Ask Mr. Smarty Plants

It is discouraging to hear that your serviceberries and red maple have withered away under your black walnut tree, as they are two plants that are on the list of juglone tolerant plants in this very helpful article from the Virginia Cooperative Extension Service. 

Funny how deer don't read the lists of resistant plants and eat them anyhow and it seems your walnut tree hasn't read the list of juglone tolerant plants either.  Most of the time a gardener only succeeds by trial and error, which can be time consuming and expensive.

Something in the article caught my eye: "Soil under black walnuts tends to be alkaline, with the pH often 0.7 points higher than beyond the roots, thus influencing the growth of many different plants."  You commented that you are also close to a limekiln, which may be increasing the alkalinity of your soil even more.  This may exacerbate the effects of the juglone.  You should have your soil pH tested by your local extension service.  They will make recommendations for acidifying the soil and probably have suggestions/lists of juglone tolerant plants for your area.

Ceanothus americanus (New jersey tea) is a plant that is tolerant of alkaline soil so it is worth giving it a try.

Although potentilla Dasiphora fruticosa ssp. floribunda (Golden-hardhack) is a very tough plant and tolerant of may soil conditions, it requires full sun.  So it would have to be planted well away from the shade of the tree.

Paxistima canbyi (Canby's mountain-lover) requires acid soil, so it will probably not thrive unless you make an adjustment to your soil pH.

The best advice we can give you is to look around your neighborhood, see what is gorwing under walnut trees and talk to other gardeners who have black walnut trees in their yards.  We do know that they grow in abundance in Pennsylvania. You should also take your cues about grasses and wildflowers from the meadow that already exists. Visit our Native Plant Database and do a Combination search for Pennsylvania to see what plants are native to your area.  You can sort according to plant type and light and soil moisture conditions.  You cannot sort according to soil pH but each plant listed is linked to a detailed page where you will find that information.  You can look at lists of shrubs, herbaceous plants and grasses.

You should also take the opportunity to attend the Native Plants in the Landscape Conference (and plant and book sale) that takes place in your town every June.  It is a great opportunity to meet other gardeners interested in native plants, particpate in workshops and tours and hear world renowned speakers lecture on many native plant related topics.