To mark my targets, I jammed golf tees into the ground right above each one (completely into the dirt). I mixed a few colored ones in there so I knew when a new group of 5 started. The Lawn Mower has never pulled any of the wooden tees out, but some work their way out occasionally. Mine are all 18 inches apart. Mine is a large garden and I use it to refresh my skills when working with a detector I don't use that often, and to teach people detecting (as well as test their detectors after they tell me that the detector doesn't work, most often to find that they really had no idea what the detector was telling them or how to use it).
I also have targets that have trash next to them (pull tabs, nails, etc), because these types of targets tend to get written off as trash by people with VDIs when encountered in the field - they are not always bad targets.