Answer: Echinacea, or coneflower, are rugged plants that grow best in full sunshine and average to poor soils that drain quickly. They need regular watering during the hottest part of the summer but tolerate soils on the dry side once they've become established. I wonder if the soil amendments are holding too much water close to the roots? The planting hole filled with organic amendments could well be holding water like a well in your clay soil. Dig around in the planting hole and see if this is true. Rather than watering on a schedule, base the plants' need for moisture on this "feel" test.
Though generally echinacea prefer not to be disturbed once they're established, it may be better for them if you replant them in a raised bed or terrace on the slope to aid in soil drainage. Cut back the spent flowering stems and remove the obviously dead leaves, but wait until frost to remove the rest of the foliage. Echinacea are tough plants and yours will probably come back next spring, producing stout stems and leaves and lovely flowers all summer long.
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