Answer: It is a good idea to "transition" soil from one type to another. Plant roots will cross over into another soil type better if the interface is not so sudden. I would place about 4" of new topsoil in the bed area, rototill or spade it into the native soil to mix them a bit, and then add another 4-6" and repeat the mixing of the surface 8" or so. Mix some well decomposed compost (or composted bark) into the new topsoil you are adding if it doesn't already have some included. Your beds can be about 10-12" high for most annuals and perennials. If the lower clay layers are truly impervious, I would suggest 12-16" for the shrubs.
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