The NHS and RHS have helpful articles on poisonous plants – try a Google search for ‘hemlock poison’. The list of harmful plants on the RHS site (https://www.rhs.org.uk/advice/profile?pid=524) is daunting and it’s a wonder any of us gardeners are still alive.
I have no experience of hemlock itself and can’t comment on the time needed to kill it off completely. I can’t see that roots left in the soil would poison any other crops.