I think it's important to be able to make cordage in whatever environment you're in and cordage is such an important facet of the bushcrafters armoury that it should be given a higher place than it is, after all it's one of Dave Canterbury's 5 C's..
Some roots freshly extracted
If the roots are too thick they will need to be split, this one is about 1/2 inch thick so we decide to quarter it. First cut a split in one end with your knife then gently start to work the split down the length of the root, if it starts to run off centre then bend the thicker bit slightly more and the spli will run back to the centre
Lots of split roots hung up ready for use
If you need a long length of root cordage it won't reverse twist well so it's best to braid or platt it adding in extra as you go along, this also makes it more flexible and allows a bigger variety of knots to be used.
It's finished length is limited only by your stock of roots and personal patience!!