What flowers grow best in clay-like soil,and in northeastern P.A.?

Question

Its still prety cold here is around 35 degrese

Answer 1

Yarrow (Achillea), Liriope.

Answer 2

Hemerocallis are great on clay. I remember seeing these wild all over the Philly area.
Doronicum Leopard's Bane spring bloomer, one of the earliest of yellow daisies.
Bergenia should be fine. This flowers in whites, blends or pink. In England it was easier to find various species but there are a few here. B. crassifolia 'Bressingham White'
Campanula the taller species do better than shorter, so do try C persificolia.
Asters will grow anywhere. Clay or concrete does not matter to this plant.
Burning bush Euonymus alatus,
Red osier dogwood Cornus sericea,
Witch hazel Hamamelis virginiana, very fragrant and blooms Feb
Winterberry Ilex verticillata,
Common ninebark Physocarpus opulifolius,
Rugosa roses, the one with lots of spines and textured leaves. Though roses do not like wet feet this is the most tolerant rose.

Answer 3

Check with your local greenhouse or home center garden area.