I have a narrow garden bed where the hostas have never grown well. I believe the reason is (1) tree roots (2) compacted soil (3) poor drainage. Undulata albomarginata grows pretty well there, but not other hosta. I have had the other hosta in spin out bags. That didn't help much. So I think drainage is the problem.
The area to be raised is approximately 10 feet x 2 feet, and it will be about 10 to 12 inches deep. Some gravel will be added to the bottom of this area to aid in drainage. Half the soil removed will be put back. Any bad clay soil will be removed. What should I add to the old soil to improve it? We don`t have soil conditioner here or small pine bark. Common soil products would be black soil, composted manure, triple mix, promix BX etc. I would like to improve the soil so I can get on with growing some decent hosta and perennial.