Earlier in the summer, these yellow-topped, celery-stalk-like plants can be seen colonizing many roadsides. Not only are they objectionable as invasives that drive out native flowers, they are truly poisonous in that their sap can cause a serious, scarring skin rash. Especially avoid any contact when the plants are wet, and if any sap does get on you, wash it off because it is exposed to direct sunlight, which will greatly worsen the effects.
Milkweed plants such as those arrayed behind the poison parsnips have flowers much utilized by monarch butterflies, which become extremely distasteful to birds afterward. Milkweed sap is not harmful, but tastes terrible--though putting flowers or pods through successive changes of boiling water can make milkweed edible (edible but not incredible).