Vegetables that may harm dogs when ingested are onions, garlic and mushrooms. Avocados, which are a fruit often served as a vegetable, can also be problematic for dogs. Raw potatoes may cause severe gastrointestinal symptoms when the green plant portion is eaten.
While dogs can eat raw onions and garlic and not experience severe issues, negative health effects usually result from eating high amounts of these common vegetables. They contain toxins that damage healthy red blood cells in dogs. The pungent nature of raw onions and garlic prevent dogs from eating too much. However, when ingested in concentrated chopped or powdered form, dogs are more likely to experience toxic symptoms. For instance, a dog may lap up some onion soup mix and become lethargic and weak several days later.
Some mushrooms contain toxins harmful to dogs, which can cause a dog to suddenly go into shock. Alternatively, avocados usually don't pose much of a health risk to dogs, and are found in some dog food mixtures. However, certain types of fresh avocados may cause dogs to experience stomach distress, but only if a large portion is ingested. Dog owners with potato gardens must keep dogs away from raw potato plants because the green parts of the plant can cause central nervous system problems and other health issues.