Answer: Planting wildflowers is harder than one might expect. You will definitely need to remove the grass first, the seedling flowers will not be able to compete with the grass.
Usually it is more successful to treat the area like a flower bed at first, planting small started perennials and annual transplants that you know are well suited to the growing location. Over time they would hopefully self seed and replenish the planting. You might also need to supplement that with additional seeding and/or transplants each year.
In the meantime you would need to weed out undesirables that grow up from the soil seed bank or blow in on the wind. Eventually you might have a somewhat self sustaining planting.
Prior to planting the above you would need to remove the existing grass and prepare the soil just as you would for any flower planting. This seems like a lot of work, I know, but if you don't do it there will most certainly be more weeds than flowers.
You might also want to consult with a local naturalist and/or your county extension for suggestions as to the best wildflowers to plant. In an all purpose mix you will likely find just a few do well. Good luck with your project!
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