The herb spiral is one of those mainstays of permaculture, as iconic as comfrey or nonlinear beds. Like other permaculture techniques they can be misused, and I've only seen one out of the dozen or so herb spirals I have seen to perform well.
In theory, the spiral conserves space, while the mound mimics the heterogeneous forest floor with its pits and mounds, increasing the surface area of the land into the third dimension. the mound creates microclimates as the top and southwest growing surfaces are drier and hotter than the bottom and east south growing surfaces, accommodating different sorts of herbs with different niches.
Most of the time when I see herb spirals I see a mostly bare garden, with herbs doing poorly. The walls of the spiral are in decay, the bare soil is slipping and unhealthy, and the spiral is not incorporated into the rest of the garden design.
Part of the problem seems to be that the spiral is often in urban environments with small lots, and the mound is not large enough to generate true microclimates. I often see the space gained by verticality finally lost in the greater wall space taken up by the vertical element. That is, the walls holding up the spiral take up so much space that it might as well be flat. In permaculture we make thoughtful use of edge. In this case I see edge between the soil growing medium and the wall reducing production.
Pit and mound
Often the rest of the design doesn't incorporate pit-and-mound dimensionality elsewhere in the garden or in a continuous landscape as in the forest floor. In nature, pits and mounds are formed when life colonizes a fallen tree stump or boulder, or the berm of debris left by flooding rivers. In the garden, the spiral is alone.
Usually in western culture we are not taught how to manage planting slopes until very late in life. This contributes immensely to our risk of civilization crisis through soil loss. I have recently seen fields and pastures on as steep as 50 degree slopes. The gardener must shore up the slopes, further terracing the spiral in a perpendicular manner, and always keep the soil planted and covered with mulch.
I wonder how suitable this technology is to the temperate climate where so many of our herbs die down to the root much of the year (parsley) or are annual (cilantro). The tropical clamate is surely more suited to having perennial living solutions for holding soil like Lemongrass and veviter (Carbon Farming Handbook, p77). I can see a spiral working where the herbs are all perennial and consistently alive and growing.
How to do herb spirals well
First of all, decide if an herb spiral is suited to your unique growing conditions. What about an herb keyholl bed, an herb square, an herb terrace or an herb wall? Second, make the spiral large to produce true microclimates. Start at 4' wide and at least 2.5 feet tall. Finally make the spiral out of durable material like stone so that soil doesn't slip, and be sure to mulch well during the winter.
Should herb spirals be used in the temperate zone at all? Have you had or seen herb spirals that work well? If so, please tell me what you've noticed that make this function.