A herb spiral is a great idea to grow herbs on a mound of dirt. However, in hot and dry weather with extra radiant heat from stones even herbs struggle to survive. To keep them alive a regular watering and a fair a bit of water is required. For that reason a self-watering way sounds like the way to go. But how to make it work in a spiral with steeping soil for herbs to grow? Being an engineer I came up with a very clever but simple design idea which should work marvellously. So here is a description how we designed and built our herb spiral.
The spiral has three water reservoir levels as shown on the schematic. Each level comprises of the following layers (described from bottom to top).
- Softening layer (carpet) to smooth ground surface to protect black plastic
- Concrete underlay (black plastic 0.2mm thickness)
- Protective layer (carpet) to avoid punching holes in plastic
- Ag pipe across an entire reservoir and bended upwards to supply and spread water
- Water holding medium (stones)
- Soil around perimeter and in fill soil in the middle
- Stone to build outside wall
The pictures show step by step how we built each water level. The plastic underlay is as high as the stone water storage so the excess can overflow. From the top level it overflows into a middle level, from there to the bottom level which then overflows into the ground underneath. Each level watering pipe has got a float (a wooden skewer in a styrofoam rounded piece) to indicate the water level.
At the bottom the soil is about 10cm deep. When it curves around the spiral the soil depth increases to about 40cm. That’s where the next level starts with 10cm deep soil again. So in that way we achieved continuously raising soil without any interruption.
Our self-watering herb spiral is about 2.5 metres in diameter and 1 metre high. We planted over 20 different herbs around it according to their needs (e.g. mint in shallow soil thus more moist facing east; oregano in deep – less moist soil facing north).
We (two people) built the entire herb spiral project in just over one day with mostly freely available material on our property. It cost us a couple of dollars for the black plastic underlay. We located it in our newly built kitchen garden less than 10 metres from our kitchen. It’s already been well used for every day cooking.