Here's the gopher plant I've got. It's not the same thing as castor bean, but it is still poisonous:
"All parts of the plant, including the seeds and roots are poisonous. Handling may cause skin irritation as the plant produces latex. While poisonous to humans and most livestock, goats sometimes...
Sorry - I think everyone has so many more problems with hawks, that your ground squirrel question sorta got hijacked.
I don't have any experience with ground squirrels, but we do have moles and gophers. My cats help a bit with the problem, but not much. They're lazy.
Gopher plant also helps...
So I have a hawk question that's been rattling around in my head for a while.
Do ravens typically keep hawks away?
Earlier this year, I had a couple of hawks hanging around. Then several ravens moved in to the area after discovering my compost pile. There were a few spectacular aerial battles...