Principal countries are Japan, Germany, Spain, Italy and Israel. All Juglans spp are monoecious, bearing male and female reproductive organs on separate flowers on the same tree. Although J. regia is self-fertile, it is heterodichogamous, either protandrous or protogynous depending on cultivar. Payne' has good bloom overlap, and can be planted in solid blocks. Each catkin may produce 2 million pollen grains.
Pistillate flowers lack visible sepals and petals, and are small and green.
As with most catkin-bearing species, walnuts are wind pollinated. Pollinizers should therefore be placed on the upwind side of the main cv in orchards. However, higher ratios are normally used since pollinizing cvs also bear high quality nuts. There are fewer nuts per cluster, but more nut clusters compared to pecan. The best soils are deep, well-drained silt loams with pH 6 to 8, as found in central valleys of California.