There are many opinions as when and how to cut a walnut tree. That is already shortly mentioned in the article about the background knowledge for pruning (Pruning trees - Background Knowledge). In fact, the walnut tree is very sensitive, if you cut it at the wrong time or too strong. In this case it will lose its juices and can literally bleed to death. A precipitated or capped walnut tree, where the root is still present, produces for weeks true sources of liquids from the cut surface. So never cut a Walnut tree in spring!
The linked video proves that with logic has to be appliede for all different cutting instructions. And especially the basic principle also applies to the walnut tree: A cut has to be done when it is causing the least damage to trees. And this ist the time when the least juice circulates and / or the trees lose the least leaf mass. For the majority of the plants and trees that is indeed not so important, because they do not bleed excessively. But for the walnut tree it is a problem that should not be underestimated.
Similar to the extremely dripping capped roots of a Walnut tree in the video, at the wrong time cut branches lose their juice. The best time to trim a walnut tree or reduce it consequently is a period with little juice pressure. The juice circulation of a Walnut tree already reduces at the end of summer. It is therefore often recommended to cut it in August, although it is still leafy. Thus, although it loses a lot of foliage, the damage is less than the loss of vast amounts of plant sap. After a summer pruning the wounds have still enough time to heal until winter.
The phase with the least juice pressure continues for the walnut tree with autumn. That means that there can be found other times until the winter. According to our experience a walnut tree will not bleed until a few weeks before the new year, as well. It depends on the temperatures and the general weather and that you can easily test it with a small cut. BUT: In winter the walnut tree is losing no valuable foliage, but the mechanisms of healing for the wounds are not active, either. AND: If the wounds can not be closed naturally it can happen, that it starts to bleed months later, in spring.