Answer: Typically, the area under pine trees is very difficult to plant because it is so dry due to the canopy and the tree roots, infertile due to the competing tree, and because the soil tends to be acid due to the steady dropping of pine needles.
Ornamental grasses as a rule prefer sun and at least average moisture levels to an evenly moist soil, so this would not be a good location for them. they are also generally quite tall.
Unfortunately, among the groundcovers, too, there are few that would tolerate the conditions you described. You might experiment with the lower growing creeping sedums to see if they can handle it or perhaps simply use a layer of mulch and/or gravel to make things look tidier. In nature's way, the ground would stay bare except for the pine needles and to some gardeners this is an acceptable look.
Another approach might be to place an attractive bench under the trees and use a large container of colorful annuals to dress up the area without having to try to plant it.
I'm sorry I can't be more helpful.
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