Too many years ago to share, my best friend and I put up a prefab greenhouse in my backyard to grow African Violets. We were, and she still is, into growing the fancy ones for shows and flower arrangements. I share this not because of the greenhouse or the violets but because of a technique we used back then that I have applied to my herb garden. About four years ago I bought a galvanized horse trough, thinking it would make a good container for some of my plants. What I learned with African violets was the concept of wicking moisture from a reservoir under the flower pot. I don't know if they still make them but the Volkmann company made a special wicking pot just for violets. When I was building my herb garden I used this principle in the following way.
1. I drilled drainage holes in several places around the sides of the trough about three inches from the ground. This keeps rain from saturating the soil.
2. I put a layer of large rocks and bricks in the bottom - about 1/3 full.
3. I stood three lengths of pvc pipe up evenly spaced through the trough. The pipes go from the bottom of the trough to about six inches above the top of the trough.
4. I used nylon cordage for the wicks with the bottom of the cord in the bottom of the trough and frayed the top and spread it out through the soil as I placed a layer of good potting soil with fertilizer on the top of the rocks to fill up the trough to about two inches from the top.
5. I planted my seedlings, sprinkled water water on them and then filled the water reservoir through the pvc pipes.
Except for rosemary, which so far never even goes dormant in our area, and sage which comes back every year, I have to put in new seedlings each spring. Until this year I have not had to add new soil but this year I'm going to work the soil and add new to it before I put in the new plants. I'll put up pictures when I get it all done.