The Cowbell spike is 5.5 inches long, so we recommend at least 5 inches of soil depth so your spike does not wobble around and your system can stay stable and anchored.
We recommend burying the spike so you can no longer see the white cone, but it will work even if part of the cone is exposed to air.
You'll want to ideally bury the spike as close to the plant trunk or main root systems as possible. If your plants have established root systems or are somewhat root bound, we recommend burying your spike all the way into the container until you can no longer see the top of the spike at all.
In fact, you can bury half of your reservoir deep into your planter if you want to see less of your Cowbell, as long as the lid stays above ground to allow for easy refills.