It is a minor miracle that walnuts exist. Their reproduction is slap-dash and haphazard. In spring, first, catkins appear. Catkins contain the pollen.
Small and closed at first; they lengthen and open, dispersing pollen.
Walnut trees do not need bees. The catkins release pollen which is taken to the flower by the wind. Flowers appear after catkins. This mismatch has not led to the extinction, but in the orchard, we plant pollenators: trees of species that produce catkins at the same time as our chandler trees are in flower.