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. Walnuts are extremely intolerant of soil flooding and salinity. As the dominance of the California industry suggests, Persian walnuts are best adapted to Mediterranean climates, with dry, hot summers and mild winters.
Cold hardiness is a major limiting factor for northern production.
However, cold acclimation must occur slowly in the western US, since trees can be severely injured or killed by much higher temperatures in Autumn. Chilling requirement is highly variable in walnut, from 400 to 1600 hr. This is commonly done in spring on 1-yr seedling rootstocks in nurseries, but some growers prefer to plant ungrafted stocks and graft in the orchard, after the rootstock becomes established. Manregian' is the selection most tolerant of blackline disease. Trees usually lack vigor and yield efficiency in California, so it is not used there.