The season between mid August and end of September is well suited for a cut back of your walnut tree. This will keep it strong and healthy. At this time of year, walnut trees prepare for dormancy. That is why they produce less juice. The wounds have enough time to heal before the first frost arrives.
If you have a young tree you should carry out a formative structural prunning. This is how it works:
Walnut trees often build leader branches at the top of the tree which compete with the leading branch or main stem. Prune back these competing branches at the top. This will help to shape a uniform tree crown.
Elder trees need a preserving pruning:
Every second branch of the external area of the crown should be shortened about 150 centimeters at a branch side. In the following year you can shorten the other limbs. This process keeps the three crown airy so that it receives a lot of sunlight. The crown will not get too big and stays healthy.
The walnut tree cuts will heal by themselves. You simply have to make sure to use clean and well sharpened tools.