—story by Matt Milkovich
—illustration by TJ Mullinax
When training crews to renew limbs in apple trees, teaching a uniform stub length might be the simplest approach, but that doesn’t necessarily make it the best answer.
Michigan State University tree fruit physiologist Todd Einhorn always assumed that the ideal stub length varies depending on multiple factors. Interactions among cultivar, vigor, management strategy and tree spacing are complicated, and he didn’t think a uniform renewal length would be optimal in every situation. Once he started hearing Michigan growers talk about the advantages of making uniform 4-inch renewal cuts, however, he knew he needed data to back up his assumption.
“Branch renewal is a simple concept — whack off a branch and get a new one — but the technical details haven’t been well researched,” Einhorn said. “We wanted to look at longer stubs in a systematic way.”
Einhorn tested varying stub lengths for two years at MSU’s Clarksville Research Center. He and assistant professor Mokhles Elsysy studied the effects of leaving 1-inch, 2-inch, 3-inch and 4-inch limbs on third- and fourth-leaf Honeycrisp, Gala and Fuji trees trained to a tall spindle system on Budagovsky 9 rootstock.
Two years of data confirmed Einhorn’s assumption: Some cultivars benefit from a longer 4-inch stub, while others do not. Stub length should be considered on the basis of cultivar and site vigor, he said. One size does not fit all.
Stub lengths
Limb renewal practices vary in Michigan, but growers typically remove a couple of big branches per tree every year to grow more balanced limbs with higher yield potential. One of the decisions they need to make when cutting off the old limb is how long of a stub to leave. If they want a roughly 4-inch stub, for example, they can train workers to put four fingers between the crotch and branch and then make the cut there. But 4 inches is fairly long for a stub.
Einhorn’s study demonstrated a generally positive relationship between increasing stub length and the number of shoots and total vegetative growth per stub, but there are tradeoffs, too. Leaving a 4-inch stub “induces a chain reaction of responses that require intervention,” Einhorn said. It requires additional pruning after dormant season to eliminate superfluous shoots on the longer stubs.
The study also found that scion vigor varies in response to stub length. Growers have known for some time that low-vigor Honeycrisp needs at least a 2-inch stub. But Einhorn discovered that a 4-inch stub is no better for Honeycrisp than a 3-inch stub. A large number of 1-inch Honeycrisp stubs did not produce any shoots, while most of the 2-inch stubs produced a single shoot. Three- and 4-inch stubs generally produced one or two renewal shoots.
Previous work done at Penn State University showed that a 1-inch stub can work well for Gala. Einhorn’s study suggested that 2-inch stubs might lead to more consistent renewal growth for Gala but would require more pruning. Most stubs of any length produced at least one renewal shoot. The percentage of stubs returning two or more shoots increased with longer stub length. In general, a longer renewal shoot (12 inches) was achieved more reliably with 1-inch to 2-inch stubs.
Fuji, a “beast of a tree,” Einhorn said, grows an overly vigorous limb no matter the stub length. In his study, however, a 4-inch stub increased the probability of generating an optimum, low-vigor replacement limb, compared to shorter stub lengths. A significant percentage of 1-inch stubs didn’t produce any shoots in the two-year study. Renewal was excellent for longer stubs, however, and a long stub length encouraged growth of a longer renewal shoot. •
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