Forgot to mention, my technique for planting this area is going to be:

1. take an established plant (I dont' do so well with seeds).
2. dig a hole in the ditch
3. fill hole with compost or other organic soil amendment
4. pop plant in hole, water, mulch (maybe)
5. pray
6. repeat in 3-4 other locations of the ditch.

If I get some good seed, I might throw it out there, but I'm mostly interested in putting plants in, since I feel like my chances are better of having SOME plants come up. They are going to have to fight with the existing wild flowers, weeds, grasses & some moss that's out there anyhow.

It would help if you said what state you live in and the type of soil in your ditch. Is it sandy or clay? Is it wet or mostly dry? Is it part shade or sun? Also, do you care if the plants are North American natives or not? Coloured yarrow, most large poppies, and so-called "wild" phlox (really hesperis matronalis) are not true US natives. Rudbeckia, echinacea, many types of native asters, goldenrod, vernonia, coreopsis, helianthus are only a few suggestions for showy, low maintenance native plants. Check on the net or in person for some nurseries in your state that feature native plants and that way you will know what plants grow best in your region.