I'm in need of some wise gardening advice. I have a roughly 10'-30' wet area in my back yard that I want to turn into a water garden. I've already purchased Swamp Rose seeds and two mature Swamp Roses. In the middle I planted a 5' Bald Cypress in May of this year. I'm also looking to plant a couple of Black Lace Elderberries for diversity. However, my big challenge is the existence of cattails. I'm currently using a pickaxe to dig out the first 8-10 inches of ground in the hopes of eliminating the cattails. I also sprayed them with weed killer before I began my backbreaking work using the pickaxe. Once done, my plan is to cover the ground with lots of cardboard and backfill with 8-10 inches of top soil. But I'm not sure if this is sufficient to keep the cattails out next spring if they are not dead from the weed killer or tearing at the roots. I'm considering cardboard since I want something to block cattail growth initially, but then to degrade over time to allow what I plant to grow down into the soil. I'm hoping any cattails not dead will die the first summer if the stalks can't reach sunlight. So, here are some thoughts I have had:

(1) Should I wait to lay cardboard and backfill next spring to see if the cattails are indeed dead or do it right now?

(2) Should I use a natural weed killer like salt or vinegar now--or is this a waste of time and money?

I'd love to wrap-up this project this fall, but I do not want to see cattails growing through my fresh top soil and competing with my swamp roses.

Thanks for your help!