My experience is that they just like to play with them, like any other grass-like plant. Given the choice between crab grass and daylily to eat, they'll choose crab grass. Some might regard this as justification for not weeding.
My little cat Minnie has eaten tomato seedlings, which are also considered poisonous. But usually she just plays with them. I'm also growing datura this year--they have shown absolutely no interest in eating the leaves.
A short movie of my cat Minnie mangling my irises.
http://www.youtube.com/watch?v...
Dora
