I'd suggest it depends on the nature of your 'native' soil.

Miscanthus likes fast draining lean soil. If the soil is too fertile, and/or too moist, the plant will flop.

They don't need fertilizer, and once established, are fairly drought resistant.

I too will be planting Japanese Silver grass. My yard is very fertile. It used to be an old pig farm way,way back in the day. I was planning on adding Miarcle Grow Tree and Shrub dirt to my plantings to add more dirt to the ground. So will this make my grass flop? What do I need to do?

Kellygelly,
I think that you will probably be all right. Your new Miscanthus won't be getting all that huge in the first year or two so flopping shouldn't be an issue. By the third year when you have a giant grass (since it is getting such good dirt), it will have used up some of the nutrients in the soil. Don't add any more!

Seth, I find my grasses do fine if planted in 'native soil' as long as it isn't constantly wet or in my case pure sand. The pure sand lacks most nutrients & drys out too fast in the summer. That said, I do have two or three Miscanthus seedlings growing on the side of our gravel road! My theory, if they cost nothing & start small they will grow anywhere!
A.