Strongly agree with Rita - you should pick up every bit of that poison that you can. Not only might it poison your plants (don't know) but the rodents that eat it then go off and die somewhere else, and could be consumed by predators (before they die) or other carrion eating animals or birds. So you can cause a widespread problem with a lot of species by using rodent poison irresponsibly.
My daughter's cat almost died last year because a neighbor put out rat poison in covered "stations" and she ate part of one of the poisoned rats.
Please DO get out there and remove the stuff as best you can, even scraping up and replacing the tainted soil if necessary! Using simple traps is the way to go, and that way you know what you're dealing with. Usually a large population of mice will attract predators like owls, hawks and snakes very quickly to take care of the problem for you. But if you poison them all . . the problem will persist and you'll forever be having to deal with the aftermath.