I would imagine that if your growing medium had some chunks of decomposed wood chips or bark in it, the nettles would thrive in root pouches
Your idea of adding wood chips seems good.
I also read that nettles like nitrogen, so I could add so called "horn meal", which is a natural nitrogen-rich fertilizer.
So basically what basic growing medium should I chose and what percentages of wood chips and horn meal should I add? What comes on the bottom, what at the top?
Thank you all again!