There has been considerable uncertainty amongst botanists as to the best way of treating the genus Toxicodendron, with some viewing it as a genus distinct from Rhus, whilst others see insufficient differences and lump the two genera under Rhus. Toxicodendron is treated here as distinct, with two of the major differences from a gardener’s viewpoint being that Toxicodendron has a toxic sap (not toxic in Rhus) and the fruits have a thick, waxy mesocarp.

Common Name: Western Poison Oak

General Information

Toxicodendron diversilobum is a many-stemmed, deciduous shrub that can adopt a climbing habit. Shrubby forms have erect stems usually 1 - 2 metres tall, whilst climbing forms typically have stems 3 - 10 metres long, exceptionally up to 30 metres. As a climbier, the plant supports itself amongst other vegetation by means of adventitious roots and/or wedging stems within grooves or crevices of the support. The plant spreads at the roots by means of rhizomes and can form thickets[].
The plant is harvested from the wild for local use as a medicine and source of materials.

Known Hazards

All parts of the plant are known to contain resinous phenolic compounds known as urushiols that can cause severe skin problems in humans. Direct contacr with the plant, exposure to smoke or fumes from a burning plant or even contact with pets, animals or clothing that have touched the plant can cause severe allergic dermatitis in some individuals. There is usually a latent period of about 12 - 24 hours from the moment of contact, this is followed by a reddening and severe blistering of the skin. Even plant specimens 100 or more years old have been known to cause problems[].
The plant does not cause skin problems with wildlife or livestock - the foliage and fruits are eaten by several other species of animals and birds - though some pets are sensitive to it[].

Botanical References

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Range

Western N. America - British Colombia, Washington, Oregon, California and Nevada

Habitat

Mixed evergreen forests, coastal sage scrub, thickets and wooded slopes in foothills, along streams, in washes and hedgerows; at elevations up to 1,500 metres[, , ].

Properties

Medicinal Rating
Other Uses Rating
Habit	Deciduous Shrub
Height	2.50 m
Pollinators	Bees
Self-fertile	No
Cultivation Status	Wild

Cultivation Details

Toxicodendron diversilobum is a very cold-hardy plant, able to tolerate temperatures down to around -25°c when fully dormant[].. The young growth in spring is more sensitive, however, and can be damaged by late frosts.
Species in this genus generally succeed in a variety of well-drained fertile or moderately fertile soils in full sun or light shade[, ]. This species is found mainly in acid soils in the wild[].
This species is closely related to Toxicodendrom pubescens[].
Plants in this genus are notably resistant to honey fungus[].
A dioecious species - both male and female forms must be grown if fruit and seed are required.

Edible Uses

None known

Medicinal

Western poison oak was employed medicinally by some native North American Indian tribes who used it to treat various complaints[]. In view of the potential toxicity of the plant, extreme caution is advised in any use of it. See the notes above on toxicity.

A leaf has been swallowed in the spring as a contraceptive[]. A tincture of the fresh leaves has been used in the treatment of eczema and skin diseases[]. It is also used in the treatment of warts, ringworm etc[]. A poultice of the fresh leaves has been applied to rattlesnake bites[].
The leaf buds have been eaten in the spring in order to obtain immunity from the plant poisons[]

A moxa of the plant has been used in the treatment of warts and ringworm[].

The juice of the plant has been used as a treatment for warts, corns, calluses and ringworm[, ].

An infusion of the dried roots has been taken in order to give immunity against any further poisoning[]. A decoction of the roots has been used as drops in the eyes to heal tiny sores inside the eyelids and to improve vision[].

Other Uses

The leaves are rich in tannin. They can be collected as they fall in the autumn and used as a brown dye or as a mordant[].

An oil is extracted from the seeds[]. It attains a tallow-like consistency on standing and is used to make candles. These burn brilliantly, though they emit a pungent smoke[].

The supple stems are used as the warp in basket making[]. Slender stems are used as circular withes in basket making[].

An excellent black dye is obtained by exposing the sap to air[].

Propagation

Seed - best sown in a cold frame as soon as it is ripe. Pre-soak the seed for 24 hours in hot water (starting at a temperature of 80 - 90c and allowing it to cool) prior to sowing in order to leach out any germination inhibitors[]. The stored seed also needs hot water treatment and can be sown in early spring in a cold frame[]. When they are large enough to handle, prick the seedlings out into individual pots and grow them on in the greenhouse for their first winter. Plant them out into their permanent positions in late spring or early summer, after the last expected frosts.
Cuttings of half-ripe wood, 10cm with a heel, mid summer in a frame[].
Root cuttings 4cm long taken in December and potted up vertically in a greenhouse. Good percentage[, ].
Suckers in late autumn to winter[].