"There's nothing here to even take except for books, anyway. And plants can't read."

So why would they want those? Still, Susan is happy to be ready for a fight, if necessary, although unlike some of the others he doesn't have to do anything to prepare. It's easy when all you have to do is blast a fire spell at them, and plants are easily flammable. This shouldn't be much of a problem if they aren't able to convince them away or go around.