Lovage.

Lovage: A big plant with an even bigger flavor.
- You only need one leaf to flavor a dish!
The taste of the foliage is, admittedly, unsubtle -- much like celery's, but stronger.
Nonetheless, it has its place in the kitchen.
Early in the season, before bloom, it's a bit milder and you can toss a handful of the leaves into a green salad.
They are also good in soups, stews, sauces and other cooked dishes, used the way you would celery.
Even the hollow stems can be cooked, though I prefer the freshness of the leaves.