Answer: Most plants including trees suffer terribly when a change in grade of any kind is made, and perennials will literally smother if buried in soil so you should lift them and plant them in a holding bed while you do the work. Then allow the new soil to settle before finally replanting.
With regard to your project, in general it is better to use topsoil from your own property if possible so that it matches the existing soil characteristics. In addition, for perennials it is always a good idea to add ample quantities of organic matter to the soil. Other amendments might also be needed for optimum plant performance, depending on the results of soil tests. Your County Extension (454-0900) should be able to help you with the tests and interpreting the results.
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