I am unsure of what you are saying but if I understand this right. I would dig the DG into the soil with some compost, plant some xeric stuff and then mulch with granite.

I'm no expert, but I would dig the holes where you want to plant the new xeri. plants. That will tell you how much of the old dead roots are there, and if they will need to be dug out. Then, I would plant.

You'll need to water to keep the new plants going this year. Some of the old stuff may revive. That's when I would deal with it. It sounds like a huge job to dig out a H. strip. Of course, if you want it to be relatively smooth or even you;ll need to scrape a little.

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Thanks!