Beware of Toxic plants Lining Rural Roads



By Karen Graham

If you suddenly notice unusual brown discolouration of your skin with little or no accompanying discomfort, or weeping blisters from an unknown source, you may have come in contact with the sap from a plant containing a chemical compound called “furanocoumarin”.  Common plants containing this chemical include celery, wild parsnip, fennel, dill, parsley, wild rhubarb, lovage, mustard and chrysanthemums. Some of the furanocoumarin compounds are photoactive, which means that their toxicity is enhanced by ultraviolet radiation, and their impact is magnified by humidity and perspiration, so gardeners, beware!

Wild Parsnip can cause blistering. Photo: Karen Graham

Wild Parsnip can cause blistering. Photo: Karen Graham

At the moment, many of our rural roads are lined with culprits such as wild parsnip, which while beautiful, can leave permanent scars on exposed skin.It is also commonly found along fencerows, in pastures and at the edges of cropped fields. Another more dangerous culprit is the giant hogweed, a poisonous and invasive plant reaching 5 metres in height which resembles Queen Anne’s Lace on steroids.  The sap of this plant can also cause temporary or permanent blindness if it gets into the eye.  Both plants are on Ontario Ministry of Agriculture, Food and Rural Affair’s list of toxic weeds.

When skin comes into contact with the sap from these plants and is then exposed to sunlight, it reacts to UV radiation.  Symptoms begin with significant pigmentation changeswhich usually take 24 hours to appear, making it difficult to connect the cause to the effect.  The exposed skin turns brown, and the resulting marks resemble scars from a burn, and in fact they are burns.The skin is reacting to a component of the plant which causes a hyper-sensitivity to long-wave ultraviolet light.  The irregular marks resulting from exposure have been known to resemble handprints, and have been occasionally been mistaken for evidence of child abuse.  Another common pattern is a splatter formation resulting from the use of a weed wacker.  For those who are very sensitive, the burning of the skin can persist for 48 hours, by which time the exposed area may swell and develop itchy, weeping blisters resembling the reaction to poison ivy.  Unlike poison ivy, which creates an allergic reaction, the sap of wild parsnip causes a skin reaction in a more permanent way by destroying skin cells.Reactions to this compound typically lastfor several months, but affected area may remain scarred and hypersensitive to ultraviolet light for many years. The severity of the reaction depends on the individual’s sensitivity to the compound.

What damages the skin of the unwitting victim is a potent defense mechanism for the plant.  The furanocoumarins found in wild parsnip enhances the plants’ disease resistance and offers chemical defense against the plant’s principal enemy; the parsnip webworm.

There are obvious ways to avoid exposure to these compounds without being able to identify the culprits.  Wearing gloves and covering skin when handling plant material or using a weed eater prevents direct contact with the sap, and using a high-level sunblock reduces the risk of UV damage.  If you do come into contact with plants containing this chemical, wash the exposed skin and limit sun exposure for the following 48 hours.  For most there is little if any discomfort, but the area can be treated with cool wet dressings, topical steroids, and nonsteroidal anti-inflammatory drugs (NSAIDs).

Do not try to remove suspicious plants without expert advice which can be found at


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