Poison ivy (Toxicodendron radicans, (L.) Kuntze 1891)

Identity

Poison ivy (Toxicodendron radicans (L.) Kuntze, 1891), also known as eastern poison ivy, is a deciduous vine or small shrub in the genus Toxicodendron, family Anacardiaceae. This family also includes ornamental sumacs (Rhus), mango and cashew. That relationship helps explain some food allergies in people already sensitized to urushiol. ^{1 6}

Despite its familiar English name, poison ivy is unrelated to true ivy (Hedera), which belongs to a different botanical family. The Quebec French name herbe à puce refers to the itching caused by contact, compared with flea bites, rather than to any connection with the insect. ⁵

Recognition

Look for alternate leaves with three leaflets, the basis of the North American warning “leaves of three, let it be.” The central leaflet has a longer stalk than the two lateral ones. Leaflet margins may be smooth, toothed or lobed, so shape alone can make quick identification difficult. ^{3 2}

No thorns occur on this plant, unlike several vines mistaken for it. Depending on available support, it grows as an upright bush about 0.3–1 m (1–3 ft) tall, a trailing groundcover, or a vine attached to trunks by hairy, reddish-brown aerial roots. Climbing stems can sometimes exceed 18 m (59 ft). ²

Small yellowish-green to whitish flowers open in loose clusters in the leaf axils in spring. They are followed by white, waxy, hairless drupes, about 0.5–0.8 cm (0.2–0.3 in) across, held in clusters that often remain through winter. ^{2 5}

Useful taxonomy

For poison ivy, the accepted POWO/WCVP name is Toxicodendron radicans (L.) Kuntze. Otto Kuntze transferred it to Toxicodendron in 1891 after its long use under Linnaeus’s name Rhus radicans. Several geographic subspecies help explain regional forms:

• Toxicodendron radicans subsp. rydbergii (Small ex Rydb.) Á.Löve & D.Löve (regional subspecies)
• Toxicodendron radicans subsp. eximium (Greene) Gillis (regional subspecies)
• Toxicodendron radicans subsp. verrucosum (Scheele) Gillis (regional subspecies)

The subspecies rydbergii is bushy and climbs little or not at all. It reaches northwestern Mexico and is sometimes treated as a separate species, Toxicodendron rydbergii. Such diversity explains why poison ivy may be a shrub, groundcover or vine in different regions without losing its botanical identity. ¹

Habitat and range

From eastern Canada, poison ivy ranges south to Guatemala, across almost all US states east of the Rockies, northern Mexico, the Bahamas and Bermuda. It colonizes moist woodland edges as well as dry disturbed ground, hedges, coastal dunes and roadsides. Tolerance of pH 5–8.5 allows it to grow in both acidic and calcareous soils. ^{1 2}

Cold tolerance reaches USDA zone 4, with winter temperatures down to −30 °C (−22 °F), a threshold covering almost all of France. Together with its tolerance of different soils, this hardiness helps explain its broad North American distribution.

Waxy white fruits provide winter food for birds, including thrushes, woodpeckers and many songbirds. White-tailed deer and rabbits browse the leaves without a skin reaction; sensitivity to urushiol is characteristic of primates. ²

French occurrences remain largely outside extensive naturalized populations. The plant is still occasionally sold there as an ornamental wall climber for its colorful fall foliage. In that country, sale requires warning labels rather than being completely prohibited. ⁵

Life cycle and reproduction

Spring brings clusters of inconspicuous yellowish-green flowers. Birds eat the white fruits in winter and disperse seeds after they pass through the digestive tract, often encouraging germination the following spring. ^{5 2}

After becoming anchored to a vertical support, a vine can add several meters of growth each year. This speed lets it cover an entire trunk within just a few seasons.

Root suckers also allow vegetative spread. A root fragment left in the soil after removal can regenerate a new plant the following year, making lasting eradication from a garden difficult. ³

Uses and risks

Roots, bark, unripe fruits and every other part contain urushiol, a resinous oil that causes allergic contact dermatitis in most exposed people. In sensitive individuals, redness, blisters and intense itching usually develop 12–48 hours after exposure. ^{6 5}

The ASPCA lists poison ivy as non-toxic to dogs, cats and horses. A pet can nevertheless carry urushiol on its coat and transfer it by contact to someone who later pets the animal. ⁴

Cut stems and uncleaned tools can retain active urushiol for months, sometimes up to two years. Burning the plant is especially dangerous because smoke vaporizes the oil and inhalation can cause a severe respiratory reaction. Washing exposed skin with cold water and soap within minutes remains the best preventive measure. ³

Varieties, look-alikes and related plants

Harmless climbing or shrubby plants account for the main look-alikes. Virginia creeper (Parthenocissus quinquefolia) has five leaflets instead of three. Young box elder (Acer negundo) seedlings also have three leaflets, but their leaves are opposite rather than alternate on the stem. ³

Two relatives deserve separate recognition: poison sumac (Toxicodendron vernix), a wetland species with seven to thirteen leaflets and reportedly even more concentrated urushiol, and staghorn sumac (Rhus typhina), a harmless ornamental whose similar name can be misleading. ⁶

Cross-reactions with mango skin and cashew shells have been documented in people already sensitized to urushiol. The family relationship described above can therefore help explain some otherwise puzzling food allergies. ⁵

Main sources

• Plants of the World Online / Kew, Toxicodendron radicans (L.) Kuntze
• North Carolina Extension Gardener Plant Toolbox, Toxicodendron radicans
• University of Wisconsin Extension, Poison Ivy
• ASPCA, Poison Ivy toxicity entry
• Wikipedia (French), poison ivy
• Wikipedia, Toxicodendron radicans