Small to medium tree, 6–23 m, typically with a single straight trunk. Bark light or medium gray, splitting into plates with age. Branchlets stout, spreading to pendulous, red-brown pubescent at first; leaf scars broadly triangular with rounded corners, the upper margin shallowly to deeply notched, not bordered by well-defined band of pubescence; pith light brown. Terminal buds ellipsoid to oblong, flattened, 6–8 mm. Leaves 22–45 cm; petiole 3–8 cm, terminal leaflet well developed. Leaflets 13–21, (5.6–)7.3–13 × (1–)1.9–2.8 cm, narrowly triangular to lanceolate, sometimes somewhat curved, tapering or rounded at base; margins serrate, apex acuminate; upper surface more or less glabrous, dark green; lower surface paler with tufts of hair in axils of major veins, sometimes more widespread. Male catkins 6–15 cm; stamens 20–40 per flower; female flowers 1–3. Fruits 1–2, globose, 3–5 cm diameter; nuts ovoid to globose, 2.4–3.2 cm, smooth or with occasional slight longitudinal grooves. Sheel thick, kernel small and sweet. Flowering April–May (California). (Whittemore & Stone 1997; Grimshaw 2004).
Distribution United States Central Valley of Northern California, Southern Oregon
Habitat Along streams, sometimes on disturbed slopes; 0–300 m.
USDA Hardiness Zone 8-9
RHS Hardiness Rating H4
The Northern California Walnut became known to European science through specimens collected by the English naval surgeon and naturalist Richard Hinds, when the survey ship HMS Sulphur explored the lower Sacramento River in 1837 as part of a survey of the Pacific (Thomsen 1963). Willis Jepson, an early authority on the Californian flora, named it Hinds’s Walnut in his honour as a subspecies of J. californica in 1908, maintaining this treatment in his influential Flora of California (Jepson 1909). However, Smith (1909) stated that Jepson now considered it a full species and gave diagnostic characters. These two treatments appear to have been used in parallel long into the 20th century giving rise to confusion in records.
Juglans hindsii is distinguishable from J. californica in leaflet shape, being more or less lanceolate, with acuminate tips; in J. californica they are more or less elliptic and with less clearly pointed tips. It is most helpful to compare mature leaves. Further, J. californica lacks clusters of hairs in the axils of major veins on the underside (Whittemore & Stone 1997). The fruits of J. hindsii are larger. Both were included in the wider phylogenetic studies of Stanford et al. (2000); Aradhya et al. (2007) and Zhang et al. (2019); although none of these studies focussed explicitly on them; the results are somewhat ambiguous, but a careful reading certainly does not exclude a very close relationship, as sister taxa.
There has been controversy, summarised by Potter et al. (2018), over the extent to which present day populations are truly wild, and whether those outside a few documented pre-colonial sites are pure J. hindsii. The recent discovery of stands as far north as southern Oregon (Callahan 2008) was subject to the same doubt. Using molecular markers, Potter et al. (2018) were able to show that over 70% of trees sampled in putative wild populations in northern California and Oregon were pure J. hindsii, the remainder showing introgression from other black walnut species, perhaps planted in the landscape. No evidence of introgression from J. regia was seen, perhaps because Paradox hybrids generally have low fertility.
This is a useful and culturally significant tree. The Kashya Pomo people of Northern California ate the nuts both fresh and dried, using the husks to make a black dye (Moerman 2019). Jepson (1909) noted that the tree was associated with Native American village sites, something which would be hard to confirm nowadays, given the widespread planting of walnuts including J. hindsii and hybrids over the 20th century. He went on to develop the hypothesis that this was not a truly indigenous plant, being derived from Southern California Walnut brought hundreds of miles north by Native Americans. Thomsen (1963) thoroughly scotched this idea.
J. hindsii is an important species in the Californian walnut industry, not for its nuts but as a disease resistant rootstock for J. regia. Most commercial walnuts are grafted onto either J. hindsii itself or a Paradox hybrid, either seed raised or clonal (see below): each has its own pattern of resistance and susceptibility to fungal and bacterial diseases (University of California, Davis 2013).
Known in the timber trade as Claro Walnut, its wood is used in furniture making, for gunstocks and veneers. Wood taken from the graft area of orchard walnuts is sometimes used by woodworkers, exploiting the colour change from hindsii to regia wood: an example can be seen at McFadden (2019).
Juglans hindsii is the parent of two significant and vigorous hybrids. The Paradox hybrids (J. regia × hindsii) are variable and may (rather controversially) involve other black walnut species. ‘Royal’ is a hybrid with J. nigra. In Trees and Shrubs Online we give Paradox hybrids a species-level treatment, and mention ‘Royal’ within it.
Beyond the Pacific states, where J. hindsii and perhaps even more its hybrids are planted in the landscape, there is little evidence of this species being cultivated in North America. In Europe there are a few records from England, where any without wild provenance should be examined with an open mind, considering the confusion surrounding the western black walnuts. At Howick Hall, Northumberland, there are two specimens planted in 1988 from WAHO 207, the larger 8 m × 28 cm in 2019 (The Tree Register 2019). The Yorkshire Arboretum has a specimen from the same collection, which was about 4 m tall in 2019 and not thriving (J. Grimshaw, pers. comm.). A number of much younger trees with wild provenance are growing at Westonbirt Arboretum, Gloucestershire (Forestry England 2019). The largest specimen claimed as J. hindsii is at the Valley Gardens, Windsor Great Park (16 m × 45 cm in 2008); it is recorded as ‘from Seattle’ and hence not directly from a wild source (The Tree Register 2019). There seems little doubt that this species should be hardy elsewhere on the Atlantic fringes of Europe.