Wild parsnip is most irritating while flowering.
Poison oakWhen bruised or damaged, Poison ivy, poison oak and poison sumac release an oil called "urushiol" which can cause contact dermatitis, an uncomfortable allergic rash. Most people exposed to the oil will develop an itchy red rash with bumps or blisters.
Severe reactions may require medical attention, according to the CDC. Burning the plants is especially dangerous, as the allergens can irritate the lungs when inhaled.
Poison oak is found primarily in the southeast and west coast. Its leaves have three parts and the plant can take the form of a shrub or vine. The plant may have yellow or green flowers and clusters of greenish or white berries.
Poison ivyPoison ivy, perhaps the most notorious of the allergenic plants, is found in nearly every state with the exception of Alaska, Hawaii and California.
The plant can be recognized by its three shiny leaves, which can be green or red depending on the season. Some varieties grow on a climbing vine and the plant may have yellow or green flowers and white, green-yellow or amber berries.
Poison sumacPoison sumac is most common near the Mississippi River and boggy areas of the southeast. Each stem on this woody plant has seven to thirteen leaves, arranged in pairs. The plant may also have shiny, pale yellow or cream berries.
Giant hogweedGiant hogweed is a noxious weed that can grow up to 14 feet tall. The plant has thick leaves that can stretch five feet wide and large clusters of white flowers. Its stems are green with purple blotches and white hairs.
Giant hogweed sap contains toxic chemicals known as photosensitizing furanocoumarins. When these chemicals come into contact with the human skin, it can cause a skin reaction that's extremely sensitive to light which leads to dark painful blisters that form within 48 hours, and result in scars that can last anywhere from a few months to six years. Touching giant hogweed can also cause long-term sunlight sensitivity and blindness if sap gets into a person's eye.
Read more about Giant Hogweed here
Stinging nettleFar more benign than others on this list, the stinging nettle is still no fun if handled incorrectly.
Nettle plants grow three to four feet high and are found in large clusters. Nettles can be recognized by their jagged, deep green leaves and greenish-white flowers. The leaves and stems are covered with stinging hairs, that when brushed up against, can cause a painful, burning sensation.
Despite the danger of touching nettles, once cooked, the greens can be a delicious and healthy ingredient> in soups and other recipes.