Rhus glabra would probably win the war. It forms thickets and is very bright red in fall. Maximillian sunflower is another that will form a big tall clump, I see thick stands of it around here that are very beautiful in fall. Artemisia ludoviciana (prairie sage) is a good aggressive spreader for some silver and looks very pretty in a mass.
The local asters around here also form dense thickets, either the white or the purple, both are nice.

Thanks Texas! The sunflowers and asters sound like great ideas! There are asters here and since they are flowering that indicates to me that they are not at the top of the local herds (deer) menu so that might work! Don't know if sunflowers are tasty or not. I live in a cove forest so soil is always moist and artemesia turns into a monster....I just spent half a day removing a 7 ft specimen...not same species but I'm a little wary of that genus :) What about liatris? Would that seed and clump easily?

You can pull the asters right out of wet ground or dig them up if its dry and transplant them, they hardly seem to notice they've been relocated in my experience. I got several starts that way. I've been using Silver King artemisia like crazy here on a bank, its filling in nice, not too aggressive and it does well in partial sun. There is a mugwort that grows wild which is very aggressive, sounds like you might have that one, there is a silver version and a green one. I've heard it called the Mugwort from Hell.

I pulled some Maximillian sunflowers straight out of a ditch a couple years back, its like the whole thing is one big plant underground and the hefty roots just break free from the clump. I hauled the dripping mess to my trunk one Sunday in my best clothes. I put them in a bucket when I got home and they transplanted with hardly a whimper and they form a big clump that gets bigger in diameter every year with more stalks coming up. These were from a place where they are putting in a new road to some apartments on the edge of the city. (I don't need to be lectured by someone jumping in and saying its wrong to dig up plants from the wild)

Liatris don't really form clumps although I see lots of them growing together in spots sometimes very thickly. They have long taproots. Be sure to get a type good in wetter soil, otherwise they will snake along the ground and not do well.

What about Amorpha fruticosa (Indigo Bush)? It likes extra moisture. 4 x 8 ft shrub.

If it was me, I'd go for some varieties of Switchgrass. Its so pretty in the wind and looks good all season and in winter. It likes wet. Santa Rosa has their fall sale going on, I ordered three more myself, it looks gorgeous massed.

All good suggestions. On the sunflower tip, sawtooth sunflower-Helianthus grosseserratus- is another colonial species-fast spreader, pleasant to look upon, likes moisture. We have it all over our prairie plantings, and if you've got one, you've got a bunch.

+oM