Water Irises are going to be as aggressive as Horsetail Reed in mushy soil. Biodiversity and competition are healthy. If one of those species isn't suited for your conditions, the other hopefully will be. My bet is that they'll both work together. I had both of those in a pond garden and they worked together. Try it.

Always hard to say which would win. If there were a lot of Irises to start it might be hard on the Horsetail. Horsetail can spread but isn't always too good at pushing out other plants. Irises leaves are taller, block more light, have a very dense rhizome mat so I'd probably bet on the Irises doing fine.

Depends on how wet, kind of soil, etc... of course and if you plan on digging the entire bed every 2-3 years to divide it probably wouldn't matter.

It's great to hear native species like these being used! A lot of folks still seem to use aliens like Elephant Ears/Taro, Umbrella Grass, Bamboo, and Philodendron. And while those look kind of interesting in a tropical way, they do absolutely nothing for our Louisiana ecosystem. The insects won't even touch them. Even worse, they occasionally escape out of yards and invade riversides and lakesides.

If you plant the Irises and Horsetail in "clumps", they will definitely both survive.

Another great native bog plant is Pickerel Rush. In fact, I got rid of my Horsetail and Irises and I only have Pickerel Rush now. It looks like an ordinary green stalk and green leaves at first...but then it has these gorgeous purple flowers that appear and stay for the entire warm half of the year. They are a crazy magnet for bees, butterflies, dragonflies, and even hummingbirds. Some sort of colorful wild visitors will enjoy them almost every day.

One more great native bog plant is the Dwarf Palmetto. It looks super tropical. It's easy to find in commercial nurseries. It doesn't have the fast-spreading underground runners like these other plants, but it's just as beautiful in my opinion.

And I stumbled onto an amazing native ground cover for bog gardens. It's called the Pennywort or Dollarweed and you see it in ditches. I've never seen it sold anywhere. I've never heard of anyone trying to cultivate it intentionally. But it popped up in my bog garden, spread its tiny round leaves all over the ground underneath my trees and shrubs, and the result is a beautiful "carpet" of what looks like large clover.

Apologies for responding so late to these comments, which are most appreciated! Glad to hear it is worth a try, i think they'd look lovely together... and i had to look up Pickerel Rush, s8us89us- i didn't know it by name but i know it by sight! i love the leaf shape and how they grow!

As for pennyroyal...aquarium stores / suppliers tend to sell it! [though offering to "weed" someones garden for them in return for the ground cover sounds like a better plan.

I'll try planting them in clumps when the time comes, and see how it goes. i don't mind the irises taking over, i just thought it would be nice to have the contrast of the horsetail in the softer iris beds... and i also was not aware that horsetail was a native species- very nice to hear! i've been trying to be conscientious of that.

again- thank you both for your comments!