Proper Walnut Spacing For Light Exposure

By Bruce Lampinen and Janine Hasey

Editor’s Note: Bruce Lampinen ([email protected]) is a walnut and almond specialist at the University of California-Davis. Janine Hasey ([email protected]) is a University of California farm advisor in Sutter, Yuba, and Colusa counties.

There are many things to consider before planting a walnut orchard. One of the most important is determining the orchard design system, and following that, the difficult decision of determining tree spacing (distance between trees within a row) and row spacing (distance between rows). A planting design should provide the tree canopy with maximum exposure to sunlight and allow ease of equipment operation. This article provides information to help you with the first decision — whether to plant in a standard spaced or hedgerow configuration, and secondly, walnut spacings to consider for optimum production.

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How To Quantify Canopy Light Interception

Using a second generation ATV Mule equipped with a light bar for measuring midday photosynthetically active radiation (PAR), infrared thermometers, a GPS antenna, and a datalogger, we can drive down rows in mid-summer to measure canopy light interception. At harvest, we pick up and weigh all nuts from the same area driven down with the light bar. From this we can correlate percent midday PAR (light) interception with yield in tons per acre. The predominate variety in this multi-year study has been Chandler, but some Tulare, Howard, Forde, and Gillet were also measured. The majority of orchards in the study have been on Paradox rootstock.

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How Canopy Light Interception Relates To Yield

• Walnut limbs need 30 to 45 minutes of direct sunlight to keep lower wood productive.
• The upper limit for yield (potential production) is 0.05 in-shell tons per acre (or 100 in-shell pounds) for each 1% of total midday PAR light intercepted.
• The fastest growing orchards can increase by 10% per year in light interception, reaching about 90% cover by the ninth year.
• Yield appears to peak at about 10 to 12 years of age and then gradually decrease.

How Tree Spacing Influences Canopy Development And Yield Potential

• Light interception continues to increase with increasing tree density.
• Yield tends to peak at 65 to 80 trees per acre.
• Standard spaced orchards have about 80% to 85% light interception which results in a mature orchard yield potential close to 4 tons per acre.
• Most walnut hedgerows have 65% to 75% light interception which would suggest a yield potential of 3.2 to 3.7 tons per acre. Since vegetative growth is stimulated in response to hedging, yield is slightly less than light interception would suggest. Hedgerow orchards actually produce about 0.5 tons per acre less, so yield potential is about 2.7 to 3.2 tons per acre.
• Optimum tree spacing appears to be in a standard square or offset square spacing design at about 56 to 75 trees per acre (e.g. 25 feet x 25 feet = 70 trees per acre, 28 feet x 28 feet = 56 trees per acre).
• Remember, when determining optimum walnut tree spacing, always consider soil type, rootstock, variety vigor and growth habit, and management style/design system (e.g. closer spacing is recommended if planting walnuts on more marginal soil or if using black rootstock).

How We Can Use This Information

Chandler, a vigorous variety, predominates in the walnut industry. In this canopy light interception and yield study and a previous study in the 1990s comparing hedging intervals in a Chandler hedgerow, results indicated that it takes three to four years to redevelop the complexity of branching that existed before hedging. Therefore, production will be lost for the first few years after hedging Chandler and other varieties. We recommend planting Chandler in a standard spaced orchard for long-term maximum yields (e.g. a minimum spacing on poorer soils would be 25 feet x 25 feet with wider spacing on better soils). If a hedgerow design is still desired, then consider Howard, which is a less vigorous, smaller tree than is Chandler.

Walnut orchard 45% light interception. Photo credit: Bruce Lampinen

Walnut orchard 45% light interception. Photo credit: Bruce Lampinen

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Walnut orchard with 90% light interception.

Walnut orchard with 90% light interception.

 

 

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