Alberto Dorigoni, Pomologist at the Istituto Agrario di San Michele all’Adige in Italy’s South Tyrol province, and Tom Auvil, Horticulturist at North American Plants in Lafayette, OR, offer some suggestions on what growers need to understand about transitioning to a multileader system.
Q: What do growers need to consider before going to a multileader system? Does it vary by crop?
Alberto Dorigoni: Do I want to build a fruit wall or large trees? Well, in the first case most of the time the size of my spindle trees is too large for it with some exceptions. In other words, multileader training is a quantitative tool: I can vary the number of leaders to shrink my tree canopy. The consequences are manifold, on management and mechanization. Most fruit trees species can be grown multileader.
Tom Auvil: The three basic systems in the Northwest are tall spindle, formal trained to horizontal wires, and multileader — two or more leaders per tree stump. The multileader is a very short fruiting limb up the leader/axis. The target number of leaders per acre is 3,200 to 3,800 leaders/axis per acre with a spacing of 16 to 24 inches between leaders in V-trellis canopies. Vertical would be about half and may not have the volume potential per acre as V. Strong-growing varieties such as ‘Granny Smith’ or ‘Fuji’ may need more space between leaders/axis than with compact varieties such as ‘Honeycrisp.’ The multileader systems may be more easily adapted to robotic harvest.
Q: What is the biggest mistake growers might make early on with a multileader system?
Dorigoni: [They] forget that a multileader is not meant to form ordinary big branches. Therefore, apple growers, worried about leaving enough room for tractors, choose wrong spacing between rows, 12-14 feet instead of 8-10 feet. [They] forget that that multileader needs more spacing on the row (about 1.5 feet/leader)
Auvil: Starting early to establish consistent even spacing. Some use heavy braided twine, others bamboo. In windy locations, anchoring the vertical guide to the trellis is important. If the leaders/axis slide down the trellis, “clumps” of leaders lead to clumps of leaves and fruit creating shade and production/quality problems. A “hack” is to apply a sticky, gripping paint material such as grafting paint (Doc Farwell’s yellow cap as an example). A ketchup/mustard squeeze bottle dispenser is quick way to apply.
Q: Are there any varieties or rootstocks that are ill-fitted for multileader systems?
Dorigoni: No. Red and bi-color apple cultivars respond better than ‘Granny Smith’ of course. As for rootstocks, it is just a matter of choosing the right combination with plant spacing.
Auvil: High vigor scions on large canopy rootstocks will encourage feathers and branches to develop off the leaders. The goal is to form bourse/fruiting spurs [cluster bases or knobs] at every leaf node on the vertical leader, not branches. Good vertical growth is important, but not so much vigor that big branches are formed.
Q: What else can cause problems in production when it comes to multileader systems?
Dorigoni: [Growers] underestimate the importance of pruning in both winter and summer. Multileader is suitable to hand pruning and to mechanical pruning because trees produce weak branches that react mildly to headback cuts. On the other hand, weak branches, if left long, can cause unwanted fruit shading. It is like buying a race car to drive it off the road.
Auvil: As the number of leaders per stump/rootstock increase, so does the amount of manipulation and training required to establish a full, uniform canopy. One trick is to keep the growing points of the vertical leaders dominant, pinch competing growing points so there is limited competition to the vertical shoot. Keep the leaders/axis supported and the growing point upright. In second and third leaf trees remove bloom near the growing point from tight cluster to first bloom to encourage growth of the leader. Hand-held string thinners have been effective, though the results look scary for a couple weeks following application. Looks like a heard of deer/elk has grazed the block, but the result is surprising compared to untreated check.
Q: Is there anything else growers need to understand about multileader systems?
Dorigoni: Multileader requires major rethinking of the all standard parameters. New plant distances, different crop protection systems, and new simple machinery will eventually replace today’s sprayers, platforms, and big trellis. Small will be more efficient than big and powerful.
Auvil: One of the things we are still wrestling with is the cost of trees versus cost/availability of labor at the right time. If three or four leaders require trading efficient (time and resource) canopy development for fewer trees, it can be a business decision of how much to spend on trees, versus the delay in production and the increase in doing the right thing at the right time. Craig Hornblow [Horticulturalist with Ag First New Zealand] summarized it well: “The biggest expense in orchard establishment is the time out of production,” especially when looking at the marketplace and high value of many new varieties.
I have planted multileader systems in 2012, 2015, and 2016, returning to single leader trees in 2017, 2018. Grafting into a multileader system is much easier than growing trees into a multileader system. I think two leaders per stump is twice as difficult as growing one leader per rootstock/stump. Three leaders per stump are 10 times more difficult than one leader per stump to get a uniform, easy-to-manage system. Four leaders per rootstock/stump are 100 times more difficult than one leader per stump.