The Western Chestnut

by Ken Hunt, University of Missouri Center for Agroforestry

Backyard Trees

Author next to a 21-year old backyard tree (‘Auburn Homestead ‘ Chinese Chestnut) that was slowly limbed up over many years.

Pruning for the homeowner backyard planting should be minimal. Pruning should start after the trees come into bearing, taking off a bottom limb or two a year until there is enough clearance for mowing and hand harvest. Once the trunk is developed, allow the tree to develop its natural spreading form. If a homeowner wants trees to withstand wind and heavy crop loads then removal of branches with "narrow crotches" should occur during the first few years. These branches tend to have included bark in the crotch causing a weak branch with a tendency to split out at the crotch. Weak structured branches should be removed each spring during the first few years of tree growth to help develop strong scaffold branches along the trunk of the tree.

Low-Input Orchard
Larger plantings under medium intensity orchard culture (50-500 trees) should utilize the modified central-leader style training system used in the California walnut industry (Aldrich et al., 1982). Many cultivars do not naturally grow vertical central leaders. To ensure they do, a strong stake should be placed near each tree and a leader chosen and loosely tied to the stake. Central leaders grown vertically develop leaves and buds in a spiral fashion allowing good choices for scaffold branches growing in the correct direction. Central leaders allowed to bend over will develop buds on opposite sides of the shoot, not in a spiral fashion. The scaffold branches are selected during the second through sixth growing season, allowing for good spacing between branches in a spiral fashion along the main trunk. The height of the lowest scaffold branch depends on the individual growers equipment needs. Tree thinning will be required once crowding occurs in order to maintain high yields and uniform bearing throughout each tree. Crowded trees tend to create excessive shading on lower branches causing bearing only in the tops of the trees. Severe shading causes lower branches to die out and compromises general tree vigor (Vossen, 2000).

Intensively-Managed Orchard
A commercial grower (over 500 trees) needs to develop strategies to produce large quantities of high quality nuts per unit area. Currently many retail nuts are sold in-shell therefore a high percentage of large nuts is important. Three different pruning strategies will be reviewed that involve intensive pruning to contain the size of the tree which allows high planting densities and no need for tree removal.

Open Center pruning - Japan
An intensive level of culture has been developed in Japan and is now also being practiced in Australia and New Zealand providing a tight spacing within the row. Intensive culture allows for a shorter time frame to reach full production and possibly maximize nut production and nut size on a grower’s acreage. Intensive pruning contains the size and shape of the trees so that tree thinning is not necessary (Araki, 1998; Bennett et al., 1999).

Figure 1: Measurements of relative solar radiation within the tree canopy

Dr. Hitoshi Araki developed the Japanese pruning system. The Japanese chestnut in Japan has similar tree form but perhaps a bit smaller than the Chinese chestnut. Small, densely planted chestnut orchards designed to maximize production from the limited land available characterize the pruning techniques designed in Japan. Initial planting densities of 13 by 13 feet and a final tree density of 13 by 26 is currently the Japanese industry standard. For use of typical farm machinery in Missouri, an initial density of 13 x 26 is recommended. Dr. Araki’s research has shown that chestnuts require a relatively high level of light intensity to be able to set and develop fruit (Araki, 1998). He has shown a direct relationship between the intensity of sunlight within the tree canopy and the number of female flowers per cubic foot of canopy (fig. 1).

Figure 2: Re-shaping of chestnut trees in Japan.

He also correlated light intensity and the proportion of early flower drop, and determined that the diameter of bearing shoots was correlated to light intensity and the subsequent fruit size was directly related to bearing shoot diameter. Dr. Araki’s pruning methods limit tree height to 12 - 13 feet and restrict the distance from the edge of the canopy to the center of the tree. To achieve this, an elliptical tree shape was adopted (fig. 2)

To achieve needed tree structure for the Japanese pruning method, pruning needs to start in the first year and continue regularly throughout the tree’s life. The target initially should be to promote a leader and two main branches. The third year, the aim should be to encourage the tree to spread out (fig. 3).

Figure 3: Pruning a young tree in three successive years.

Thinning the inward growing branches is necessary to further encourage branch spreading into the alley. By approximately the 5th growing season, commercial cropping should begin. Cut out the central leader in the winter, leaving the two permanent branches that reach out into the alleyways, which become the permanent framework of the tree, and reducing tree height (fig. 4).

Figure 4: De-horning the central leader.

The tree will fill the gaps left by removal of the central leader. From this point forward thin out interior branches as they begin to crowd, and also remove branches to maintain a reduced tree height and elliptical tree shape.

Depending on the cultivar and management inputs, time will determine whether close 13 x 26 foot spacing can be maintained in the Midwest. If not, every other tree will need to be removed to allow the necessary light penetration into the canopy. As mentioned, a Chinese chestnut production orchard has been established at the Univ. of Missouri, Horticulture and Agroforestry Center and at Kansas State (Wichita) to help determine the feasibility of the Japanese pruning method at 13 x 26 foot spacing, with the option of thinning to 26 x 26 if necessary. The Japanese style of pruning has been introduced into the commercial chestnut industry in South Korea due to its efficiency for both fruit quality and cultural efficiency (fig. 5) (Kim, 2005).

Figure 5: Japanese style pruning showing "low tree-form" in South Korea. Source: Kim 2005.

Hedge row pruning — California
Biennial mechanized pruning (tree hedging) has been shown to be one method to reduce labor requirements and yet control tree size, sustain crop production, and improve nut quality. An attractive alternative to hand pruning to maintain productivity of closely planted orchards is mechanized tree hedging. Experimental results have shown that once California walnut orchards become crowded, hedging to allow light between trees does not effectively alleviate the condition. Mechanical hedging, however, may be valuable in maintaining tree size and production of precocious lateral-bearing varieties where trees have not yet become crowded.

Hedgerow planting of lateral-bearing walnut cultivars is another method to increase early returns on investments and develop a more efficient high-density management system for walnuts. The trees are trained to produce a continual wall of foliage and nut-producing shoots in response to mechanical side hedging. The wall (not the single tree) becomes the management unit in the orchard. Higher early yields and the use of mechanized hedging machines for pruning are the major advantages. High initial cost is the primary disadvantage due to the increased number of trees/ha (Ramos et.al. 1997).

Hedgerow pruning is experimental in chestnut with two research programs exploring the feasibility, at the University of Trais-os-Montes and Alto Douro, Vila Real, Portugal (Gomes-Laranjo, et. al. 2004) and at the University of Missouri Horticulture and Agroforestry Research Center. Both studies have north-south rows to allow for the sun’s movement to give equal amounts of radiation to the east and west facing sides of the pruned vegetative wall.

Figure 6: Example of European chestnut trees in their 9th leaf and with hedgerow style pruning in its 4th year in Portugal. (Poster from 2004 International Chestnut Congress)

In figure 6, trees are on a 10 x 10 meter grid with hedging performed on the east and west sides of the trees starting when the trees were in their 5th leaf. The first year of hedging, trees were pruned within 0.5 meter of the trunk, and the second year of hedging trees were pruned within 0.8 meter of the trunk to limit the shady and unproductive interior regions of the canopy. The trees at the Horticulture and Agroforestry Center at New Franklin, MO are on a 13 x 26 foot grid and are in their 5th leaf this coming spring, so will be pruned in a similar fashion to the Portugal research study starting this coming spring.

Preliminary results of the Portugal research show that leaf morphology, photosynthetic efficiency, and water relations in the interior part of the developing vegetative wall canopy are less affected by shading than conventionally pruned canopies, and that the trees have adapted well to intensive pruning. The north-south orientation completely eliminates shady regions and reduces midday heat stress because neither side is fully irradiated.

Figure 7. Older branches are removed after about 3-4 years of age ( stub on right), then the next older branch is allowed to fruit (branch on left) while the "renewal" shoot is allowed to grow on the stubbed off branch. The branch on the left also has a "renewal" shoot developing for the future. The jagged stub on the left was an older branch that had been removed and the current fruiting branch was the previous "renewal" shoot.

Time will be needed to find out results on the effect this pruning system will have on the fruiting ability in these experimental orchards.

Author’s Note: John Ireland, Research and Product Director of Fowler Nurseries in Newcastle, California has told me that a few of his clients are beginning to experiment with mechanical hedging in the Central Valley region with Colossal chestnut orchards. It would be very interesting to get to tour one of these orchards at one of our association’s future meetings.

Shoot renewal pruning — China

Figure 8. A fifteen year old orchard where shoot renewal pruning is being practiced to contain the size of the trees. Sustenance crops are planted under the tree rows, such as beans and cereal grains, using a form of alley cropping.

Greg Miller of Empire Chestnut Company near Carrollton, Ohio has observed intensely managed chestnut orchards in north-central China. Instead of tree removal as trees become crowded, they are instead contained in size by what Greg calls by "shoot renewal pruning" that is similar to what is done to blueberries (fig. 7).

Selected cultivars have been developed for this shoot renewal system that bear heavy crops of medium sized nuts with the texture and flavor that is favored for the roasted chestnut market. The orchard is heavily fertilized with nitrogen fertilizer to promote vegetative growth. (Figure 8)

Literature:
Aldrich, T.M., D.E. Ramos and A.D. Rizzi. 1982. Training Young Walnut Trees by the Modified Central-Leader System. UC Division of Ag. Sciences Leaflet No. 2471.

Araki, H. 1998. Proc. Of the Chestnut Pruning Conference, Beechworth, Victoria, Australia, 29-30 August, 1998. Bennett, R., S. Carmichael, A. Ridley, H. Schneider, and L. Trapnell. 1999. The Australian Chestnut Growers’ Resource Manual.

Ridley, D. and J Beaumont (eds.) Dept. of Nat. Resources and Environment, Victoria, Australia.

Gomes-Laranjo, J.C.E. 2001. Photosynthesis in Chestnut. Integrated Study at Different Biological Organisation Levels. PhD Thesis. UTAD. Vila Real, pp. 194.

Kim, Mahn-Jo 2005. Chestnut Cultivation and Breeding in Korea. In Press.

Ramos, D.E., W.H. Olson, G.S. Sibbett, and W. Reil 1997. Hedgerow vs. Standard High-Density Management Systems for Walnuts. Acta Hort. (ISHS) 442:333-338.

Vossen, P. 2000. Chestnut Culture in California. University of California-Davis, Division of Ag. and Nat. Resources, Publication 8010.