I also wanted to add, that any tie I start a new garden the first growing season after the area is prepared is always the worst for weeds germinating. This is because you've recently disturbed the soil and made a perfect bed for them to germinate. You've also brought seeds that may have been buried to deep in the soil to germinate up to the perfect depth for germination. Every information I've found about starting a new garden strongly recommends preparing the bed ahead of time. Now I never follow this practice personally, but I am also prepared to hand weed the new bed because I know they will appear because I short cutted the preparation. I had found an interesting article on starting a wildflower meadow which I feel applies to what you are trying to do. So I thought I'd paste some of the info about preparation here. I know you may not want it to be so involved but at least you'll understand why you are having so many weeds before you make your final decisions..
Here are some additional tips about soil preparation, planting and care to help ensure your success:
When to sow. Wildflower seed can be planted in either spring or fall. In colder climates, fall-seeded wildflowers will usually remain dormant until the following spring; in warmer areas, plants will germinate and start getting established before winter.
Selecting the planting site. When thinking about where to locate your wildflower meadow, consider starting with a section of your lawn that has been difficult to mow or that seems poorly suited to grass. Another option is to convert an overgrown flower bed. A sunny site is usually best. Most wildflower meadow seed mixes contain sun-loving varieties that require five to eight hours of direct light each day. Well drained soil is preferred, but if you have a problem site with overly wet or extremely dry soil, you just need to select seed varieties that are particularly suited to those conditions.
Preparing the ground. This is probably the most important step in the process. Wildflower seeds need to be sown on well-prepared soil that—as much as possible—is free of weeds. Mow existing vegetation as close to the ground as possible. Remove any woody plants. Strip off any sod, or rototill the area and then rake out the clods of grass. You can also consider applying an herbicide, such as Roundup. Once the grass and weeds are gone, till the area to a shallow depth of 1-2 inches.
Eliminating annual weeds. Do not plant wildflower seeds immediately after preparing the soil. Tilling the soil usually brings weed seeds to the surface, where they will germinate. Wait two weeks or so after tilling, then go over the ground again with the tiller or a hoe, to expose the roots of newly sprouted weeds. A second application of Roundup or a similar herbicide is also an option at this point.
Sowing the seed. Before seeding, mix the seed in a pail with fine-grade builder's sand, which will help ensure even coverage. Use four parts sand to one part seed. Broadcast one-half of the seed/sand mix using a handheld broadcast seeder, or by hand, using an even, sweeping motion. Then walk across the seedbed at right angles to the path you took before, sowing the other half of the seed. Sow seed rather thickly; this will help the wildflowers out-compete weeds and grasses.
Firm in the seed. Seed that is buried too deeply will not germinate. Instead, simply walk across the seedbed, tamping it down with your foot. You can also use the head of a garden rake to ensure good contact between the seeds and the ground. Another option is to rent a lawn roller, a large cylinder that you fill with water and then push over the seedbed to firm it down.
Water and, if needed, fertilize. Seeds require moisture to germinate. You should water for an hour or so every few days until the plants are about 1 to 2 inches tall. After that point, water only if plants look wilted or stressed. Wildflowers normally grow well in average soil and don't require fertilizer. If your soil is nutrient-poor, however, spread an all-purpose, organic fertilizer at the same time you plant seeds.
Control weeds. If your wildflowers germinate well and grow thickly, they should choke out most weeds. When weeds do spring up in the midst of the wildflowers, pull them by hand before they have a chance to flower and disperse their seeds.
Mow annually. A few weeks after the flowers have faded in your meadow planting, you can do a high mowing of the field (set the mower to 4 to 6 inches above the ground). This helps control unwanted weeds and grasses, and also helps disperse the mature seeds of your wildflowers.
