As with any perennial, proper site selection is critical for success with ornamental grasses. Soil preparation in a perennial bed is completed only one time, just prior to planting and must be done properly. First, kill and remove any persistent weeds, especially grasses. Such weeds are difficult to manage once the beds are planted. This may take several months and multiple applications of a systemic herbicide. Except in native naturalized plantings, amend the soil by adding 2-3 inches of well-aged compost or manure. This is especially important in the arid, calcareous soils of southern Idaho. Add the equivalent of 3 lb/1000 sq. ft. of nitrogen in the form of a complete fertilizer, such as 5-10-5.
After amendments are added, the soil should be tilled to a depth of at least 8 inches, leveled, and smoothed (but not packed). Just prior too or after planting, it is a good idea to add two or three inches of mulch (wood chips, bark, etc.) to the soil surface. It may also pay dividends to place some type of edging or border around the bed to slow encroachment of grass or other weeds.