to a large crop of small fruit, he says.
Deficit irrigation is another way to control vigor, but not one Lang recommends. "I'd rather work with the tree instead of fighting its natural tendencies," he says.
Dwarfing rootstocks are more susceptible to water stress than standard Mazzard and Mahaleb rootstocks, simply because of their size. Smaller trunks and root systems have a correspondingly smaller storage capacity, and require more spoon-feeding when it comes to water and nutrients, he says.
Training systems also affect results when working with dwarfing rootstocks, Whiting says. An upright system such as a Y-trellis or Spanish bush works best; a central leader bent to the horizontal plane, on the other hand, encourages more fruiting.