Answer: Twelve inches of soil will be fine for your hostas. I'd amend the planting bed with some compost or other organic matter prior to planting the hostas. They like rich, moist soils so adding organic matter will help with both preferences. Hostas for the most part like shade so if the site gets some sunshine, be sure to choose sun-tolerant types. Or, interplant with perennials that will grow tall enough to provide some shade. Astilbes might be a good choice for the site.
Enjoy your garden!
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