Are zinnia leaves poisonous to dogs?

My dog has been eating zinnia leaves for years. She is now 14 years old and is still taking a nibble when the feeling moves her. She is a herbalist in dog's clothing. She eats Echinacia and Comfrey leaves. This she does at certain times of the year which she knows better than I do. At other times of the year she will not touch them. She is part border collie and I think she has the instinct of a breed that lived on the land with the sheep.