A spiral herb garden will give the urban gardening a huge bonus in the ability to grow many different kinds of herbs in a small area, generally five to six feet in diameter, rising to three to four feet in height, but can vary with your building technique. The spiral is a ramp that rises with the center of the spiral at the tallest point. It is built with a spiral of rocks holding the dirt, and due to the variations as it rises, you will have variations in conditions, which allows different herbs to thrive according to their needs.
It is one of the vertical gardens that small space gardeners can enjoy. Your plants will have vertical and horizontal space to grow. A spiral of rocks is the support, and the rocks themselves help to retain warmth in the soil, with the height making it simple to care for. The common goal is to be able to grow as many herbs as possible for your kitchen garden in a convenient and efficient way.
Permaculture growth uses natural processes to assist with the growth of the garden. With the vertical aspect of the spiral, watering is done from the top and drains down through the spiral, allowing for hardier herbs in the upper environment and herbs that desire more moisture in the lower levels. With the nature of the spiral you will also be able to place some herbs in different areas according to their sunlight preferences. Think of it as micro climates for a huge variety!
The rocks (or bricks or blocks) will absorb heat through the day and keep the soil warmer throughout the night, essential in increasing your growing season in more northern areas where soil temperature can be a challenge.
Here is a video that will help you see this idea and let you see how you can build it also for vegetables in a larger oval (or just add herb growing space)!