Identifying Poisonous Plants

We all love hitting the trails, getting some exercise and soaking up the sun, but it’s important to be informed of new risks when entering a new area. After settling onto Manitoulin Island for the summer, I was plagued with a brutal reaction to the poisonous plant “Rhus Radicans” A.K.A Poison Ivy. Spending the last few years camping and hiking in the Sudbury area, I was totally naïve to identifying the plant in its many forms. Sudbury soil is rather rocky acidic, an unfavourable environment for these generally low-lying plants. On the contrary, the area surrounding Georgian Bay provides the nutrient-rich southern Ontario soil that is ideal for the plant to grow and spread. I had faced an incident with Poison Ivy last time I bush crashed on the Island, and am now committed to making sure it does not happen a third time. Since recently guiding a trip in Bruce Peninsula National Park, I realized how many other folks are as bad at identifying poison ivy and other burn-inducing plants as I once was. This post is meant to help you brush up on your basic poisonous plant identification skills to avoid a nasty case of burn rash. Please note that this post is not meant to cover the wide variety of other plants that are also toxic to the human body when contact is made in any way, shape or form.

Poison Ivy: Rhus Radicans

            In general, the age old adage: “leaves of three – let them be”, is good rule to follow. Poison Ivy is a perennial that spreads through seed distribution and underground stems, resulting in what is usually dense, vast patches of the plant. So when you’re looking for it…think big sections of it, not small solo-standing plants (although you should be on alert for those too). The first and most common stem type for Poison Ivy is woody, horizontal stems that lead to upright leafy stalks (a low-lying cover or mid-size shrub anywhere from 10cm-100cm off the ground). The second type of stems you’ll see are of the climbing vine variety (most commonly visible in the lower Ottawa Valley) which will wind themselves around trees and can climb to be 5 or 6ft tall! It is usually a bad idea to generalize poison ivy by leaf/stem colour or leaf shape only. Colour can be green, red-orange, brown, purple-red, waxy or matte depending on time of year; and leaf shape can vary from long and pointed, or round and curved with smooth edges to serrated (toothed) or bluntly wavy (lobed) edges. Poison Ivy flowers are small and white/greenish in colour, but usually remain unseen as many plants do not flower year-to-year and many flowers grow on the underside of the leaf sprouting. You can spy berries that are white-green-yellow on plants during September straight through the winter. They are small (about 5mm in diameter) and resemble a peeled orange in texture.

Poison Oak: Toxicodendron Diversilobum

            Contrary to popular belief, Poison Oak does not grow in Ontario. It is a plant to watch for if you are ever visiting our neighbours in the States or in some parts of British Columbia. Most people mistake Poison Ivy that has serrated leaves for Poison Oak, however this is a case of mistaken identity.

Poison Sumac: Toxicodendron Vernix

            Poison Sumac typically grows into a shrub or tree that is anywhere from 5-20ft tall, but can grow even higher (or lower) depending on growth conditions. Branches of the tree are sometimes covered with leaves along their length, however the general pattern of growth for leaves “leaves” (haha) the tree or shrub looking fairly sparse. Branches on large trees are usually thin and downwards pointing….but small plants have upwards pointing leaves. Poison Sumac has a “pinnate” leaf structure – meaning leaves grow parallel on either side of the stem. Each stem has between 6-12 parallel leaves, with an additional leaf on the end. Stem colour is generally reddish-brown, and leaf colour ranges from green to red to brown (it is a deciduous plant) depending on time of year. Leaf shape is generally oblong, and a sure fire way to identify Poison Sumac from other types of Sumac is the smooth edge of its leaves. Other non-poisonous types of Sumac leaves will have a serrated or toothed edge. Small pale yellow or green flowers may grow in clusters on Poison Sumac shrubs or trees. They grow on a separate green stem than the red stem of the leaves and usually hang below the branches. In the fall and winter, the flowers are replaced by small whitish/greenish berries. If you see berries on a Sumac that are red, it is not Poison Sumac. Berries and flowers may or may not be present on the shrub as they provide sustenance to a variety of woodland animals. The ideal environment for Poison Sumac is in moist or wet soil, so it is unlikely that you will run into this species in areas where soil is dry for the better part of the year. Be on the lookout for Poison Sumac trees if you plan on burning firewood. Burning Poison Sumac will produce a smoke that can be fatal if inhaled!

What they look like 

What they look like 


Giant Hogweed: Heracleum Mantegazzianum

            Giant Hogweed is a perennial member of the carrot family that is popping up in ditches, streams, fields, and open woodlands in central and southern Ontario. Originally an ornamental plant from Asia, this invasive species is beginning to naturalize in areas south of Manitoulin Island and Ottawa. It resembles many other non-threatening plants in Ontario however it is most recognizable by its size. Giant Hogweed can grow to be 5.5m tall in ideal conditions with a white-green flower head that can grow up to 1m in diameter. In its first year, the plant will produce a rosette (leaves growing out of stem in all directions) of leaves up to 1m high. In 2-5 years it will begin producing flowers. Older plants will have a thick, hollow stem that is green with reddish purple flecks. Leaves are generally large and lobed (curvy-wavy). The stems are covered in thick hairs that are filled with the toxic watery sap. When sap makes contact with skin and is exposed to sunlight, it can cause severe burns.

            Keep an eye out for these species as you make your way into the wild. I can guarantee you will have more fun in the outdoors if you are able to minimize your contact with some of Ontario’s scorching species. Good luck identifying, and happy exploring!