I think that you are going to have a wildflower garden that includes some of the grass that was there before you planted. There is probably no way to get rid of it now, and even if you did, it might just move in later. you'll have to accept this, which means you don't have to worry about pulling up all the grass. I would instead try to make sure that really noxious weeds are kept out, and keep pulling larger weeds or trailing grasses which seem ot be crowding out wildflowers. Things like Canada Thistle, Spotted Knapweed, and similar invasive plants that could really take over should be killed. You can pull out the larger clumps of grass, but probably most of the wildflowers will be able to coexist with grass pretty well. I am assuming that the grass is one of the common exotic pasture or lawn grasses - orchard grass, timothy, etc. Eventually you should be able to have a mix of wildflowers, some grasses, and some weeds that still gives you the look and feel of a prairie despite the presence of some non-natives. Keep in mind that it would be almost impossible to have zero non-natives. You are instead trying for a balance that includes all the flowers you want and not too much stuff that you don't want.

The answer depends on many factors, and keep in mind I am not from your region. Pure wildflower stands do not exist in nature. In my region the tallgrass prairie is natural. I used a combo of short and tall grasses with my flowers. Using the more aggressive wildflowers helps a lot. I helped mine along as ladyslipper said. Pulling the more noxious weeds. My naturals were able to shade out most of the lawn type grasses. Originally I had about 5000 FT and it looked good for years. My fatal error was planting in a low area with fertile soil. Eventually we had a moist year, and portions of it were lost to 6 ft weeds. Not to mention it needed to be burnt off annually to keep it tidy. That was a bit unnerving with the tallgrasses.

I now have a smaller 1000 FT patch I planted this year. Small enough that I can help it survive if I use the right plants. In my case rudbeckia, various coneflowers, daisies, little bluestem grass.

Its commonly recommended to keep a new planting mowed fairly short, rather than weeding. The first couple years, the weeds are opportunistic, and mowing keeps the weed growth and reproduction in check, without hurting the native wildflowers.

As you have discovered, keeping up the weeds is time consuming in a very large planting. Which is why some people just keep it mowed, to keep anything from going to seed for a couple years.

Another thing that I wanted to add.... for anyone else reading or if you decide to expand your planting in future.

I do not recommend that the ground be tilled. I know that some of the nurseries that sell these seed mixes recommend tilling as part of the preparation for the planting area. I think that is a mistake. Its important to kill everything off before you seed. But tilling just exposes buried and dormant weed seeds. The less soil that is disturbed will result in less weeds.

Of course, depending upon the site, tilling might be necessary, but many times its not. If you are planting into an area that was once grass, I would mow it short and then kill the grass. The dead grass left in place acts as an excellent mulch. The largest draw back to leaving the dead turf in place, is possible reduction of soil/seed contact. However, if the seed is planted in the fall, rain, snow, freeze and thaw will usually work the seed down into the soil. And of course if you have access to slit seeder that's even better.

You all have given me great information! Thank you! I am going to continue trying to keep the invasive weed population down in that area. I do think I can reduce the Wildflower meadow a little and I'm going to do that now that I see how difficult it is to maintain. And, in the fall, I am going to mow it down to about 4" with a regular mower. Hopefully, next year the perinnials will grow large and not as many weeds will grow in and I will just continue trying to keep the more invasive weeds out! I don't mind the grasses in there, just not the large sprawling kind.

I'm planning to spend a few hours tomorrow morning(4th of July)going through it. I KNOW it will pay off. Next spring and summer it will be beautiful!

the easiest way i have found is with a weed whip. it is an L shaped sickle.it's light weight. swings like a golf club
it allows you to thin out the tall grass and maneauver around your flowers and gives them more room to compete.it's fast. i know it sounds like a lot of work but isn't. no stooping, doesn't weigh you down.the more you use it you'll find out how versatile they are.

actually some of the wildflowers depend on the tall grasses to hold them up, protect them from the wind and mulch