Q.How Can I Eliminate An Invasive Plant From A Steep Hillside? I Believe It’s Hyperium Calcynium.
It has tubers that form 1- 1 1/2’ deep in rock and clay. It’s almost impossible to find and dig them all out. It’s been in place for decades so the patch is large. A landscaper told me that it was popular in the 1970’s but that something killed off most of it. He doesn’t know what did it.

Certified GKH Gardening Expert
Since there may be quite a few restrictions on what methods you are permitted to use in your area, I would opt for a treatment that is without chemical means. Solarizing soil can be a very good way to do so.
https://www.gardeningknowhow.com/edible/herbs/st-johns-wort/controlling-st-johns-wort.htm