RE: Sunflowers - for fun.
yeoman,
i used treflan and prowl. i don't remember the rates off the top of my head but i think it was 1.75 quarts/acre.i disk it a bit so that there are alot of clods. spray it, disk it twice in different directions, then plant. there is established johnsongrass in my field too. i used a herbicide called Select to knock it back. It's $205/gallon, but I found someone from the grain elevator that would sell me a quart privately. Select did a very good job of suppressing the grasses, but it did not kill them.