I don't have any specific recommendations for
flowers, but here are some thoughts. First, I'd be sure to include
ONLY native species-- wildflowers that naturally occur in your area. If you introduce non-native species, you run a HUGE risk of loosing those
flowers that you already have because non-natives compete for resources and often over-take the natives. Think kudzu in the South! That said...
http://www.westernpawildflowers.com/ is a pretty indepth site with information about western PA wildflowers. You'd want to consider time of bloom, annual vs perennial (but annual wildflowers self-
seed), color, etc. Looking just briefly, this site doesn't give you exactly all the info you'd need, but it's a good start.
WVU has a similar site that you might be able to gather some of the missing info from: http://www.wvu.edu/~Agexten/hortcult/
flowers/wldflwrs.htm
Wildseed Farms is a
seed company that offers "regional mixes". There are six mixes recommended for your area (Northeastern, Butterfly, and so on). I don't know if all of the
seeds in a mix would be native species, but you could easily cross-reference with the sites I mentioned above. Their main website is:
http://www.wildseedfarms.com/ and the map with
seed mixes is:
http://www.wildseedfarms.com/mix_main_page.htm[I've been to Wildseed Farms in Fredericksburg, TX. I thought I'd died and gone to Heaven! Acres and acres of wildflowers. Go if you ever get the chance.]
One other comment-- after poking around a bit (googling "sow wildflowers") I see that most folks are telling you to clear the "bed" of
weeds first. By
weeds, they are probably referring to your native species, which of course you want to keep! Fall is the best time to sow the
seed, perhaps after the Fall hay cut?
Hope some of this helps! Have fun!!