Mine are a very dark, so I guess it's normal. Same for unknown footed fern. I don't know if most fern roots are similar to this.

I have a fern which I feel must be the kind you inquire about. My friend, who gave it to me several years ago when it was about to die, called it an alligator birdnest fern. Today I was checking to try to identify it more exactly, and your post came up. Is this what you refer to? and it seems to me that it does multiply by clumping, because I am about to have a local nurseryman separate and repot it for me. I do not want to chance losing it now. Actually, the long fronds are greater than 2 feet long. And while we are on the subject, I found a different style birdnest fern at the nursery I spoke of, sort of a clubfoot version, different from my other wavy edged and straight edged bird nest ferns. I am quite fond of these plants, and give them a place of respect in the limited shade of my garden. Hope this helps.

Al, I planted my fern as a house plant, and I did not know what to do. I just used what I had around, so I used coir instead of peat moss then I added some perlite, some bark 1/4 that I had around, and some truface 1/4 and some bonsai gravel 1/4 that I had around. I don't know if I did right or not, but it was supposed to like wet soil and also for the OP, note you can treat a fern with a fungicide if the ferns is not too far gone already. I have seen some ferns growing around my building that have black smut, it's like a black mold and I told the owners, but I doubt they will bother to treat those ferns. Ferns do get fungus and aphids also. They had these tiny white spots, people think ferns are care free, but they are much harder then they look.

I repotted it -- in the nick of time! The root system was vainly trying to penetrate about 2" of densely compacted clay-ey mud, the bottom sides of the pot had the beginnings of fungus, and I found a freaking EARTHWORM in that mess. (I let him go into the backyard, so he can aerate the soil there -- maybe he saved the sans!) These plants must have the most cast-iron constitution ever, to be able to propagate in that. I gave the whole plant a really careful rinse and once-over, and miraculously there was only one tiny piece of root that needed to be trimmed off (looked like it had died from sitting in perched water). No signs of mold or creepy-crawlies. Here's a pic of the sans in her new lodgings -- the blue glass things are just there to keep the perlite from blowing out of the pot if I turn the fan on :)