Black walnut trees are known for their beautiful, dark hardwood timber. It is an extremely hard wood and is used in cabinetry and furniture making. It also bears nuts that are wrapped in a thick fleshy coating and a rock-hard shell, that take a hammer to open. This is a uniquely flavored walnut, not the same as the English walnut commonly sold in grocery stores. It is wonderful when used sparingly in preparations like banana bread. If you have a tree and are wondering why it is not producing nuts, there are a couple of things you can check.
Plant more than one walnut tree if you wish to grow the trees for the nuts. The flowers are self-sterile and will need the pollen from another tree to actually travel to them to be pollinated. This is one of nature's ways of causing a species to survive by continuously adding variation to the DNA, making it more resistant to existing diseases.
Make sure that your walnut trees are planted close enough together that the wind can actually carry the pollen from one tree to another. The trees will bear lots of yellow-green catkins that will hang down from the branches about the same time the whole tree is leafing out. The most of them are pretty thick, being the male flowers, while the thinner ones are the females. The pollen is not carried by bees or insects, but by the wind.
Give the tree a couple of years before you expects nuts. Black walnut trees often take about 15 years before they are mature enough to produce fruit. Once they have reached maturity, they will often bear lightly one year and then heavily the next. Early hard freezes can also destroy the catkins resulting in a lack of nuts for the year.