Green and Mean!

Green and mean: House plants can be poisonous
from The Cat Bible

Cats love to dig in the earth in household potted plants - and much to your dismay you may find them using it as a litter box. But a more serious issue is that cats like to nibble on household plants, many of which are toxic to cats. The "Danger List" is so long (it's in "The Cat Bible," if you're curious) - and cats are naturally drawn to greenery - that the only way to be really safe is to make sure that you designate your potted plants as "off limits" to your cats.