Intriguing World of Weeds iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii Poison-Ivy/Poison-Oak/Poison-Sumac-The Virulent Weeds 1 LARRY W. MITICH2 I I; INTRODUCTION I, ii The word poison entered the English language in 1387 as 'poysoun" (18), and in Memoirs ofAmerican Academy of Arts and Sciences, v. 1, 1785, the word poison-ivy was used for the first time: "Poison ivy ... produces the same kind of inflammation and eruptions . as poison wood tree" (18). · The first known reference to poison-ivy, Toxicodendron radicans (L.) Ktze., dates from the 7th century in China and the 10th century in Japan. Since Toxicodendron species do not grow in Europe, the plants re­ mained unknown to Western civiliza­ tion until explorers visited the New World seven centuries later (7). Capt. John Smith (1579-1631) wrote the first description of poison-ivy and origi­ nated its common name; he noted a similarity in the climbing habit of They used that name for a shrub of the Southern States, North American poison-ivy to English with crenately-lobed, very pubescent leaflets (1). ivy (Hedera helix L.) (7). Toxicodendron, a pre-Linnaean name, was not accepted Joseph Pitton de Tournefort (1656-1708) established at the generic level by Linnaeus. Tournefort limited the the genus for poison-ivy in lnstitutiones rei herbariae, v. genus to ternate-leaved plants, thus omitting such close 1, p. 610, 1700 (7). The name is from the Latin for poison­ relatives of poison-ivy as poison-sumac and the oriental ous tree (9). lacquer tree (7). The lacquer tree has been known for In 1635 Jacques Philippe Cornut ( 1601-1651) wrote the first book on Canadian botany (Canadensium Plantarum thousands of years, and descriptions of the inflammations aliarumque nondum editarum historia, Parisiis [Paris]). produced by lacquer made from its sap go back to before This work is also the first one to illustrate poison-ivy Christ (17). (reproduced in this article), which he listed under the name Poison-ivy is also used figuratively for an unpleasant "Edera [Hedera] trifolia canadensis" (17). person. For example, B. Gray wrote in Miss Dynamite Linnaeus described poison-ivy in Hortus Ciffortianus (1939): "So this is the charming little prairie flower that ( 1737) as Rhus radicans; he later edited his description of Norman's fallen in love with! ... Primrose, my foot! Her the plant to include the climbing habit and added a descrip­ name's Poison Ivy" (18). tion of poison-oak, R. toxicarium (Salisb.) Gillis, when he In the U.S. there are· about 2 million cases of skin wrote Species Plantarum, 1753 (7). While Linnaeus re­ poisoning each year. Poison-ivy and its allies are the great­ corded 13 species of Rhus, 14 more had been assigned to est single cause of workers' compensation claims, and they the genus by the end of the 18th century, including species are a leading cause of field injuries among workers of the of Rhus, Toxicodendron, and others (7). Britton and Brown U.S. Forest Service (12). refer to R. toxicodendron as being "of American authors." During a single year in California, the working hours lost as a result of dermatitis and the discomfort experienced 1No. 50 of the series "Intriguing World of Weeds." by sensitive individuals make poison-oak the most hazard­ 2Ext. Weed Sci. Emeritus, Dep. Vegetable Crops, Univ. California, Davis, CA 95616. ous of all the poisonous plants in the state (5). 653 Weed Technology. 1995. Volume 9:653-656 J INTRIGUING WORLD OF WEEDS CLASSIFICATION, fruit. The cashew nut is obtained from Anacardium occi­ Poison-ivy and poison-oak are neither ivy nor oak spe­ dentale; an oil obtained from its shell is valuable in resin cies. These plants, as well as poison-sumac, belong to the manufacture and, of course, the meat of the nut is a world­ cashew family, Anacardiaceae, and are known by several wide favorite. During the U.S. occupation of Japan follow­ local names. The leaf shape and growth habits of poison­ ing World War II, sensitized American soldiers ivy and poison-oak plants vary greatly throughout the U.S. unexpectedly suffered dermatitis when they inevitably Their common features are their poisonous nature and their came in contact with toilet seats and bars beautifully fin­ compound leaves, which consist of three leaflets. The old ished with Japanese lacquer (11). saying "Leaflets three, let it be" confirms the plant's nearly consistent leaf aspect (12). POISON-IVY Depending on area, poison-ivy, poison-oak, and poison- . Poison-ivy is one of two New Worl9 species; it also sumac are all generally known under the same name. The occurs in Eastern Asia. It is the most common and most proper classification of Toxicodendron has long been a widespread species in the Anacardiaceae. It is distributed bone of contention among botanists (11). Some classify from southern Canada to western Guatemala, the eastern poison-ivy and poison-oak, and infrequently poison-su­ third of the U.S. and throughout Mexico, on Bermuda, in mac, as varieties of one species, pointing out that all three the western Bahamas, Japan, western and central China have leaves divided into three ovate leaflets, variously and Taiwan, the Kurile Islands, and Sakhalin of Russia indented and lobed and downy beneath. And in June they (II). all bear slender clusters or panicles of inconspicuous Poison-ivy distribution in the U.S. is below the 44th greenish-white flowers in the axils of the leaves, followed parallel, extending from Maine to Florida and westward by clusters of small globular, greyish-white, waxy, berry­ into Nebraska, Kansas;Oklahoma, and Texas, It also grows like fruits (13). in a small area in southern Arizona and New Mexico (12). This very plasticity has given the "splitters" a chance to It is a low straggling shrub, sometimes occurring as a stout create more than 30 species! But the more conservative weedy vine climbing over trees. maintain that poison-ivy is a single variable species, with The compound leaves of poison-ivy are alternate on the several taxonomic varieties and forms (11, 14). Immu­ stem. Each leaflet is 5 to 10 cm long and is pointed at the nologically all species behave the same (17). tip. The supporting stem of the terminal leaflet is much In 1859, Darlington mentions both the erect and climb­ longer than the stems of the two lateral leaflets. Leaflets ing varieties of poison-ivy. Wrote he: "This species ... may be glossy or dull green and usually have smooth should not only be known to the farmer, but diligently surfaces. The edges of the leaflets vary considerably; some expelled from his premises" (4). George offered this ad­ are toothed or even lobed but most are not (12). vise: "Some fortunate persons are quite immune to the bad effects of the plant, and one of these might be hired to grub POISON-OAK it out and bum it" (6). The Anacardiaceae includes many widely distributed The Toumefort name Toxicodendron, a generic segre­ species which are both noxious and useful. The baleful gate of· Rhus, was chosen by Linnaeus as the specific species have in common the capacity to induce a severe epithet for poison-oak, R. toxicarium Salish. (7). Now contact dermatitis. All the dermatitogenic Anacardiaceae classified as T. toxicarium (Salish.) Gillis, this North are members of an immunologic cross reacting group American native shrub grows in dry woodland from New owing to biochemical similarities of their antigens. Sensi­ Jersey to Georgia and Texas, and the West Coast (13). tization to one carries with it the verdict of allergic reac­ Hickman uses T. diversilobum (Torrey & A. Gray) E. tivity to the rest (11). Greene for Western poison-oak, a shrub which can some­ Some dermatitis-producing Anacardiaceae possess time become tree-like (8). The common name is related to amiable or economically useful properties. The Orientals the typical leaflet shape, which resembles a white oak leaf prepare a rich furniture lacquer from R; verniciflua, the (7). ' lacquer tree; the marking nut or Indian ink tree, Semecar­ Poison-oak is very common in California, western Ore­ pus anacardium, is used to stain clothing, often as a laundry gon, and western Washington. It becomes a dense leafy mark. The mango, Mangifera indica, produces a delectable shrub when growing in the open or in filtered sun, but in 654 Volume 9. Issue 3 (July-September) 1995 WEED TECHNOLOGY shaded areas, as in coast redwoods, it becomes a tall-climb­ Both poison-ivy and poison-oak may cause severe der­ ing vine. Foliage turns a striking bright orange or scarlet matitis on contact; even breathing smoke from burning in the fall (3). plants is harmful (3). Transmission from smoke is by droplets on particles of dust and ash in the smoke, rather POISON-SUMAC than from gas. Smoke from burning poison-ivy plants or contaminated articles can carry the poison in a dispersed The generic name for sumac is Rhus; it is from the Greek form (13). rhous and the Latin rhus, both being names for sumac. The With time, sensitivity to poisoning can change. Gener­ Greek rhous is from rhodos, red, which gave rise to the ally, persons repeatedly exposed to poison-ivy and poison­ Celtic rhudd, red, the prevailing color of the sumac fruit oak become more sensitive because they react to lower (4, 9). concentrations of the oily toxicant. And people usually There are approximately 150 species of Rhus which are become less sensitive as they get older. But this rule does native to the temperate and subtropical regions of North not always hold true. Persons who have waded through and South America. It is a rather baffling genus, consisting patches of poison-oak numerous times without effect can of deciduous and evergreen trees, shrubs, sub-shrubs, and suddenly develop a severe rash after such contact.