If I were going to do this project for myself I would start by incorporating at least 3 inches of bark dust or compost into the soil with a large rototiller. Then I would plant clusters of three to seven 4-inch to gallon-size plants selected from the following improved selections of native wildflowers.
For early spring bloom I would plant three low-growing ground covers, Alyssum Basket of Gold, Aubrieta Purple Rock Cress and Lamium Pink Pewter. Lamium continues to bloom through the entire summer.
For early summer to mid-summer bloom I would plant varieties that grow 12 to 30 inches tall: Geranium Rozanne, Coreopsis Moonbeam, Black Eyed Susan (Rudbeckia) Goldsturm, Lavender Hidcote, Purple Coneflower (Echinacea) and Peruvian Lily (Alstroemeria). Rozanne has sky blue flowers, Moonbeam has lemon yellow flowers, Goldsturm has deep yellow flowers and Hidcote has deep lavender blue flowers, Peuvian Lily comes in a wide range of colors. Rozanne, Moonbeam and Peruvian Lily will continue blooming into late summer to early fall.
Then I would plant China Aster for fall bloom. I would place plants a little less than their full diameter apart so they will grow together leaving no space for weeds.
Another approach would be to plant a seed mixture of grass and wildflowers. Pro Time Lawn Seed Company in Portland (ptlawnseed.com) has an excellent mixture which they call Fleur de Lawn. This mix contains the proper selection of grass seed varieties and low-growing flowers to achieve good results with low maintenance. It even contains a dwarf clover so no fertilizer is required. An occasional, well-timed mowing at 3-inch height is all the maintenance needed.