Slight correction at the end- green nettle cordage will ply just fine, though it ends up a bit "lumpier" than retted fiber as the fibres don't lie together so well. However as it's literally the same fiber but less decayed, it lasts a lot longer when used outdoors. You can rub cord with fat (e.g beeswax or tallow)- this helps it slip over itself, e.g on a reel of cord or in a slipknot, and also makes it more durable.

Nettle cordage is really simple and uncomplicated, to refine the product you can add a tonne of extra steps but it's not necessary to get a basic tying product. In temperate climates where you can't pull a vine off a tree I suppose it's a nice plant fibre that's more flexible than most others you can find, and pretty quick to make the green stuff too :).