It's not too late. There are many to choose from. It would probably help us to know if there were certain colors you were hoping for. Personally I like hot colors. Lots of cosmos, nasturtium, zinnias, poppies and marigolds for me. Although, the poppies are pretty well spent by August here.
It might also help to know approximately where you are in zone 5b and what kind of growing conditions you are looking at (sun/ shade, clay soil?)

MARY

Thanks for the reply shinyalloy. We're in a microclimate close to Spolane Wa.Z5. I think I've gone a little overboard on buying seeds since I read about Ws.but here goes. Here's what I have-alyssum,african daisy,ambosia-for green,baby's breath,bells of ireland-again for green,marguerite carnations.1st yr. blooming delphinium,foxy foxglove,gaillardia,godetia,lobelia,morning glory,peony poppy.petunia wave,snapdragons,sweet peas,verbascum southern charm and zinnia. If you're wondering, I have a large area to cover in sun and part sun and some of this will be for large floral arrangement. And the most important- I'll have a lot of help planting.

Do you have a florist lined up "just in case?"

I find if I deadhead all summer, my annuals will continue to bloom right into September.

Pippi lol. that's a scary question but no I don't. I'm leaving that to the bride and groom(my grandson)parents. Do I hear a little apprehension in that question. If so, maybe I should sow some indoors?

Here are great annual bloomers in my garden in August that I frequently gather for bouquets:

Amaranths (many different kinds)
Dahlia (mid-August)
Tithonia (would be in fantastic in August)
Strawflowers (short for beds, tall for cutting)
Zinnias (I early spring sow some and do another round at the beginning of July to replace any that are petering out. This way I have Zinnias right into fall.
Cleosia (plume varieties) look great
Cutting Asters

What's the date in August? Early, mid, late?

Nancy

Nancy thanks for reply..I've got the dahlia, zinnias,and asters. I'll look for the amaranths, tithonia ,amd cleosia tomorrow. Any others that I don't have on my list above? The date is Aug.28th. Mary S

With wintersowing you will get awesome flowers. I think what pippi21 was saying is that you just never know what the summer will bring and that it would never hurt to have a "back up" plan. (Please correct me if I am wrong pippi21). Your list looks great to me. I wish you good luck in your sowing!