However, a 2-inch layer is good, too. During their dormancy, you can safely cover herbaceous plants with topdressing. Plants aren't dormant anymore, though, so spread compost between the plants rather than over them now. Plopping compost over the tops of sprouted herbaceous perennials can cause two kinds of trouble. First, tender new shoots may be broken.
Exceptions are evergreen spreading ground covers like myrtle, pachysandra and ivy.
They are never truly dormant, and their stems root readily in contact with moist, loose soil so they can be topdressed any time. Often, ground covers are raked or blown clean for neatness' sake. One cubic yard of material will cover 300 square of bed when spread 1 inch deep. One cubic yard is 27 cubic feet. You live in a city that makes and sells compost at a very reasonable rate.