I've got some large areas of mixed weed, fern and grass that I wish to clear and later seed with various ground covers like phlox. Pulling it all is impossible and removes too much good soil. I've seen black plastic recommended to just starve the weeds and preserve the organic material for later planting. I'd be worried using this method that I'd be leaving live weed seeds and spores behind.

I've myself had good results with regular salt which does clear annoying ferns in particular. Contrary to popular belief salt doesn't sterilize the soil it just makes it more alkaline thus unfriendly to acidic plants like most ferns.

Anyone got ideas?

How large an area and as you have ferns just what is the local? (THey need water, lots of it.)
I have ferns wanted , but they are also growing in the yard unwanted.
An area where they are supposed to be/were that area is too close to bare.
So I am curious why you have fern problem as the ones in the yard I just mow over but then that area of the yard now has 75 percent more shade than it did even just 20 years ago.