Dangerous Plants

There is a variety of dangerous trees and plants worldwide. The following sites have informa­tion on noxious and poisonous plants throughout the world.

The most common poisonous plants you will find in Canada are:

  • Poison Ivy
  • Poison Oak
  • Poison Sumac
  • Giant Hogweed

Poison Ivy, Oak, and Sumac:

Poison ivy, poison oak, and poison sumac grow almost everywhere in the United States, except Hawaii, Alaska, and some desert areas in the Western U.S. Poison ivy usually grows east of the Rocky Mountains and in Canada. Poison oak grows in the Western Uni­ted States, Canada, Mexico (western poison oak), and in the Southeastern states (eastern poison oak). Poison sumac grows in the Eastern states and southern Canada.


A plant induced rash is an allergic contact dermatitis caused by contact with oil called urushiol. Urushiol is found in the sap of the poisonous plants like poison ivy, poison oak, and poison sumac. It is a colourless or pale yellow oil that oozes from any cut or crushed part of the plant. Once exposure to air, urushiol turns brownish-black in colour. Damaged leaves look like they have spots of black enamel thus making it easier to recognize and identify the plant. Contact with urushiol can occur in three ways:

  • Direct contact – by touching the sap of the toxic plant.
  • Indirect contact – by touching something on which urushiol is present. This oil can stick to the fur of animals, to garden tools, sports equipment, or to any objects that may have come into contact with it.
  • Airborne contact – by burning the poison plants. This will release the urushiol particles into the air.

When urushiol gets on the skin, it will begin to penetrate in minutes. A reaction appears, usually within 12 to 48 hours. There is severe itching, redness, and swelling, followed by blisters. This rash is often arranged in streaks or lines where the person brushed against the plant. In a few days, the blisters will become crusted and take 10 days or longer to heal. Poison plant dermatitis can affect any part of the body. The rash does not spread by touching it, although it may seem to when it breaks out in new areas. This may happen because the urushiol absorbs more slowly into skin that is thicker, skin such as on the forearms, legs, and trunk.

About 85 percent of the population will develop an allergic reaction when exposed to poison ivy. The sensitivity will vary from person to person. For people who reach adulthood without becoming sensitive, they will then have only a 50 percent chance of developing an allergy to poison ivy. However, only about 15 percent of the population seems to be resistant.


Identifying the plant is the first step to avoid the rash. The popular saying leaves of three, beware of me is a good rule of thumb for poison ivy and poison oak, but is only partly correct. A more exact saying would be leaflets of three, beware of me, because each for poison ivy and poison oak each leaf has three leaflets. Poison sumac has a row of paired leaves. The middle or end leaf is on a longer stalk than the other two or more leaves.

Poison Ivy:

Poison Ivy
  • Grows as vines or low shrubs
  • Grows in in fertile, well drained soil
  • In spring has yellow-green flowers

Poison Oak:

Poison Oak
  • Oak-like leaves
  • Low shrub in the East, prefers sandy soil
  • High shrub in the West, prefers moist soil

Poison sumac:

Poison Sumac
  • Can be a tall shrub or a small tree
  • Grows in standing water, ex: peat bogs

For all three:

  • Most common in the spring and summer
  • Black marks where they have been bruised
  • Ooze lots of sap
  • Cases may occur in winter when people burn wood to clear yards that has urushiol on it or cut poison ivy vines for wreaths
  • Early fall, their leaves can turn colors such as yellow or red while other plants are still green.
  • Berry-like fruit on the mature female plants will also change color in the fall; from green to off-white

Prevention of Poison Rashes:

  • Stay away from them
  • Destroy with herbicides in your own backyard
  • Wear long pants, long sleeves, boots, and gloves
  • Do not burn plants that look like poison ivy, poison oak or poison sumac
  • Apply barrier cream containing bentoquatum


If you think you’ve had a brush with poison ivy, poison oak, or poison sumac, follow these simple steps:

  • Wash all exposed areas with cold running water as soon as you can reach a stream, lake, or garden hose
    • If done within five minutes: the water may keep the urushiol from contacting your skin and spreading to other parts of your body
    • If done within the first 30 minute: soap and water are helpful to eliminate or reduce the rash
  • Wash your clothing in a washing machine with detergent and equipment with dish soap.
  • Relieve the itching of mild rashes by taking cool showers
  • Applying over-the-counter preparations like calamine lotion or Burrow’s solution
  • Soaking in a lukewarm bath with an oatmeal or baking soda solution may also ease itching and dry oozing blisters

If you know you have been exposed and developed severe reactions in the past, consult a dermatologist or doctor. They may prescribe cortisone or other meds that prevent blisters from forming.

Giant Hogweed

Giant Hogweed is a dangerous plant. The sap from all parts of the Giant Hogweed causes a Phyto-photodermatitis affect when the sap gets onto your skin. At first the skin will turn red and become itchy: then once the skin is exposed to sunlight or UV rays the sap will cause deep blisters. These blisters can form black or purplish scars that can last for several years. Even a tiny amount of the sap in the eyes can cause temporary to permanent blindness. Giant Hogweed First Aid treatment will need to be applied if you come into direct contact with Giant Hogweed.

Giant Hogweed is an invasive species from Central Asia and has spread all across Canada and the US. In Alberta, as with many other provinces, Giant Hogweed is being confused with its smaller and less toxic cousin, Cow Parsnip.