I could have my botany all wrong, but...

I think catnip is a dicot, so the veins in the leaves should be all branching, like you'd find on a tree leaf or other normal leaf.

Grass and grains are monocots, so their veins will all run roughly parallel with each other along the leaf, and not branch from a central spine.

But like I said, my botany could be bogus.

It's definitely not catnip. Catnip looks like http://tinyurl.com/4yetg8

Does your planter have drain holes on the bottom? Wouldn't hurt to drill a few if it doesn't. Then you don't have to worry about the rain flooding it; any excess will just flow out the bottom.