I used to do this in regular gardening and thought it would to well in a media bed.
Use a bunch of green onions from the grocery store. Cut the green part away from the white, leaving about a 3 to 4 inch segment of the white part, including the roots. Stick the root end into the growbed, leaving about a 1/2 inch exposed on the top. You'll see very noticeable growth in a week. After a month or more, you'll have enough green tops to cut off and use again.