zinnias, marigolds, your vigorous flowers will in a two
inch mulch, no thicker, you will have to pull up any
weeds comming through, good luck

Direct seeding has its pros and cons. Handweeding is your best bet until the seedlings are large enough to mulch, and yes, pinestraw is a wonderful mulch. After trying just about all types, I've gone back to pinestraw as the "most garden friendly" mulch around. Free, looks nice, water and air permeable, and organic, it's really the best choice for beds.
I used pine straw last fall while my beds rested, and now the areas that didn't have quite as much coverage (1 inch or so) have millions of noxious little weeds sprouting underneath. Next time must remember to give 3" coverage just to be on safe side.
This early hot weather is making things JUMP!
Good luck!