Mystery Plant #2

I'm thinking of starting a new series "The Mystery Plants of Spring". Here's another little mystery I uncovered today:

It's in my oldest border. I don't recall planting anything new there last year, but this is obviously a perennial because I cut back some old stalks.

Update: I started this post earlier today. Since then, I remembered that I transplanted a few scraggly catnip plants last fall from the pot where I had (tried) to grow them to this border. They were in such bad shape that I didn't expect them to survive the winter. I'll keep an eye on these and if the stray cat that I am feeding in my backyard takes a sudden interest in them, I'll know for sure!