There probably isn't one clear cut answer. A depth that would be barely adequate in one situation might be too deep in another, and the resources that you have to bear on the project constitute another factor.

I have found, in the heavy clay soil in my area, that even a 6 inch depth can mean the difference between overwinter survival for some plants. The beds I "constructed" were simply edged with 6 inch wide sections from a downed tree, essentially filled with partially decomposed leaves, with extra topsoil added around plants and put into place. That worked fine for tomato plants, not so well for herbs. (By the next Spring, all of that had turned into very nice topsoil).

No matter how deep your bed is, eventually it will merge with the soil below. The worms and other garden soil occupants see to that. However, if the bed is too tall, your plants may be more vulnerable to summer dry spells, and require more watering than lower beds would.

If you were growing root crops, such as carrots, which need a certain depth to grow properly, I would say make the extra expenditure for deeper beds. With herbs, and their varying needs, it's a bit more open to your own preferences and resources, I would think.

If you have a budget in mind for your project, you might calculate the amount that the total square footage of soil required will add up to at various depths, and go with whatever you feel is a reasonable expenditure.

Even really packed-down hardpan will eventually soften up a bit under a raised bed, allowing additional room for root growth. And, as already suggested, if you have to be away for a few weeks during the summer, your plants might be less susceptible to drought conditions if they are rooted partially in the soil under the bed. Still, being in a somewhat raised condition will make them less susceptible to drowning as well.

The best answer to your question is probably to be had from you. What are the reasons you are choosing to have raised beds in the first place? That might give you a better answer as to what depth would be optimal in your conditions.

I suspect the soil composition, amount of water, and drainage will be more important to how well the herbs grow than the exact depth of the bed. Eibren is right, the raised bed and the soil underneath are merged together so the plants can grow as deep as they need to.

FataMorgana

Thanks so much for your quick and very thoughtful replies. It's an experiment, so we'll start with the 12 inches and go from there. I'll keep an eye out here at the Herbs Forum, though, to see how the REAL herb gardeners are doing things:) Thanks again. Happy spring!