I have read numerous threads about how to calculate the amount of mulch needed for a given size. What I have not seen is how to deduct the amount of square footage occupied by the plants within the bed. I recently was asked to give an estimate for edging and mulching many existing beds. The overall square footage would suggest I need 36 yards. However, when I spoke with the homeowner they indicated that prior contractors usually use 18 yds of mulch. Many of the beds are filled with plants that take up most of the bed. How do I compensate for all the plantings? Any help would be greatly appreciated.