Training young walnut trees
Desirably shaped mature trees are the result of years of careful selection and training. A well-shaped tree provides convenience and economy in orchard management. If pruning is neglected until a tree is several years old, many severe cuts are often necessary to bring the tree into somewhat of the desired shape.
First winter pruning
At the end of the first summer, the tree may be from 4'–8' tall. Usually, no heading is necessary. Heading means a cut such as (a) in Figure 1. A tall trunk is an advantage. Heading would tend to force out laterals too low on the trunk and therefore not desirable. Permanent lateral branches should be selected so that the lowest shall be no closer than 4 1 /2' from the ground and preferably 5'–6'. Stakes to hold the tree into the wind are usually needed.
Shoots developing below the desired level of the first permanent scaffold or branch should not be removed when they have developed. It’s better to pinch back the shoots when they attain a length of about 15". Pinching back simply means to break off the tip. This leaf surface that is maintained will help to build the trunk and root system while at the same time protect the trunk from sunburn.
Second dormant pruning
At the second dormant pruning, all laterals below 5' should be cut back to a 6" stub as shown in Figure 2. At this pruning, there may or may not be any branches which are high enough to become permanent laterals. By not removing them completely at this time, the tree will benefit by the extra leaf surface the following season and will also receive sunburn protection.
It’s desirable that the lowest permanent lateral should be on the windward side of the tree. In Stanislaus County this branch is best located on the northwest side of the tree. This process of selecting the desired framework may take a two or three-year period. The tendency for most walnut varieties to spread and droop makes a high head desirable and does not result in a higher tree. A wide vertical spacing between the branches is preferred. A distance of about 2' seems advisable.
Branches should be selected from wood that is at least one year younger than the trunk. Branches that are same age as the trunk are poor and easily split off. They should be removed entirely at the dormant pruning. Permanent branches should form a fairly wide angle with the trunk. These branches are more strongly attached and will form a sturdier framework.
Third Growing Season
During the third growing season, summer pinching may again be beneficial. Shoots arising from the lower portion of the trunk or from the stubs that were left at the winter pruning, should be pinched back when they attain a length of about 15". Here again the extra leaf surface means a sturdier trunk and affords sunburn protection.
3rd Dormant Pruning
During the third dormant pruning the selection of the permanent branches is usually completed. 4 or 5 branches that are well- distributed vertically along the trunk, such as in Figure 3, are enough. These should be evenly distributed around the trunk. Certain varieties, such as the Payne and Hartley, tend to set nuts laterally along the shoots. Long growing branches on these 2 varieties may have the tips headed back to prevent excessive weight during the 4th growing season. The growth resulting below the first permanent branch may again be cut back to a 6" stub.
If the stubs from the previous year are getting very large, they should be removed completely. As to whether or not to remove a stub, a good rule to follow is this: A stub should be removed while it can still be done with pruning shears. If the stub is left for another year and a pruning saw will be needed, it should be cut off now.
Fourth and Subsequent Dormant Prunings
During, the fourth and subsequent dormant prunings, crossing and interfering branches should be removed. Usually all stubs should be removed by this time. Laterals from the permanent branches should be selected so as to form a tree such as in Figure 4.