You are asking the wrong first question(s). The first is, "Are wildflowers going to work as a 15-acre planting?" The universal answer is, "No." -- unless you anticipate a 15-acre garden.

Wildflowers can't sustain themeselves in pure stands. They require supporting grasses. Otherwise, you will have a giant weed patch after a year or two. You'd best plant a native prairie, with both native grasses and forbs ("wildflowers").

The second first question is, "Is the planting site properly prepared?" If it's not, nothing will succeed. To plant either a wildflower garden (good for one or two years, then weeds forever), or a real native prairie meadow, the site has to first be cleared of all existing vegetation. This is best done by planting a year of Roundup Ready soybeans (and use the Roundup to kill the weeds).

After all of this, you can begin to contemplate planting 15 acres. But that's the next question. How are you going to get the seeds properly onto the site? Just scattering them won't work. You will have to engage someone with a prairie grass and forb seed drill.

Sorry to throw so much water on the idea. But to make it work, quite a number of things have to be perfectly aligned.

Sorry John, I should have been more detailed in my initial post. I think I have most of the things you mentioned covered.

The ground is currently planted in "Roundup Ready" soybeans. I will end my cash rent agreement after the crops are harvested in October. I will have the farmer disk the ground to prepare the bed for my grass/wildflower mix. I realize over 90% of the mix will be grass or other cover with the rest in wildflower seeds. I plan on evenly mixing the seeds with the grass/cover crop seeds and using a rotary spreader behind my tractor. I will follow with a roller to provide good soil/seed contact.

After sowing the grass/wildflower seeds, I will be planting 5000 very small tree seedlings spaced at 12' intervals. Therefore, the wildflowers will be in mostly sun the 1st 10-15 and then the trees will slowly create a shaded canopy from there on out.

I have a few follow up questions:
-When is the best time to plant?
-What's the best cheap low growing "cover crop" (tall fescue, alfalfa, clover, something else)?
-Any recommendations for a known mix that produces 12 inch or high perennials?