You should first get a good field guide and identify the "weeds" in the ditch. Some may be native plants and it would be a shame to kill them.

Most natives do best in poor soil, so do not enrich. Enriching the soil will likely encourage the weeds to get very big. It is very difficult to change the pH of soil, adding lime once will not suffice. Plant things that grow well in wet, acid soil and forget the lime.

Anemone canadensis is very invasive, if you don't want wildflowers spreading to your lawn, do not plant it. Actually, if you do plant it, don't bother planting the other ones as it will choke them out.

How much sun does your ditch get? The Iris and the cardinal flower like more sun than does the snake root and the blue lobelia. The snakeroot is not well suited to constant wet feet whereas the iris and cardinal flower do like wet feet. They also like acid conditions more than blue lobelia does.

Planting seed without mulching will lead to the same sedges and "weeds" next year. The seedlings you plant will be relatively small next spring and may not show up until the spring after that.

Two books I would suggest from your local library
Newcombs Wildflower Guide
William Cullina's Guide to Growing and Propagating Wildflowers.

The first will help identify what you already have, the second provides great information about the growing requirements of your plants, how to grow from seed and provides list of recommended plants for various sun/ moisture/ soil conditions.

I'm already able to identify what's in my ditch. Yellow nut sedge and quack-grass.

It's full sun.

How do I go about planting? Should I plant this fall? How deep?

Quack grass is difficult to eradicate, you may have to use Round up three or more times, spraying anything that remains green every two weeks until nothing remains. start now to make sure all roots are dead by the fall when you hope to plant.

Cardinal flower, snake root and blue lobelia do not like full sun. Cardinal flower, lobelia and iris should not be allowed to dry out ever; especially in full sun their soil must be always moist or wet. Blue flag is suited to full sun. Canada anemone grows in partial to full shade, growing in sun will slow it down but I still would not recommend planting it at all unless you plant it as the only species.

Plants suited to moist, full sun conditions include Joe Pye Weed, Bidens (beggar ticks), Veronia (Ironweed), Monarda didyma, Turtlehead, bog goldenrod, rushes, cattails and sedges.

The Iris seeds may not be viable if they have been allowed to dry out since collection, they are best sown immediately after collection. Sowing after drying out means that germination will be slower, if it happens at all, likely two years.

All your seeds need to be cold treated before they will germinate, that is they will need at least two to three months of cold, moist conditions before returning to warmth. You can achieve that by storing them in the fridge in moist vermiculite until the spring and sowing then or by sowing in the ground now and letting Mother Nature do the rest. Scattering the seed on the prepared surface (kill the grass/sedges, don't fertilize or add soil) then covering with a light dusting of sand or fine soil will work. All your plants will produce small seedlings and grow slowly. You will need to mulch them and remove competing weeds for the first year and likely the second year as well.

An alternative to sowing in place is to sow into seedling trays around March next year, after the seeds have spent August to March in moist vermiculite in a baggie in the fridge. Place the trays in a sunny window. When the seedlings have true leaves, pot them on into 2" pots filled with good potting soil and keep in bright light for them to grow on.

Round Up the ditch again in April and May if anything green appears, then cover it with 4" of mulch or straw (do not use hay, use straw -- hay has seeds). When danger of frost is past in late May or June, plant out the plants. With this method, the plants get a small head start, you know where the plants are so you can pull the weeds and you have mulch in place immediately after using Round Up.

Wow. Great advice. Thank you.

Now... to decide what to do...

;)

If you eliminate all the sedges and don't replant sedges, grasses, and rushes, as suggested, you will be leaving a great big niche for weeds to take over. In nature, forbs (flowering plants) never exist without grasses. It's as if you were to try to create a forest without the trees. I suggest that you take a look, as a first step, at Prairie Nursery's catalog. They have seed mixes for every possible site, including the wet site you're describing. They also describe how to prepare ground for a prairie plantings.

Here is a link that might be useful: Prairie Nursery

Your ditch sounds like a good site for swamp milkweed, Asclepias incarnata. I can supply seed this fall, if needed.

Hi

My ditch is also too wet to cut on a regular basis and the peonies that were there have pretty well died (since they changed the grading of the road)

I'd suggest you check out the winter sowing forum - that would be perfect.
You can get the bed ready - spray again with round-up in the spring and have your plants ready in early spring.

I'm going to try a few of the plants suggested in place of the peonies.

Acorus is great - if you want a spreader. I keep mine in check with a lawnmore and looooooove the scent of the bruised/cut roots. Takes full sun and varying wetness.