Lemongrass (also spelled lemon grass) is an Asian grass used throughout Thailand and Southeast Asia. It is commercially grown in Thailand, Australia, Mexico, and the US mainly in California and Florida.
I had to research this since some viewers may not have fresh lemongrass available but can typically be found in the spice section of most grocery stores, and the Asian stores should stock this regularly.
To use fresh lemongrass it is just stated by number of stalks, the stalks are then pounded and tied in a knot and are removed before serving the dish.
I found several sites that have stated a ratio and all seem to agree.
The ratio is 1 teaspoon powdered = 1 stalk fresh.