Common Name: California Walnut

General Information

Juglans californica is a deciduous shrub or a small tree that usually grows around 6 - 9 metres tall, occasionally reaching 18 metres[].
The plant is harvested from the wild for local use as a food, medicine and source of materials. The tree is grown as an ornamental and sometimes used to restore native woodlands in California[]
Juglans californica is restricted to the southwest of Calicornia. The population size is unknown and the species can be dominant in woodland. Despite this there is continuing decline in population size due to threats from habitat loss and conversion, grazing of the habitat and increased occurrence of fire. There are also concerns for the future regeneration of the species and potential habitat availability due to a local growing human population which could put pressure on the species and its habitat. The plant is classified as 'Near Threatened' in the IUCN Red List of Threatened Species(2018)[].

Known Hazards

None known

Botanical References

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Range

South-western N. America - southwestern California.

Habitat

Hillsides and canyons, growing in moist or dry gravelly soils along river courses and bottomlands[, ]. Locally common in oak woods below 750 metres[].

Properties

Conservation Status	Near Threatened
Edibility Rating
Medicinal Rating
Habit	Deciduous Tree
Height	6.00 m
Growth Rate	Medium
Pollinators	Wind
Self-fertile	Yes
Cultivation Status	Ornamental, Wild

Cultivation Details

Juglans californica is native to the more or less Mediterranean climate of southern California, with its hot, dry summers and cooler, moist winters. It is not very hardy in most of the temperate zone, being able to tolerate short periods with temperatures down to between -5 and -10°c when dormant so long as the summer has been hot enough to fully ripen the wood. The young growth in spring is very susceptible to frost damage[].
Requires a deep well-drained loam and a sunny position sheltered from strong winds[, ]. Prefers a slightly alkaline soil[].
A fairly fast-growing tree in its native habitat, but quite short-lived, dying after about 125 - 150 years[].
Plants produce a deep taproot and they are intolerant of root disturbance[, ]. Seedlings should be planted out into their permanent positions as soon as possible and given some protection since they are somewhat tender when young[, ].
Flower initiation depends upon suitable conditions in the previous summer[]. The flowers and young growths can be destroyed by even short periods down to -2°c, but fortunately plants are usually late coming into leaf[].
Any pruning should only be carried out in late summer to early autumn or when the plant is fully dormant otherwise wounds will bleed profusely and this will severely weaken the tree[].
Plants will generally resprout if top growth is cut back by fire[].
Closely related to Juglans hindsii[, ]

Edible Uses

Seed - raw, cooked or used in pies, cakes, biscuits, confections etc[]. A sweet flavour[]. The seed is small with a thick shell but it makes good eating and is said to have a better flavour than Juglans nigra[]. The seed is up to 15mm in diameter[]. The seed ripens in late autumn and, when stored in its shell in a cool place, will keep for at least 6 months[].

An edible oil is obtained from the seed, it tends to go rancid quickly.

Medicinal

An infusion of the leaves is used to treat thin blood[].

Agroforestry Uses:

This species is sometimes used as a rootstock for the common walnut, Juglans regia[].
Plants produce chemicals which can inhibit the growth of other plants. These chemicals are dissolved out of the leaves when it rains and are washed down to the ground below, reducing the growth of plants under the tree[, , ]. The roots of many members of this genus produce substances that are toxic to many plant species, especially apples (Malus species), members of the Ericaceae, Potentilla spp and the white pines (certain Pinus spp.)[].

Other Uses

Wood - moderately coarse-grained. An attractive wood, but the frequent branching pattern of the trunk limits the use of this wood commercially[]

Propagation

The seed is best sown as soon as it is ripe in individual deep pots in a cold frame[]. You need to protect it from mice, birds, squirrels etc. The seed usually germinates in late winter or the spring. Plant out the seedlings into their permanent positions in early summer and give some protection from the cold for their first winter or two.
The seed can also be stored in cool moist conditions (such s the salad compartment of a fridge) over the winter and sown in early spring but it may then require a period of cold stratification before it will germinate[, ].