Dahlias are indeed poisonous to dogs. All parts of the dahlia plant—the leaves, stems, flowers, and tubers—contain toxins that can cause adverse reactions in dogs, primarily gastrointestinal issues such as vomiting, diarrhea, and abdominal pain. Contact with dahlias can also cause skin irritation. The exact toxin is not fully identified but may include glycoside toxins and other compounds causing these effects. Severe ingestion could potentially lead to more serious symptoms, but usually, mild stomach upset is common. If a dog ingests dahlias, it is recommended to seek veterinary care promptly. Preventative measures include keeping dahlias out of a dog's reach and avoiding planting them in accessible areas for pets.