During about the third week in June, young Howard walnut trees experienced nut drop of perhaps as many as 200-300 nuts per tree.  These nuts were full size with the shell beginning to harden.   Cutting open an immediately dropped nut, you could see darkening of the inside watery kernel material, and termination of shell development that appears to have been triggered about 10 days before the drop was observed.  This nut drop, wide spread with the Howard variety, was similarly noted in 2003 and 2011. Several possible causes are explored in this article.