black Walnut tree
According to experts at the University of Missouri, "black walnut trees are sensitive to spring frost injury. Freezing temperatures after the onset of bud break can result in the loss of a tree’s entire nut crop."
Black walnut trees from wild stock tend to fruit heavily one year and lightly or not at all the next (biennial bearing). Squirrels taking nuts from a light crop could also account for crop failure.
Our guess is that unfavorable growing conditions account for the lack of nuts this year. Many observers have also reported few or no acorns.
Learn more here: http://www.centerforagroforestry.org/pubs/walnutNuts.pdf