Marigolds or Calendula are considered poisonous to dogs. Marigolds may cause diarrhea, vomiting, depression, and abdominal pain. The active ingredient in Marigolds is Calendula Officinalis Extract. This component is generally not well tolerated by dogs. In addition, it may cause the dog to become drowsy which may lead to difficulty in breathing. If your pet has ingested marigolds, call the veterinarian immediately.