I went out and tried some Trailing Blackberry today. TB is strong and pretty flexible, but when it gets actually bent, as in folded over, it becomes breakable in that spot – if I yanked it hard, it would not break unless it had been folded over, then it would break at that spot.
Then I tried braiding three pieces together and that resisted folding over and was strong, seemed like it would be usable as “rope” for some purposes.
Stinging Nettle used to be used for string and fiber purposes. It used to be used for making fishnets, in fact. It takes some processing (can’t just use the stems as is) and I don’t know just what the process is. I don’t have Stinging Nettle growing where I live, either, so can’t experiment. Hope Selkie can tell us more about this.