I've seen clumps of these locally that get around 6 feet tall. I need the true perennial type that'll survive a MO (zone 5) winter. What is the type I'd need?
I doubt you're seeing "Fountain Grass" (Pennisetum) growing to 6 feet tall in your zone. The tallest I know of is Pennisetum orientale 'Tall Tails' which can hit 5 feet in warmer climates, but is hardy to zone 5.
fountain grasses/cutting them back in March
"fountain grasses/cutting them back in March"
If that's a question, the answer is yes.