The rash that Stevie and David get is said to be from the poison oak that Johnny picked behind the motel and put into the flower arrangements. Since the town location is understood to be in southern Ontario, Canada, or maybe upstate New York, the plant wouldn't be poison oak.
The range of Atlantic poison oak doesn't extend northward into New York. The range of Pacific poison oak extends eastward from the Pacific coast to about the crest of the Rocky Mountains. The actual plant might be one of the related species, poison sumac or poison ivy.