Poison oak is a deciduous shrub,
which like POISON IVY IS in the SUMAC FAMILY, native to North America.
Its
leaves contain a compound that causes a rash on human skin.
Poison Oak can Specifically Refer to:
Toxicodendron pubescens, which grows in Eastern
North America
Toxicodendron diversilobum, which grows in Western
Coast of North America
Poison oak (Toxicodendron diversilobum) and its EASTERN
COUNTERPART poison ivy (T. radicans) are two of the North American plants most
painful to humans.
Note: These
species were formerly placed in the genus Rhus. Poison oak and a related,
look-alike shrub, Rhus trilobata belong to the SUMAC family (Anacardiaceae).
Poison oak is widespread throughout the mountains
and valleys of the western USA. It thrives in shady canyons and riparian habitats.
It commonly grows as a climbing vine with aerial (adventitious) roots that
adhere to the trunks of oaks and sycamores.
Rocky Mountain poison oak (Toxicodendron rydbergii)
occurs in canyons throughout western Canada. Because the two species of
western poison oak look like a vine as they develop, some
authors list poison oak as a subspecies of eastern poison ivy.
Poison oak often
grows like a climbing vine.
The cautionary rhyme "leaves of three, let it
be" applies to poison oak, as well as to poison ivy (Toxicodendron
radicans). Toxicodendron diversilobum, commonly named Pacific poison oak or
western poison oak (syn. Rhus diversiloba), is in the Anacardiaceae family (the
sumac family).
The woody vine or shrub is widely distributed in
western North America, inhabiting conifer and mixed broadleaf forests,
woodlands, grasslands, and chaparral biomes. It is known for causing itching
and allergic rashes in many humans, after contact by touch or smoke inhalation.
The Pacific poison-oak Toxicodendron species is
found in California, Nevada, Oregon, Washington, and British Columbia. The closely
related Eastern poison oak (Toxicodendron pubescens) is native to the
Southeastern United States. Pacific poison-oak and Western poison-ivy
(Toxicodendron rydbergii) hybridize in the Columbia River Gorge area.
Toxicodendron diversilobum is common in various
habitats. It thrives in shady and dappled light through full and direct
sunlight conditions, at elevations below 5,000 feet. The vine form can climb
up large shrub and tree trunks into their canopies. Sometimes it kills the
support plant by smothering or breaking it.
The plant often occurs in chaparral and woodlands,
coastal sage scrub, grasslands, and oak woodlands; and Douglas-fir (Pseudotsuga
menzesii), Hemlock—Sitka spruce, Coast redwood (Sequoia sempervirens), Yellow
Pine (Pinus ponderosa), and mixed evergreen forests.
Description
Toxicodendron diversilobum, Pacific or western
poison oak, is extremely variable in growth habit and leaf appearance. It grows
as a dense one and a half to thirteen foot, tall shrub in open sunlight; also as a tree like vine
ten to thirty feet high and may be more than more than one hundred feet long with a three to eight inch trunk. It reproduces
by spreading rhizomes and by seeds.
Pacific Poison-oak foliage.
The plant is deciduous, so that after
cold weather sets in, the stems are leafless and bear only the occasional
cluster of berries. Without leaves, poison oak stems may sometimes be
identified by occasional black marks where its milky sap may have oozed and
dried.
The leaves are divided into three (rarely five,seven or nine) leaflets, one and one a half to four inches long, with scalloped, toothed, or lobed edges. They
generally resemble the lobed leaves of a true oak, though the Pacific poison
oak leaves will tend to be glossier. Leaves are typically bronze when first
unfolding in February to March, bright green in the Spring, yellow-green to
reddish in the Summer, and bright red or pink from late July to October.
White flowers form in the spring, from March to
June. If they are fertilized, they develop into greenish-white or tan berries.
Botanist John Howell observed the toxicity of
Toxicodendron diversilobum obscures its merits:
"In spring, the ivory flowers bloom on the
sunny hill or in sheltered glade, in summer its fine green leaves contrast
refreshingly with dried and tawny grassland, in autumn its colours flame more
brilliantly than in any other native, but one great fault, its poisonous juice,
nullifies its every other virtue and renders this beautiful shrub the most
disparaged of all within our region”
Pacific poison oak leaves and twigs have a surface
oil, urushiol, which causes an allergic reaction. It causes contact dermatitis-
an immune-mediated skin inflammation- IN 4/5 OF HUMANS However, most, if not
all, will become sensitized over time with repeated or more concentrated
exposure to urushiol.
Pacific Poison-Oak grows on drier
rocky slopes at lower elevations on south-eastern Vancouver Island and nearby
Gulf Island.
Reactions:
Urushiol-induced contact dermatitis from poison
oak.
Pacific poison oak skin contact first causes
itching; then evolves into dermatitis with inflammation, colourless bumps,
severe itching, and blistering. In the dormant deciduous seasons the plant
can be difficult to recognize, however leafless branches and twigs contact also
causes allergic reactions.
Contrary to what has been written by
other authors and posted on other web-sites Urushiol volatilizes when burned;
SO IT IS, INDEED, CARRIED THROUGH THE AIR to unsuspecting victims.
And,
human exposure to Toxicodendron diversilobum smoke is extremely hazardous, from
wildfires, controlled burns, or disposal fires. The smoke can poison people who
thought they were immune. The resin can persist on pets and
clothing for months and is also ejected in fine droplets into the air when the
plants are pulled.
Branches used to toast food over campfires can
cause reactions internally and externally, although, or so it is claimed, the
Karok peoples traditionally used them as a cooking tool.
Ecology: Black-tailed deer, Mule deer, ground
squirrels, Western grey squirrels, and other indigenous fauna feed on the
leaves of the plant. It is rich in phosphorus, calcium, and sulphur. Bird
species use the berries for food, and utilize the plant structure for shelter. Native
animals, horses, livestock, canine pets, DO NOT demonstrate reactions to
urushiol.
Due to human allergic reactions, Pacific poison oak
plants are usually eradicated in gardens and public landscaped areas. It can be
a weed in agricultural fields, orchards, and vineyards. It is usually removed
by pruning, herbicides, digging out, or a combination of these.
Uses:
Native North Americans used the plant's stems and
shoots to make baskets, the sap to cure ringworm, and as a poultice of fresh
leaves applied to rattlesnake bites. The juice or soot was used as a black dye
for sedge basket elements, tattoos, and skin darkening.
An infusion of dried roots, or buds, eaten in the
spring, were taken by some native peoples for an immunity from the plant
poisons.
Chumash peoples used Pacific poison-oak sap to
remove warts, corns, and calluses; to cauterize sores; and to stop bleeding.
They also drank a decoction made from Pacific poison-oak roots to treat
dysentery.
© Al (Alex-Alexander) D Girvan> All rights reserved.
© Al (Alex-Alexander) D Girvan> All rights reserved.
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