Know Your Natives – “Leaves of three, let it be”…usually

We have all heard the advice, Leaves of three, let it be.  Two native Arkansas plants, poison-ivy and poison-oak, have three leaflets per leaf and contain urushiol, an oily allergen.  Following direct or indirect contact, many people experience allergic reactions (contact dermatitis) resulting in skin redness, itching, swelling, and blisters, with severity being dependent on an individual’s sensitivity.  Urushiol is water soluble so it can be removed from skin with soapy water, with less or no irritation if done within about 15 minutes.

For those hiking or gardening, being able to recognize poison-ivy and poison-oak is important, so that such activities can be better enjoyed.  Growth habits of these two plants is such that one or both plants could be encountered anywhere from foot-level to head-level.  Both poison-ivy and poison-oak have a leaf characteristic which helps distinguish them from other plants with three leaflets per leaf.  As shown by Photos 1 and 2, that characteristic is the presence of a petiolule (the stalk of an individual leaflet, as opposed to a petiole, the stalk of a whole leaf) at the base of the terminal leaflet.

Fragrant sumac, which has a sessile (no petiolule) terminal leaflet, is also shown in Photos 1 and 2.  It grows in similar habitats as its cousins poison-ivy and poison-oak and has leaves that look similar and may often be confused with its more notorious relatives.  However, fragrant sumac does not cause contact dermatitis in most people.

Poison-ivy, poison-oak, and fragrant sumac are all deciduous plants of the Anacardiaceae (Cashew) Family.  Leaves of these three plants vary in shape, venation and texture.  Leaf size is also somewhat variable, more so in poison-ivy which may have leaves up to two feet long, especially when the plant vines.  All three plants produce drupes (berry-like fruits with a fleshy outer layer covering a hard inner shell surrounding the seed) that are important to wildlife.  All have nice fall color.

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Photo 1:  Upper leaf surfaces of poison-ivy, poison-oak, and fragrant sumac. (Leaf stems removed for photo.)

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Photo 2:  Lower leaf surfaces of poison-ivy, poison-oak, and fragrant sumac.  (Leaf stems removed for photo.)

Poison-Ivy

Poison-ivy (Toxicodendron radicans), a woody vine, occurs throughout Arkansas.  Plants may trail along the ground, but can be sufficiently self-supporting to have upright stems several feet tall with a shrubby appearance.  Poison-ivy will attach itself to a tree trunk via numerous strong rootlets and grow quickly toward brighter light.  Vines of varying sizes, up to five inches in diameter, may surround a tree trunk and remain strongly rooted to the tree.  Older plants may grow 60 feet or more from the ground and, nearer the ground, have many horizontal limbs growing in all directions from the tree trunk, extending out for up to six feet.  Poison-ivy has smooth, white drupes that mature in the fall.

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Photo 3:  Poison-ivy with inflorescence.  Photo taken May 22nd.

Poison-Oak

Poison-oak (Toxicodendron pubescens, formerly Toxicodendron toxicarium), is a woody shrub that exhibits limited branching and may grow to four feet tall (see Photo 4).  It also may trail along the ground somewhat but does not typically vine up other vegetation.  Poison-oak is found throughout much of Arkansas, though is mostly absent from the Mississippi Alluvial Plain.  Poison-oak usually occurs in sandy to rocky soils in drier woodlands and pinelands.  Leaflets of poison-oak are usually hairy, unlike poison-ivy, and have rounded lobes somewhat resembling those of some species of oaks.  Poison-oak has small, smooth yellowish drupes that mature in the fall.

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Photo 4: Poison-oak with dried inflorescence.  Photo taken May 24th.

Fragrant Sumac – A Poison-Ivy/Poison-Oak Look-Alike

Fragrant sumac (Rhus aromatica), a low-growing woody shrub (see Photo 5), is found throughout most of Arkansas.  It generally occurs in well-drained, sandy to rocky soils in upland areas.  Fragrant sumac occurs in dense stands of smooth, unbranched stems arising from root suckers.  Plants have a slow growth rate, typically reaching two to four feet tall, but some varieties can reach six feet tall with a spread of up to 10 feet.  Leaflets are usually hairy and have rounded lobes.  One-inch catkins (male) and short panicles (female) in a cluster on twig tips form on separate plants (sometimes on same plant) at the ends of stems.  Catkins form in late summer, but don’t open until early spring.  The more common variety blooms before the plant leafs out, but another variety more common in the limestone areas of the Ozarks flowers as or after the leaves appear.  Hairy red drupes of fragrant sumac mature in late spring.  The leaves and stems have a citrus-like fragrance when crushed (this test not recommended for poison-ivy or poison-oak).

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Photo 5:  Fragrant sumac with fruit.  Photo taken May 12th.

Additional notes:  Smooth sumac (Rhus glabra) and winged sumac (Rhus copallina), both common in Arkansas, are non-allergenic.  Unlike fragrant sumac, these two sumacs would not be confused with poison-ivy or poison-oak since they have pinnately compound leaves.  Poison sumac (Toxicodendron vernix), which also has pinnately compound leaves, fortunately does not occur in Arkansas.  Some other native plants may also be confused with poison-ivy or poison-oak, such as marine-ivy (Cissus trifoliata, formerly Cissus incisa) and hop tree (Ptelea trifoliata) which also have three leaflets, but, like fragrant sumac, these two have sessile terminal leaflets.

“Leaves of three, what can it be?”

Article and photos by ANPS member Sid Vogelpohl

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