Wait a week or two after spraying before setting out your new transplants. The less soil you expose to daylight the better. The prairie wildflowers and companion grasses grow deep roots that eventually outcompete the weeds, especially the short-lived annuals. Have patience, it may take a year or two for the natives to knit together. Pay more attention to perennial, persistent weeds than annuals.
Early intervention pays off with weed control.
Plant the shorter bunchgrasses to best show off the flowers. Little bluestem, Prairie dropseed, and sideoats grama are native grasses that best show off the perennial flowering plants provide fall interest and help the wildflowers to squeeze out the weeds. You may be tempted to leave out the grasses in favor of the more showy wildflowers. I urge you not to do that for two reasons. One, the grasses have shorter, more compact root systems that combine with the deeper-rooted perennials to squeeze out the weeds, and two, the grasses are more persistent late in the season and provide nice fall colors.