Are you planning to build new raised beds for them or clean out and use the old beds? If you plan to reuse it may be a real battle to get the remnants of the old crowns dug out.

I've never planted my crowns 18" deep. I planted my last two 10" deep raised beds by double digging the existing native soil down about 12" and amending the soil well with compost and aged manure and then placing the crowns on top and adding an initial 3 inches of good top soil mixed with compost. As the spears came up I gradually added more soil until the bed was full to the top of the frame. Now, 5 years later I add 1-2" of compost to the beds each fall and then usually a couple inches of shredded leaf mulch.