Walnut Hedgerow Planting On Marginal Soils

The objective here is to investigate the feasibility of producing English walnuts on local Class II-III soils using a hedgerow planting system. We also are comparing two varieties, Howard and Chandler and two rootstocks Northern California Black and Paradox Hybrid on native soil and on slip plowed ground.

The trees were planted during the spring of 1986 as a hedgerow, 12 ft. between trees in the row and 18 ft. between the rows, giving 202 trees per acre with a north-south row orientation. Prior to planting, half of the plot was slip plowed to a depth of 5- 6 ft. on 10 ft. centers north and south. Four treatments were repeated six times in both slip plowed and unmodified soil. The treatments were Howard and Chandler on Northern California Black and Paradox.

Grafted Howard trees on Paradox were unavailable at planting time, so Paradox seedlings were planted and then grafted to Howard in 1987. Cisco and Franquette were planted around the plot for pollenizers (12%).

The trees are drip irrigated using ET_o calculations based on the nearby CIMIS weather station, and received monthly N applications via drip lines. Initially the trees were irrigated with a single drip line per row. In response to unacceptable vigor in 1990, a second drip hose was added in 1991, which approximately doubled the wetted soil area and allowed more oxygen into the rootzone between irrigations. Since this change, shoot growth has been acceptable for a developing walnut orchard. Potassium sulfate is applied at l /2 lb/emitter (500 lbs/acre) each fall.

The trees were trained and pruned by hand for 3 years to develop a framework. Both the east and west sides of the hedgerows were then mechanically hedged and topped yearly through March of 1992. In 1993, yearly alternate side mechanical hedging began by pruning one side of the hedgerow 4-5 feet from the trunk. Moderate mechanical topping was continued yearly as needed to stimulate top growth to the desired tree height of 16 feet. However, starting in 1995, only short statured trees have been topped (mainly trees on NC Black). Thus, the more vigorous areas which had achieved the desired height of 16 feet have been unheaded since then.

Results

Yield results continue to show a substantial and consistent yield advantage to Paradox Hybrid rooted trees compared to trees on NC Black rootstock. This holds true for both varieties and under both soil conditions. Also notable is the consistency of this Paradox advantage over ten consecutive years (1989-1998).

The Howard variety has again outyielded Chandler this year but the Chandler production continues to be among the highest in the state for a Chandler hedgerow. Howards generally respond (yield) better in response to yearly alternate side mechanical hedging than Chandlers. Our practice of hedging one side of every middle each year verses hedging both sides of alternate middles may help explain the more positive yield response of Chandler in this hedgerow. The effect of slip plowing in this test is unclear. Limited backhoe pit investigations have shown deeper and more extensive rooting beneath slip plowed trees. However, trunk growth measurements and yields show no advantages to the slip plowed treatment. It should be noted that soil conditions vary significantly within the trial area. About fifty percent of the test area consists of Arbuckle sandy loam to a depth of 4 - 5 feet. The Hillgate series covers the remainder with a similar sandy loam texture, but only to a depth of 2-3 feet where a dense clay layer limits deeper rooting. Given the use of high frequency (2-3 times/week) drip irrigation and monthly fertilization, mixing soil layers by slip plowing may not benefit yield. Also, current slip plow recommendations include a diagonal pass of the plow in addition to the main direction. Here the machine made only one pass in a North-South direction.

Surprisingly, this test plot continues to produce high walnut yields of desirable quality despite adverse soil conditions. More work will be required to fully evaluate this walnut production strategy in terms of economics. The eighteen foot row width is too close and impractical for commercial use. Under these soil conditions, spacing between rows should be 20-22 ft. for Howard and 22-24 ft. for Chandler . Distance down the row is less important , 12-16 ft would seem appropriate. Whether equivalent production could be achieved at these more practical wider spacings is unknown.

Table 1.1998 Walnut Yields

Lbs/Acre

Numbers followed by the same letter are not significantly different at the 5% level using Fisher’s test.

Table 2.Nickels Hedgerow Yearly and Accumulative Yields

(lbs/acre)

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Copyright © 1999 Colusa County Cooperative Extension, Univ. of California
Last modified: September 01, 2000