As everyone else here, I carry multiple ways to start fire. One of them is carrying char cloth BECAUSE I carry a high carbon steel knife and a sharp rock is easy to find.
Why go looking for a rock?
If you're going to the trouble to carry charcloth then why not also carry a decent flint as well? (I recommend a musket flint from Dixie Gun Works, it's about the size of a quarter)
We have natural flint some places here in the northeast, but considering the depth of dirt and duff and leaves, it's not easy to find, and trying to find a decent rock under the snowpack will be impossible.
Bring the rock with you, it really doesn't weigh much.
If you're going to work with charcloth, then bring the whole kit...flint, carbon steel, and tow or jute or whatever birdsnest material you want to work with.
Big +1 on milkweed...never heard it called "placenta" or "ovum"

I just call it fluff, the stuff that carries the seed airborne. It easily catches a spark and makes a great flame. The only problem is you can't collect it until late summer or early autumn when the pod splits open (I may be wrong on that, maybe you can split the pod open earlier and use the green "ovum") Anyway, I gather handfuls of it whenever I'm out and stuff it in my pockets. No special container is required.
Edit: Anybody ever tried dandelion fluff for tinder? I will make a point of trying that this spring.