27th June: On our walk with Marilyn MacKellar and Glynn Richardson, Marilyn pointed out a good example of poison ivy (toxicodendron radicans):
The plant can be seen close to the main path at several places in the ravine. A Canadian government publication says that all parts of this glossy perennial, including the roots, contain the poisonous resin urushiol, which is so potent that a nanogram (billionth of a gram) is enough to cause a rash. Contact with any broken part may cause inflammation after 24-48 hours, followed by blisters and intense itchiness. The extent of reaction depends on a person’s sensitivity and the amount of sap in contact with the skin. You can contact the sap directly, or via another surface that has picked it up e.g. the fur of a dog. (The implication seems to be that you would have to trample the plant and break it, causing sap to emerge, before you would get a rash. This interpretation turns out to be incorrect: apparently, contact with the leaves alone is enough to give you a rash).
There used to be warning signs about poison ivy in the ravine, but unfortunately the vandals first defaced them and then tore down the signs themselves. Parks replaced the signs at least once, but eventually gave up.
The message is: learn to recognize poison ivy and stay away from it. Also, keep your dog away from it, as you can pick up a rash from the dog’s fur – another excellent reason for keeping your dog on-leash.