The primary way to distinguish between poison oak and poison ivy is by where the plant grows. Poison oak generally grows along the West Coast of the United States, while poison ivy grows in the remainder of the states, according to About.com. Both plants have three-leaflet leaves and white berries.
Poison ivy usually has a leaf made of of three shiny, oval, pointed leaflets, while poison oak leaves have lobes, says Dummies.com. The lobes of the poison oak, however, are not as deep as those of many oak trees. One problem with both of these plants is that their appearance can change from season to season or plant to plant, says Missouri Botanical Garden.
For example, the leaves of poison ivy are smooth or toothed, says the Missouri Botanical Garden. The leaves are also dull or shiny. The plant grows as a vine or a shrub. It blooms from May through July, has greenish white flowers and white berries that some birds eat.
Poison oak is present in western North America from hardiness zones 5 to 9, according to the Missouri Botanical Garden. Poison ivy is present from South Canada, throughout the United States and south into Guatemala. It is also present in Japan, China and Taiwan. It thrives in zones 4 to 10.
Poison oak is also a much larger plant than poison ivy, which usually grows to about 1 to 3 feet tall with a 1- to-3-foot spread, says Missouri Botanical Garden. By contrast, poison oak grows to 10 feet tall and has up to a 7-foot spread. As a vine, it grows to 50 feet tall. Poison oak also blooms earlier than poison ivy, with its showier flowers appearing between April and June.
These plants, along with poison sumac, form an oily substance on their leaves known as urushiol, according to WebMD. This oil causes irritation to humans who touch the plant as well as to those who touch clothing or animals exposed to the plant. Burning any part of the plant releases urushiol as a vapor that has the potential to irritate the lungs.
Poison oak and poison ivy rash develop from contact with the oil. WebMD indicates it is impossible to develop the rash from touching the blister fluid from another person with a rash. The reaction is to the oil, which the immune system sees as a harmful substance.
The rash from all three plants is identical as the substance to which the body is reacting is the same, according to Parents. At home, treatment for the rash includes the use of cold compresses, cool baths and calamine lotion to relieve the itching. Sometimes the rash spreads and requires medical intervention. Dermatologists sometimes prescribe topical medications, including corticosteroids and anti-inflammatories to relieve the itching.