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Shrub or small tree, 6–9 m, usually multistemmed. Bark light grey, darkening with age, divided into rough plates. Branchlets stout and spreading, sometimes becoming pendulous; leaf scars rounded to triangular, upper margin notched, often shallowly so, glabrescent or bordered by poorly defined velvety patch; pith brown. Terminal buds ovoid to ellipsoid, somewhat flattened, 5–6 mm. Leaves 15–24 cm; petiole 2–5 cm. Leaflets (9–)11–15(–17), narrowly oblong to lanceolate, symmetric or weakly falcate, (2.5–)4.3–9.5 × 1.6–2.6 cm, margins finely serrate, apex rounded to acute; terminal leaflet well developed. Stem, petiole and midvein with small multiradiate hairs only when young; leaflets glabrous apart from a few glandular hairs on major veins, lacking non-glandular or tufted hairs. Male catkins slender 5–14 cm; stamens 15–35 per flower. Fruits 1–3, globose, 2.1–3.5 cm, smooth, at first glandular, with scattered scales, soon glabrescent; nuts nearly round, but flattened at base, 1.8–2.2(–2.5) cm, shallowly grooved, surface between grooves smooth. Flowering March–May (California) (Whittemore & Stone 1997; Grimshaw 2004).
Distribution United States SE California.
Habitat Hillsides, valley- and canyon-bottoms, alluvial terraces, 30–900 m.
USDA Hardiness Zone 8a-9b
RHS Hardiness Rating H4
Conservation status Vulnerable (VU)
Juglans californica is not a familiar tree outside its native range; even here it is a local and declining tree (Esser 1993). It has been much confused with its northern neighbour J. hindsii, and records are sometimes misleading. In cultivation the name is rare but the tree itself rarer still.
This is a Mediterranean-climate tree, experiencing periodic fire and drought. It often dominates natural woodlands, while the evergreen Quercus agrifolia is sometimes codominant (Esser 1993). Some denser stands comprise tall, single trunked trees while others are made up of broad, multistemmed specimens (Keeley 1990). This may in part be due to the vagaries of seedling recruitment – seeds are not produced in drought years – but it may also be influenced by fire history. Walnut trees persist here through sprouting from the base of burned trees; seeds and young seedlings are destroyed by fire (Esser 1993).
Juglans californica is distinguishable from J. hindsii in leaflet shape, being more or less elliptic and with less clearly pointed tips; in J. hindsii they are more or less lanceolate, with acuminate 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). It has often been assumed that these are very closely related species. The fruits of J. californica are smaller. 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 focused explicitly on them; the results are somewhat ambiguous, but a careful reading certainly does not exclude a very close relationship, as sister taxa.
Like other walnuts, J. californica has been used by indigenous people. The Costanoans of Southern California ate the nuts and took a leaf infusion as a blood medicine (Moerman 2019).
Coming from southern California, J. californica has been thought to be tender in Britain (for example Bean 1981). Nevertheless, a few specimens are recorded. Any record without wild provenance should be treated with caution. Although The Tree Register (2019) records two at the Val de la Mare Arboretum, Jersey (the larger 13 m × 25 cm in 2014) and one at Westonbirt, Gloucestershire (9 m × 23 cm in 2014). This last tree is of garden origin and on examination did not appear to be authentic J. californica (pers. obs. 2019). However, several young trees of wild provenance were planted around Westonbirt in 2017 and 2018; still untested through a severe winter, the best specimen is growing quickly in a sheltered position, without signs of winter shoot death (pers. obs. 2019). A record from further north at Thorp Perrow Arboretum, Yorkshire proves instead to be one of the Asian taxa (J. Grimshaw, pers. comm. 2019); true J. californica from Los Angeles County struggles in an open position at the Yorkshire Arboretum, having reached 3 m since 1989.
Outside of southern California where it is sometimes offered by nurseries, there is little evidence of J. californica in American arboreta or the wider landscape. It is susceptible to root diseases and has not proved useful as a rootstock in commercial walnut production (Bernard et al. 2018).