Both variants of hemlock are excellent for the purpose of poisoning bait, with water hemlock being the herb of choice when the predator is large and there is no room for error.
The white flowers sprouting from purple branches are difficult to mistake for anything else, too. The entire plant can be used – leaves, roots, seeds, anything will work.
When choosing the dose, err on the side of caution. Some creatures, even when weakened, can, and will, tear a person to shreds.