The chances that you have a plant growing in your yard that is at least mildly toxic to dogs (as in causing them to vomit, for example) are very good. And some plants poisonous to dogs have much more serious ramifications for your pets' health, should they ingest one.
Still think that the odds of your dog eating a toxic plant while outdoors are slim? Check out this list, which links to resources telling you more about the plants in question. While it's hardly exhaustive, I think the list will give you an idea of just how many different types of plants are poisonous to dogs. Note that these are common, not obscure plants!
When I say that "many different types of plants are poisonous to dogs," I mean just about every common plant classification you can think of, from trees to shrubs, from bulbs to viny plants often grown in hanging baskets. Even some weeds are toxic to dogs -- and you know how hard it is to keep track of all the weeds in your yard, let alone eliminate them!
Read article: Plants Poisonous to Dogs