Thanks for the questions. Yes, do remove the buckthorn and weeds. Of course, nature's way under evergreens is with a thick mulch layer of needles. If the lower limbs are healthy and green, I would only go that far. As so often is the case with the lower limbs looking rather sparse, I would prune those of and then do as you are suggesting, add some ferns or shade tolerant ground cover, such as periwinkle (vinca minor) or pachysandra. Ostrich ferns prefer slightly acidic soil, which is likely what you have under your evergreen tree. It's a tough site, shady and very dry, so grasses will not compete well. A visit to the MN Arb is a great plan to get ideas, and of course, it is always an enjoyable visit on a nice day!

Paul -Tree Care Advisor volunteer