Before 1850, Juglans hindsii was restricted to a few locations (J. R. Griffin and W. B. Critchfield 1972). It has been widely used as a rootstock for grafting J . regia and has been planted extensively in many parts of California for this purpose. It is now naturalized in many areas where it apparently did not occur before the introduction of commercial walnut growing. Possibly some of these naturalized populations are introgressed with J . nigra , since spontaneous hybridization between J . hindsii and J . nigra has been reported in areas where both species have been planted. These hybrids are difficult to distinguish from J . hindsii unless fruit are present. Currently most commercial walnut orchards use hybrid rootstocks, usually J . hindsii × J . regia (G. H. McGranahan and P. B. Catlin 1987).