Poison ivy

Appearance

Numerous subspecies and/or varieties of "T. radicans" are known, which can be found growing in any of the following forms; all of which have woody stems:

⤷  as a climbing vine that grows on trees or some other support
⤷  as a shrub up to 1.2 m tall
⤷  as a trailing vine that is 10–25 cm tall

The deciduous leaves of "T. radicans" are trifoliate with three almond-shaped leaflets. Leaf color ranges from light green to dark green, turning bright red in fall; though other sources say leaves are reddish when expanding, turn green through maturity, then back to red, orange, or yellow in the fall. The leaflets of mature leaves are somewhat shiny. The leaflets are 3–12 cm long, rarely up to 30 cm. Each leaflet has a few or no teeth along its edge, and the leaf surface is smooth. Leaflet clusters are alternate on the vine, and the plant has no thorns. Vines growing on the trunk of a tree become firmly attached through numerous aerial rootlets. The vines develop adventitious roots, or the plant can spread from rhizomes or root crowns. The milky sap of poison ivy darkens after exposure to the air.

Naming

Subspecies
⤷ "T. r." subsp. "barkleyi" Gillis
⤷ "T. r." subsp. "divaricatum" Gillis
⤷ "T. r." subsp. "eximium" Gillis
⤷ "T. r." subsp. "hispidum" Gillis
⤷ "T. r." subsp. "negundo" Gillis
⤷ "T. r." subsp. "pubens" Gillis
⤷ "T. r." subsp. "radicans"
⤷ "T. r." subsp. "verrucosum" Gillis

Distribution

"T. radicans" grows throughout much of North America, including the Canadian Maritime provinces, Quebec, Ontario, Manitoba, and all U.S. states east of the Rocky Mountains, as well as in the mountainous areas of Mexico up to around 1,500 m. "Caquistle" or "caxuistle" is the Nahuatl term for the species. It is normally found in wooded areas, especially along edge areas where the tree line breaks and allows sunshine to filter through. It also grows in exposed rocky areas, open fields, and disturbed areas.

Habitat

It may grow as a forest understory plant, although it is only somewhat shade-tolerant. The plant is extremely common in suburban and exurban areas of New England, the Mid-Atlantic, and the Southeastern United States. The similar species "T. diversilobum" and "T. rydbergii" are found in western North America, and "T. orientale" in Taiwan, Japan, Korea and Sakhalin.

"T. radicans" rarely grows at altitudes above 1,500 m, although the altitude limit varies in different locations. The plants can grow as a shrub up to about 1.2 m tall, as a groundcover 10–25 cm high, or as a climbing vine on various supports. Older vines on substantial supports send out lateral branches that may be mistaken for tree limbs at first glance.

It grows in a wide variety of soil types, and soil pH from 6.0 to 7.9. It is not particularly sensitive to soil moisture, although it does not grow in desert or arid conditions. It can grow in areas subject to seasonal flooding or brackish water.