Walnut Varieties

Eastern Black Walnut, Juglans Nirgra.  While its primary native region is the Midwest and east-central United States, the black walnut was introduced into Europe in 1629. It is cultivated there and in North America as a forest tree for its high-quality wood. Black walnut is more resistant to frost than the English or Persian walnut, but thrives best in the warmer regions of fertile, lowland soils with high water tables. Black walnut is primarily a pioneer species similar to red and silver maple and black cherry.

Claro Black Walnut, Juglans hindsii, with the common names Northern California walnut and Hinds' black walnut, is endemic to Northern California.  One internet source reports that Claro is not native to California but that General Bidwell, the founder of the city of Chico, imported seed stock from Greece and Turkey.  These trees were planted, mostly for shade, along highways, roads, and city streets, as well as around buildings and farm yards.  Over the course of the last 100 years or so, the nuts from these trees have been distributed by creeks and rivers, as well as by animals and man, to as far away as the San Francisco Bay area.  As a result Claro Black Walnut can now be found all over Northern California.

However Wikipedia states that it is native, and there is only one confirmed native stand remaining. It is listed as Seriously Endangered on the California Native Plant Society Rare Plant Inventory.  The IUCN classification is a Vulnerable species. It is threatened by hybridization with orchard trees, urbanization, and habitat conversion to agriculture.

English Walnut, Juglans regia, the Persian or French walnut, English walnut, especially in Great Britain, is an Old World walnut tree species native to the region stretching from the Balkans eastward to the Himalayas and southwest China. The largest forests are in Kyrgyzstan, where trees occur in extensive, nearly pure walnut forests at 1,000–2,000 m altitude—notably at Arslanbob in Jalal-Abad Province.  In California, most English walnut is grafted onto Black Walnut root stock for disease resistance.

Bastogne Walnut is a rare, naturally occurring hybrid between California English Walnut and Claro Walnut that is found in northern California.  Some Bastogne is a commercially produced fast-growing Luther Burbank hybrid, commonly called Paradox (Juglans hindsii x Juglans regia). Paradox rootstock is produced from a Northern California black walnut tree pollinated by an English walnut.